Limpets- Hardened by nature

by Matthew Norton

Who doesn’t enjoy a sunny day by the seaside? The fresh sea breeze, the crisp water washing in from the ocean’s edge, the moist sand caressing the spaces in between your toes. True, the sand can be an absolute nightmare to get rid of afterwards, but in that moment there is a pure, unspoiled bliss about it. Depending where you are, the coastline can take many other forms, from pebble and shingle beaches to jagged rocky shores that hold a splattering of saltwater pools at low tide. Even in predominantly sandy shores, there are sometimes isolated chunks of rock. And where there is rock, there is a good chance that you will find one the best known animals of the intertidal world. Limpets.

There are many species of limpets, from the seaweed munching blue-rayed limpet to the air breathing pulmonate limpets. But around the United Kingdom, and much of northwest Europe, the most common species is the appropriately named ‘Common limpet’. It might seem pointless for me to state this fact, but there are a lot of species, from a lot of animal groups, who are supposedly common. Such as the ‘Common seal’, the ‘Common starfish’ and the ‘Common sea urchin’. But many of these names were assigned to their species decades, if not centuries ago. And as we know, a lot can change in that time, especially when the human race is involved. 

It’s worth noting that these are just individual examples. Even within a species there can be variations and varying subspecies that further complicates attempt to identify and classify different animal species. 
I myself have lost count of how many times I’ve double checked these three images to make sure I’m not getting them mixed a thought. Spare a thought for those people who do this sort of thing for a living.

Common or not, these limpets are tougher and more resourceful than some might expect from a little sea snail. Like any other animal on the seashore, they have to contend with the dry air and scorching sunshine every time the tide goes out. When this happens, the aim of the game is to conserve and retain as much water as possible. The limpet strategy towards this end is to clamp themselves down to the rock with their strong, muscular foot, thereby cutting themselves off from the outside world until the sea returns.

For added security, limpets will also carve a ring into the rock that is a perfect fit for their shell, rough edges and all. This ring is called their ‘home scar’ because wherever they may venture when the tide is in, they will at least try to return to the same ring before the sea moves back again. Assuming nothing catastrophic happens, they can usually retrace their steps back by using a trail of mucus slime they leave in their wake. Some studies even suggest that limpets can think strategically and modify their feeding excursions based on certain environmental factors. For example, limpets have been observed to stray further from their home scars during spring tides, when the rising tide pushes further inland compared to the more modest neap tides. But this is just one example, foraging behaviour can also be affected by a limpet’s position on the shore, the kind of habitat they find themselves in and individual preferences. 

An exposed limpet foot, along with its entire body (left). Several limpet home scars visible as rings carved out in the rock (right). It is possible for a limpet to relocate from its original home scar in an attempt to adapt to changing circumstances, but in this interim they are vulnerable to opportunities.

Finding food is relatively straightforward for a limpet, for the rock on which they live is usually covered in a film of algae and other microscopic organisms on which they gorge themselves. Their weapon of choice for extracting their food is a radula, a feeding organ that most molluscs possess in one form or another, which in limpets is basically a tongue with built in scraping teeth. What is particularly remarkable about a limpet’s radula is the tensile strength of these teeth, which even outperforms the incredibly durable silk threads produced by spiders. The secret appears to be goethite, an iron oxide compound usually found in rusting metal, but which limpets have moulded into tiny nanofibers that reinforce their teeth. We humans have done something similar with glass fibres, which we have used to reinforce concrete and plastic polymer mixes and to create materials with a greater tensile strength than steel. 

An illustration of the radula of a common limpet (Patella vulgata; left). The marks left behind by a feeding limpet as it used its radula to remove microscopic algae from the rock (right). 
Most molluscs have a radula that they have modified to fit their particular feeding habits. The only exception appears to be bivalve molluscs (mussels, clams, oysters etc) who instead suck in their food and filter it through their gills.

In conclusion, limpets are brilliant examples of how even the smaller animals in the sea can surprise us with their ingenuity. We humans can be prone to believing that we are the smartest animals on the planet by a huge margin. We have certainly compensated for our squishy bodies and lack of sharp claws with tools and machines that suit our needs. But in some cases, our designs and inventions bear striking similarities to what the natural world had already evolved millions upon millions of years ago. Back when humanity was little more than a speck in the future of a very distant ancestor. 

From a human perspective

The ocean is, and always has been, a major influence on our existence on this planet. Even something as intrinsically man made as modern warfare can include aspects that originated in the natural world. This feels a particularly relevant topic at the moment, given the ongoing war in Ukraine. While there won’t be any further references to that particular conflict in the following paragraphs, I would like to stress that this section is not intended to celebrate, or glorify violence and death on this scale, or any scale really. Instead, this is more of an acknowledgement of how the ocean can be a source of inspiration in every human endeavour. Including those we hope never to experience first hand. 

Where do limpets fit into this picture you may ask? They are one of the most straightforward examples of ocean inspired weaponry because of the limpet mine. First deployed during the second world war, these weapons are designed to be stuck to the hull of an enemy vessel using magnets instead of powerful sucking muscles. Typically, they are applied manually by divers and fitted with a delay timer to allow them to clear out before detonation. Assuming all goes to plan, the result would be catastrophic damage to its target.

An early version of a limpet mine from 1939. Various modifications will have been in the decades following its inception and even today they are used by various navies around the world.

Some sources might claim that limpet mines are primarily used to disable a vessel rather than outright sinking it. But that didn’t exactly pan out when the Rainbow Warrior, Greenpeace’s flagship vessel, was sunk in 1985. It was due to sail out to the Mururoa Atoll to protest against French nuclear tests in the region, but instead it was hit by two explosions to the stern while docked in Waitematā Harbour, Auckland, New Zealand. 

The explosion ripped in the hull, leaving an enormous hole approximately 1.82m by 2.43m. Most were able to escape the sinking ship but one crew member drowned while attempting to recover his cameras. Given the lethal force used, both the captain of the Rainbow Warrior, Peter Willcox and the director of Greenpeace at the time, Steve Sawyer, have stated (in their own words) how fortunate it was that more lives weren’t lost.
The wreck was eventually recovered and allowed to sink further offshore to become an artificial reef.

Since there was only a small engine and no explosive materials on board, it wouldn’t have taken much imagination to suspect foul play. Especially when divers investigated the remains of the Rainbow Warrior and recovered the remnants of limpet mines. Two French secret agents were soon caught attempting to leave the country and they ultimately plead guilty to manslaughter for the attack and were sent back to France in 1986 to serve their lengthy prison sentences. 

The French government dismissed them as ‘rogue agents’ in an attempt to distance themselves from the attack. But further evidence has emerged in the years since, especially from documents released in 2005, that proves it was all an official operation to deter the nuclear protests by force. In the end, they made considerable compensation payments to both New Zealand and Greenpeace, the latter of whom put the money towards building a new Rainbow Warrior, which was eventually launched in 1989. Even so, the country’s damaged reputation from this incident wasn’t entirely salvaged. The two implicated agents only served two years of their respective seven and ten year sentences. A move that prompted a b******ing from the United Nations and probably caused further strain between the two countries.

Moving away from limpet mines, there are other examples of marine animals inspiring  real life weapons of war. Excoet missiles, which first entered service in 1975 and are designed to fly towards its target at very low altitude, are based on, and named after the flying fish. Going back further in time, or to certain museums, you might find sawfish snouts repurposed into swords. Or turtle shells repurposed into shields.

Models of three different versions of Exocet missiles on display at the Paris air show (top left) alongside their natural namesake, the flying fish (top right). Snout of an Australian sawfish (bottom left). Etching from 1633 depicting a battle in South America where native warriors are using turtle shells as shields (bottom right).

But it doesn’t always have to be this way. There can be peaceful applications for nature’s inventions. Jellyfish for example, their slow and steady swimming style has been adapted for underwater drones, complete with hydraulic ‘tentacles’. More stable and energy efficient than other designs, these jellyfish drones could be used for monitoring the oceans, particularly the environments surrounding vulnerable habitats like coral reefs, or exploring shipwrecks and other submerged structures. They could still be used for military surveillance, but unlike a gun, mine, or missile, they can be used for so much more. 

And in the end, there are very few ideas or inventions that are inherently good or evil. It’s how they are applied that really matters. Take Darwin’s theory of evolution by natural selection. For decades now, it has formed the basis of how we understand the natural world and has been applied to our benefit, such as tackling the causes of antibiotic resistant bacteria. But throughout the first half of the twentieth century, it was also exploited and used to justify discrimination, forced sterilisation and genocide. Whatever ideas we come up with, inspired or not by the world around us, it is our responsibility to use them for the right reasons.

Thanks for reading


Fish and Fish. 2011. A student’s guide to the seashore. ISBN. 978-0-521-72059-5

The Marine Life Database. 2020. Limpet, Pulmonate.,or%20overgrown%20with%20other%20species. Last accessed 17/03/2023

British Sea Fishing. Common Limpets. Last accessed 05/01/2023

Coleman and Hawkins. 2000. Limpet relocation from home-scars in relation to position in group

Garrity and Levings. 1983. Homing to scars as a defense against predators in the pulmonate limpet Siphonaria gigas (Gastropoda)

Little et al. 1988. Foraging behaviour of Patella vulgata L. in an Irish sea-lough

Santini et al. 2019. Barnacle cover modifies foraging behaviour of the intertidal limpet Patella vulgata


Gajera et al. 2022. Radular Morphology and Relationship Between Shell Size and Radula Size of Few Dominating Intertidal Gastropod Mollusks of Veraval Coast, Gujarat

Barber et al. 2015. Extreme strength observed in limpet teeth

The Editors of Encyclopaedia Britannica et al. 2019. Goethite. Last accessed 05/01/2023

Kodous. 2021. What’s the difference between glass fibre reinforced polymer (GFRP) and steel rebar?,epoxy%20or%20polyester%20thermosetting%20plastic. Last accessed 17/02/2023

Palmer. 2020. GLASS FIBER REINFORCED CONCRETE. Last accessed 17/02/2023


Ismay. 2019. What Are Limpet Mines, and How Do They Work? Last accessed 17/01/2023

Australian War Memorial. Limpet Mine Mk 1. Last accessed 17/01/2023

France 24. 2019. Limpet mines: classic maritime weapon. Last accessed 17/01/2023

BBC News. 1985: Rainbow Warrior sinks after explosion. Last accessed 04/02/2023

Bremner. 2005. Mitterrand ordered bombing of Rainbow Warrior, spy chief says. Last accessed 04/02/2023

NZHistory. Rainbow Warrior sunk by French secret agents. Last accessed 04/02/2023

Willsher. 2015. French spy who sank Greenpeace ship apologises for lethal bombing. Last accessed 04/02/2023

Centre for strategic & international studies. 2021. Exocet. Last accessed 04/02/2023

Military Leak. 2020. Exocet AM39 Airborne Anti-ship Missile. Last accessed 04/02/2023

The British Museum. Weapon. Last accessed 04/02/2023

Simmons. 2005. Sawfish sword. Last accessed 04/02/2023

Flight-toys. 2020. Last accessed 04/02/2023

Reading Museum. Sword Sawfish snout.*%7Bjoseph%20stevens%7D)&filename=REDMG&hitsStart=7. Last accessed 04/02/2023

Hermens. 2020. The turtle’s carapace: a curious painting support or a battle shield?,use%20turtle%20carapaces%20as%20shields. Last accessed 04/20/2023

BBC News. 2014. Flying drone inspired by swimming jellyfish. Last accessed 17/02/2023

Innovation news network. 2021. Engineers create a jellyfish-inspired underwater robot for marine exploration. Last accessed 17/02/2023

BBC News. 2018. Jellyfish robots to watch over endangered coral reefs. Last accessed 17/02/2023 2013. US Navy’s New Jellyfish Drone. Last accessed 17/02/2023

National Human Genome Research Institute. Eugenics and Scientific Racism. Last accessed 17/02/2023 editors. 2018. Social Darwinism. Last accessed 17/02/2023

Image sources

Muséum national d’histoire naturelle. 2020. [CC BY 4.0 (].

Muséum national d’histoire naturelle. 2020. [CC BY 4.0 (].

Muséum national d’histoire naturelle. 2020. [CC BY 4.0 (].

Thomas Schoch. 2012. [CC BY-SA 3.0 (].

Rosser1954. 2017. [CC BY-SA 4.0 (].,_Saltcoats_Harbour,_Ayrshire,_Scotland.jpg

Auguste Le Roux.2013. [CC BY-SA 3.0 (].

HM Government. 1939. [OGL v1.0OGL v1.0 (].

Ot. 2004. [CC BY-SA 4.0 (].

Tiraden. 2013. [CC BY-SA 3.0 (].

Auge=mit. 2019. [CC BY-SA 4.0 (].

All other images are public domain and don’t require attribution.

Fiddler crabs- Hot stuff

by Matthew Norton

For many of us, the seashore is an idyllic landscape where we can explore and relax with a cold ice cream. But for the sea creatures who actually live at this boundary between worlds, it’s one of the harshest environments on the planet. The dangers include overheating, drying out and environmental extremes that a watery existence would normally buffer against. But, nature always finds a way and some species have risen to this challenge so well that they seem to prefer life in the air, rather than the ocean. 

Fiddler crabs are a prime example since, unlike other seashore crabs, they wait until the tide has left the beach before emerging from their underground burrows to forage for food. They don’t often stray too far from a burrow, and so have the option to run back in if things get too hot out in the open. But they also need to scrounge enough from the sand to sustain themselves, so they employ a few tricks that allow them to bear the scorching heat. 

Fiddler crabs are often recognised from having one claw significantly larger than the other, although this is only seen in males. 
They live in various seashore habitats including salt marshes and the edges of mangrove forests (with sesarmid crabs typically dominating the very depths of the mangroves). Instead of feeding directly on land based plants, fiddler crabs scavenge on the sea soaked sediment, picking up bacteria, microalgae such as diatoms and anything else they can find.

To some extent, these crabs don’t even need to try anything new, instead using methods of temperature resistance that have been tried and tested in other animals (or vice versa). These include heat shock proteins, which protect other proteins from being deformed and rendered useless by high temperatures, and dispersing heat through the evaporation of water from their shells (this is also why we humans sweat). 

But to truly thrive in such an environment, one must know their limits and react accordingly (natural selection tends to show no mercy to individuals who ignore this advice). Fiddler crabs are well aware of this and employ behaviours and on the spot changes to regulate their body temperature. Some are quite clever, such as changing their orientation to the sun and the colour of their shells to absorb as little sunlight as possible. Other behaviours are little more than common sense, such as running to a burrow when it really is getting too hot and avoiding parts of the beach that are too inhospitable, even for them. That said, fiddler crabs who think long term can gradually acclimatise to hotter areas through experience and physical changes that will enhance their heat resistance, such as growth in body size.

All these adaptations will come in handy during courtship, where male fiddler crabs will push themselves to the boundaries of what they can tolerate to impress a female (some things never change). Specifically, the males wave their large claws in the air, sometimes alone and sometimes as part of a group effort. A particularly energetic display may woo a lady crab, but it will also be exhausting for the male and expose them to especially harsh conditions. Not to mention running the risk of attracting unwanted attention from predators. But such feats of endurance and daredevilry might just be what the females are looking for.

There is some evidence to suggest that the large claw of a male fiddler crab can be used as a heat sink to draw out some of the heat from the body, but there is still some debate about whether this claw helps or hinders their overall heat tolerance. Either way, it’s clear that the large claw is mainly used for securing mates by attracting a female and fighting off other males.

The need for male fiddler crabs to stand out with their large claws is an example of ‘sexual selection’, where a feature evolves in one gender of a species to appeal to the other. This approach tends to work because it demonstrates that the most ‘attractive’ male can still survive despite the issues that come with carrying his big claw around and waving it in the air. This suggests that they carry exceptional genes for surviving the seashore and so would be a good father for the next generation of fiddler crabs.

Different species of fiddler crabs will have their own preferences when it comes to where they live. But on the seashore, environmental conditions (temperature, pH, salinity etc) can vary wildly in very short distances. This can lead to crowds from multiple species occupying the same beach, but each species often have some distinct colours and patterns through which they can (hopefully) recognise each other.

From a human perspective

Fiddler crabs are one of my favourite sea creatures. Not only are they fascinating and important to the functioning of many seaside habitats, but they were also the subject of my first dissertation project back when I was a student studying marine biology. Which is why I saved them for this article, which is the 40th article I have published since starting this blog in late 2017. 

Their biggest influence on humanity in general is in their support of mangrove forests, which in turn can protect us from the ocean’s more destructive tendencies, such as tsunamis and coastal erosion. But since I have covered this in a previous article about mangroves, I decided to write about my experience working with fiddler crabs instead. 

The story begins at Plymouth University in the autumn of 2014, at which time I was in my second year and looking for volunteering opportunities for the following summer. This led me to an event on campus that had been organised by Operation Wallacea, who run and maintain research stations all over the world. My original intention was to join an existing research project, but the chance to do the research for my third year dissertation abroad was too good to pass up.

Operation Wallacea is named after Alfred Russel Wallace (1823-1913; left), a British naturalist (among many other things) who explored much of Malaysia and Indonesia, called the Malay Archipelago at the time (right). 

He also came very close to stealing Charles Darwin’s thunder when he arrived at near enough the same theory of natural selection. Wallace even wrote to Darwin about his insights in 1858, seemingly unaware that Darwin had been discreetly working on this theory for around twenty years. Together, they presented this theory to the Linnaean Society, but Darwin was quick to publish his famous book “The Origin of Species” the following year.
My experiment focussed on three different species of fiddler crab, Uca perplexa, Uca vocans and Uca crassipes. Each of them are typically found in different habitats on the seashore, but they can be found on the same beaches and their distributions do overlap.

Several months later, I embarked on a mammoth journey that consisted of two international flights, two domestic flights, a convoy of cars and a short (ish) boat ride. The eventual destination was Hoga Island (also known as Pulau Hoga), a remote island in Indonesia. Despite being homesick and knocked back by the time zone difference, it was a beautiful spit of land in the middle of Wakatobi National park.

A collection of photos from Hoga Island. Not only was the scenery breath-taking, but the people were extremely friendly and accommodating. I have particularly fond memories of a hut/restaurant on the beach where many of us partied with the locals on our last night on the island.

Of course, there are drawbacks to spending six weeks in such a remote location. For starters, the lack of running water meant we had to drink bottled water and shower with buckets of cold water. The heat was also pretty intense, and with no electricity in some areas, including the sleeping huts, I can guarantee there were plenty of sticky and sweaty nights.

The health and safety orientation we attended on arrival also highlighted the less trivial aspects of life on a tropical island. These ranged from diseases and dangerous animals to coral holes and the vulnerability of our location should earthquakes and tsunamis hit the region. The words “we will make you comfortable” were used on multiple occasions, although most dangers could be averted through appropriate precautions and common sense (i.e. don’t be an idiot).

We all kept to the pathways that were laid down between the communal area and our huts. The alternative was to rough it through perilous terrain and numerous coral holes. 
Falling into one of such coral holes was likely to result in deep cuts and broken holes due to all the hard and sticky out bits.

Once that was out of the way, and we’d all had a chance to sleep off the jet lag, it was straight to the job at hand. The first week or two was spent getting to know the other researchers on the island and setting up and testing the equipment for my experiments. On more than one occasion, my original research plan had to be modified according to the resources we had available. 

I performed two separate experiments with fiddler crabs during my time on Hoga Island to study the thermal ecology of the three species.

To measure heat tolerance, I placed each crab in a chamber (left) and slowly increased the temperature inside until they reached a critical thermal maximum (CTmax for short). This was temperature at which the crabs were unable to right themselves after being flipped over, indicating that they were no longer able to function properly. It would have been stressful for the crabs, but the alternative was to wait to see at what temperature they dropped dead.

To measure temperature preference, I placed each crab onto a metal tray filled with a layer of damp sand (right) with a heated plate at one end and an styrofoam box full of ice at the other. The cold end was originally supposed to be a cooling fan, but in my experience you can’t have a proper scientific experiment unless something goes wrong and you’re forced to improvise. With a collection of temperature data loggers (the metal circles) and images from a time lapse camera, I was able to estimate what temperatures the crab hung around over a 12 hour period.

With that done, it was time to hop over to the nearby island of Kaledupa to collect the crabs. On this brief expedition, I was lucky enough to be accompanied by a group of Indonesian students who were incredibly skilled at catching fiddler crabs as they bolted to the safety of their burrows. I meanwhile, was absolutely hopeless and only managed to catch a couple of them through dumb luck.

The fiddler crabs were collected from the nearby Kaledupa island (left) and transported in boxes with a bit of mud and seawater. 
These crabs didn’t really need much more than this in the wet lab back on Hoga island.

The crabs were taken back to Hoga and housed in a wet laboratory building located five minutes away (by foot) from the main site. There was a cleared path in the vegetation between the two areas, but this didn’t always prevent encounters with the island’s wildlife. Fortunately, they let me pass peacefully, as many animals will do so long as we respect their personal space. 

Monitor lizards were a common sight during my five minute commutes to and from the wet lab. 
To keep the fiddler crabs happy while they were under my care, I regularly changed their tanks with fresh seawater and mud. Collecting the seawater was a simple matter of taking a bucket and going to the sea while the ideal mud came from an existing fiddler crab habitat among the huts on Hoga island.
However, on my first mud collection trip, I foolishly leapt into the mud from a low wall and was promptly up to my knees in the stuff. When I emerged, one sandal was smothered in mud while the other was lost altogether.
Other animals found on Hoga island included snakes in trees (top left), praying mantis in the communal area (top right), big spiders in the bathrooms (no really; bottom left) and even cats around the premises (bottom right). 
Unfortunately, having chosen to skip the rabies vaccination to save money (in hindsight, not my best decision) I had stick to my self-imposed rule of not touching anything with fur and claws. No matter how cute.

There were other students working on their own research projects in the wet lab, and while there was no strict ‘quid pro quo’ arrangement in place we were encouraged to help each other if we weren’t otherwise occupied. Towards the end of our time on the island, some of us went out for a bit of snorkelling. We didn’t stray far from the shore, but we still saw some beautifully coloured fish in the crystal clear water. The spectacle made up for the state I was in as water constantly leaked into my goggles and I struggled to use my flippers in a way that was even remotely effective.

My fiddler crabs were quite happy being fed fish flakes. But for another student’s project, we went searching the sandy shores of Hoga island (left) for crabs and prawns to feed their cuttlefish (right). I’m pretty sure they were only joking when they suggested using my fiddler crabs instead, but better safe than sorry.

The six weeks ended with a big party that involved alcohol, dancing on tables and a bonfire on the beach. The result I got from my research didn’t reveal anything particularly exciting, but it was, at the very least, a well executed piece of science, which is the most important part of any study, regardless of aims, results or where in the world the data is coming from. Still, it was an incredible experience to spend six weeks away from civilisation, albeit with some modern comforts (still can’t believe there was ice cream available on the island). I’d imagine that most people would like to get away from it all from time to time. 

Thanks for reading


Eliss. 1968. Transition from water to land in decapod crustaceans

Carlson et al. 2014. Density, Shell Use and Species Composition of Juvenile Fiddler Crabs (Uca spp.) at Low and High Anthropogenic Impact Salt Marsh Sites

Emmerson and McGwynne. 1992. Feeding and assimilation of mangrove leaves by the crab Sesarma meinerti de Man in relation to leaf-litter production in Mgazana, a warm-temperate southern African mangrove swamp

Osborne and Smith. 1990. Differential predation on mangrove propagules in open and closed canopy forest habitats

Meziane et al. 2002. Role of fiddler crabs of a subtropical intertidal flat on the fate of sedimentary fatty acids

Robertson and Newell. 1982. A study of particle ingestion by three fiddler crab species foraging on sandy sediments

Feder and Hofmann. 1999. Heat-shock proteins, molecular chaperones, and the stress response: evolutionary and ecological physiology

Frenkel et al. 2008. Both heat shock and water deprivation trigger Hsp70 expression in the olfactory lobe of the crab Chasmagnathus granulatus

Somero. 2004. Adaptation of enzymes to temperature: searching for basic “strategies”

Barnwell. 1968. Comparative aspects of the chromatophoric responses to light and temperature in fiddler crabs of the genus Uca

Wilkens and Fingerman. 1965. Heat tolerance and temperature relationships of the fiddler crab, Uca pugilator, with reference to body coloration

Backwell. 2019. Synchronous waving in fiddler crabs: a review

Hews et al. 2021. Field-based body temperatures reveal behavioral thermoregulation strategies of the Atlantic marsh fiddler crab Minuca pugnax

Levinton. 2020. Thermal stress: The role of body size and the giant major claw in survival and heat transfer of a fiddler crab (Leptuca pugilator)

Darnell and Munguia. 2011. Thermoregulation as an alternate function of the sexually dimorphic fiddler crab claw

Allen and Levinton. 2014. Sexual selection and the physiological consequences of habitat choice by a fiddler crab

Takeda. 2006. Behavioural evidence for body colour signaling in the fiddler crab Uca perplexa (Brachyura: Ocypodidae)

Forward and Rittschof. 1994. Photoresponses of crab megalopae in offshore and estuarine waters: implications for transport

Frith and Brunenmeister. 1980. Ecological and population studies of fiddler crabs (Ocypodidae, genus Uca) on a mangrove shore at Phuket Island, western peninsular Thailand

Koch et al. Comparative population dynamics of four fiddler crabs (Ocypodidae, genus Uca) from a North Brazilian mangrove ecosystem

Mokhtari et al. 2015. Determination of key environmental factors responsible for distribution patterns of fiddler crabs in a tropical mangrove ecosystem

McNish. Who was Alfred Russel Wallace? Last accessed 14/09/2021

Image sources

Kathy. 2009. [CC BY 2.0 (].

Denise Chan from Hong Kong, China. 2006. [CC BY-SA 2.0 (].

Kate Perez. 2019. [CC BY-SA 4.0 (].

Bernard DUPONT from FRANCE. 2014. [CC BY-SA 2.0 (].

Thomas Brown. 2011. [CC BY 2.0 (].

All other images are public domain, or my own.

Sharks- To swim or not to swim

by Matthew Norton

The ocean is full of animals who possess incredible strength and who are capable of feats of endurance that we couldn’t hope to match, not without the help of machines. Examples include the awesome punching power of mantis shrimp, the arduous migrations made by sperm whales and the exhausting patience of a saltwater crocodile as it lies in wait below the water’s surface. Though few creatures can match the sheer power and hunting prowess of the sharks. 

These three species are prime examples of a shark’s hunting prowess. 
Bull sharks Carcharhinus leucas (top left) have awesome biting power that can rival that of the great white shark.  Whitetip reef sharks Triaenodon obesus (top right) can squeeze themselves through tight gaps, ensuring there is no escape for their prey. Common sawsharks Pristiophorus cirratus (bottom) use their saw shaped snouts to dig out prey buried in sand and slash them before consumption.

But even these powerful fish don’t spend all their time eating and making little sharks. Like many animals, they too need their down time and have no interest in burning up their energy reserves without good reason. To that end, sharks generally have a light skeleton, made mostly of cartilage, and a large liver full of oils that are lighter than water. Both of these adaptations make it easier for sharks to stay above the seafloor and out of the deep sea (unless that’s where they belong). Their skin is also covered in tiny little teeth (dermal denticles) which makes the animal more streamlined and able to swim faster.

The dermal denticles of a lemon shark Negaprion brevirostris as viewed under an electron microscope. These miniature teeth often makes the skin of a shark feel smooth if you rub your hand from head to tail, but very rough if you rub the other way.

Some sharks have gone even further in their evolution, fine tuning their biology so that they exert as little effort as possible. Sand tiger sharks (Carcharias taurus) are a great example, enhancing their buoyancy by swallowing air and holding it in their stomach. This trick allows them to hover in the water, barely moving until there is something good to eat, or they feel like stretching their fins a little. 

Sand tiger sharks are also incredibly slow at reproduction. This is due to cannibalism among the embryos until only two survivors are left, one in each uterus.  At least these surviving pups are well nourished when they eventually leave their mother.

Other species, such as the nurse shark (Ginglymostoma cirratum), settle down on the seafloor for extended rest stops. During this time, they use powerful muscles in their mouth to manually pump water through their gills so they can breathe. Should the shark be covered in sand, they also have openings just behind the eyes (called spiracles) through which they can draw in water.

Nurse sharks are nocturnal hunters, spending their nights searching coral reefs and rocky habitats for fish, shellfish and cephalopods (i.e. octopus and squid). Instead of biting into their meals, nurse sharks use powerful muscles in their mouth to suck them in. Straight out of the shell if necessary.
But during the day, many of these prey are awake, alert and tricky to catch. So the nurse sharks don’t even bother trying, instead conserving their energy as they wait for night. A time where their finely tuned senses gives the shark a significant advantage. 

For many sharks, a nap on the ground isn’t an option. They have to keep swimming to keep ramming water through their gills, otherwise they will drown. But these species can still sort of sleep by turning off one side of their brain while the other side keeps the body moving and an eye out for danger and food. 

But are any of these sharks really sleeping? Or are they just in a state of quiet wakefulness? Truth be told, we’ll probably know since the only first hand experience we have of being asleep is as human beings. Assuming that sharks sleep like we do is a flawed approach at best, as is projecting any of our thoughts and experiences onto another species, a concept known as anthropomorphisation. Nonetheless, it can help to make science more widely accessible and encourage empathy with the natural world, so long as we don’t take it too far.

From a human perspective

Whether asleep, or quietly awake, a resting shark is still a shark and they should be approached with caution and respect. Even when docile, nurse sharks have been known to retaliate against divers and snorkelers who invade their personal space. Sand tigers have also occasionally clashed with humans over a tasty fish, despite their normally sluggish behaviour. 

Such close interactions between diver and nurse shark should be avoided unless the former has a justifiable reason for approaching the shark and is taking suitable precautions.

But as large and powerful as sharks are, researchers need to get close to them, sometimes uncomfortably close, to get the data needed for wildlife management and conservation efforts. Such data is also in pretty high demand since a large number of shark species are still listed as ‘Data Deficient’ by the International Union for Conservation of Nature (IUCN). Meaning we don’t have enough information to really know if they are at risk, never mind what they might be at risk from. 

There are non-invasive methods we can use to study the lives of sharks with little to no interference. These include underwater censuses, baited cameras and environmental DNA samples, all of which can be used to estimate how many of each shark species there are in a given area. Carefully planned experiments in the wild can also reveal how sharks would behave in a given situation more accurately compared to testing their responses in an unnatural setting. 

There have been two studies, performed separately, on Caribbean reef sharks Carcharhinus perezi (left) and bull sharks Carcharhinus leucas (right) to determine how the posture and orientation of humans in the water affected their behaviour. Both found that their respective shark species were more weary of the human/s when they adopted certain postures. 

Were these experiments conducted in less natural surroundings, and with more invasive techniques, the animals would have been under more stress and their reactions would less natural. In the case of the bull shark study, the researchers chose not to fix the animals with tags (see further down the article) for this reason.

But by capturing live sharks (hopefully temporarily) we can also measure their size, determine their gender and identify the species with greater accuracy. A quick glance might not always be enough to tell apart two or more similar looking species (see image above).

Having the shark in brief custody also offers the chance for more intimate measurements through blood and tissue samples. Such samples can reveal the concentrations of hormones and heavy metal pollutants in the shark’s body, the latter of which sharks are particularly vulnerable to since they can accumulate such toxins from their prey. Admittedly, we could take more samples from the insides of dead sharks, but non-lethal samples from living specimens can still indicate what toxins they have throughout their entire body. With the added bonus of not having to wait for a corpse to wash up, or relying on being able to get there while sampling is still viable.

Captured sharks can also be fixed with tags, usually on, or around the dorsal fin, so that their movements can be tracked post-release. It’s a method that has been used on sharks for nearly a century (although fish in general have been tagged for much, much longer), although these days sharks are often fixed with satellite tags that ‘ping’ their location every time they surface. As more and more sharks are tagged, the hope is that we can identify their migration routes and seasonal hotspots, particularly for endangered and elusive species, and then protect those areas as needed.

A salmon shark Lamna ditropis with a tag in the base of the dorsal fin.

But for all the information they give us, the experience of being tagged and/or having samples taken from them is never going to be a pleasant one for the sharks. The best we can do is to carry on developing new techniques that minimise the stress and trauma inflicted on these animals. 

Fast capture techniques, such as snagging sharks by the tail, have already been tested as alternatives to more conventional techniques, such as netting and angling. But a better approach, certainly from an animal welfare perspective, would be to avoid taking sharks out of the water at all. Dart tags, attached to the end of a pole, or the bolt of a speargun, are already being used. As are biopsy probes for extracting tissue samples, both underwater and from the side of a boat, before immediately preserving them for later analysis. 

With large sharks such as the great white Carcharodon carcharias (left) and the whale shark Rhincodon typus (right) capture and restraint would be extremely difficult, not to mention stressful for the animals. This makes dart tags and biopsy probes the preferred option for everyone involved.

With all the technical challenges and ethical considerations around shark research, a top priority should be getting the very most of each set of data we get from each shark. An aim that can only be realistically achieved through collaborations between individuals and organisations from different professions and countries. The Cooperative Shark Tagging Program (CSTP) is a great example since it brings together anglers, commercial fisheries and scientists from the National Oceanic and Atmospheric Administration (NOAA). Since its inception in 1962, the program has tagged over 295,000 sharks from 33 species. 

I doubt that such a monumental feat could have been achieved without the thousands of volunteers who have contributed to this program. A true testament to what citizen science can achieve. 

Thanks for reading


Habeggar et al. 2012. Feeding biomechanics and theoretical calculations of bite force in bull sharks (Carcharhinus leucas) during ontogeny

Randall. 1977. Contribution to the biology of the whitetip reef shark (Triaenodon obesus)

Oceana. Longnose Sawshark. Last accessed 18/07/2021

Shark Trust. Shark Anatomy. Last accessed 12/07/2021

Oceana. Sand Tiger Shark. Last accessed 12/07/2021

National Geographic. Sand Tiger Shark. Last accessed 12/07/2021

French et al. 2018. Ginglymostoma cirratum. Last accessed 12/07/2021

Castro. 2013. Must Sharks Keep Swimming to Stay Alive? Last accessed 18/07/2021

Whitney et al. 2016. The effects of temperature and swimming speed on the metabolic rate of the nurse shark (Ginglymostoma cirratum, Bonaterre)

Kelly et al. 2019. Evidence for sleep in sharks and rays: behavioural, physiological, and evolutionary considerations

Ritter. 2019. Is the Shark Just Drifting, or Does It Take a Quick Nap?

Rogers. 2016. How Do Sharks Sleep? Last accessed 12/07/2021

Aucoin et al. 2017. A new underwater shark capture method used by divers to catch and release nurse sharks (Ginglymostoma cirratum)

Shark Research Institute. 2005. SHARK SPECIES INVOLVED IN INCIDENTS. Last accessed 12/07/2021

IUCN Shark Specialist Group. Frequently Asked Questions: Sharks, Rays, and chimaeras. Last accessed 09/09/2021

De Vos et al. 2015. Baited remote underwater video system (BRUVs) survey of chondrichthyan diversity in False Bay, South Africa

McCauley et al. 2012. Evaluating the performance of methods for estimating the abundance of rapidly declining coastal shark populations

Kilfoil et al. 2021. The influence of shark behavior and environmental conditions on baited remote underwater video survey results

Boussarie et al. 2018. Environmental DNA illuminates the dark diversity of sharks

Ritter and Amin. 2014. Are Caribbean reef sharks, Carcharhinus perezi, able to perceive human body orientation?

Ritter and Amin. 2012. Effect of human body position on the swimming behavior of bull sharks, Carcharhinus leucas

Martin. Shark Tissue Samples Needed to Assist Phylogenetic Research. Last accessed 03/08/2021

Save our seas foundation. Shark Share Global. Last accessed 03/08/2021

Stevens and Brown. 1974. Occurrence of heavy metals in the blue shark Prionace glauca and selected pelagic in the NE Atlantic Ocean

O’Bryhim et al. 2017. Relationships of mercury concentrations across tissue types, muscle regions and fins for two shark species

University of Miami. 2021. GPs for Sharks. Last accessed 25/07/2021

Kohler and Turner. 2001. Shark tagging: a review of conventional methods and studies

Queiroz et al. 2019. Global spatial risk assessment of sharks under the footprint of fisheries

Ocearch. Shark Tracker. Last accessed 23/08/2021

Gleiss et al. 2009. A new prospect for tagging large free-swimming sharks with motion-sensitive data-loggers

Robbins. 2006. Evaluation of two underwater biopsy probes for in situ collection of shark tissue samples

Meyer et al. 2018. Simple biopsy modification to collect muscle samples from free-swimming sharks

Daly and Smale. 2013. Evaluation of an underwater biopsy probe for collecting tissue samples from bull sharks Carcharhinus leucas

NOAA. 2020. Cooperative Shark Tagging Program. Last accessed 23/08/2021

NOAA. Tagging Instructions and Resources for Volunteers. Last accessed 23/08/2021

Image sources

Albert Kok. 2007. [CC BY-SA 3.0 (].

Pascal Deynat/Odontobase. 2011. [CC BY-SA 3.0 (].

Maniacduhockey. 2011. [CC BY-SA 3.0 (].

Aquaimages. 2006. [CC BY-SA 2.5 (].,_Monito_Island,_Puerto_Rico.jpg

Michelle Hedstrom from Arvada. 2008. [CC BY 2.0 (].

Elias Levy. 2014. [CC BY 2.0 (].

Matthew T Rader, 2018. [CC BY-SA 4.0 (].,_Mexico.jpg

All other images are public domain and do not require attribution

Starfish- Sticky feet

by Matthew Norton

The seafloor can create some rough terrain with jagged rocks, soft sands and sheer cliff edges being some of the obstacles that bottom dwelling animals must overcome to find food and escape predators. Crabs rise to the challenge with their jointed legs, as do octopuses with their sucking tentacles and lobe-finned fish with their thick, fleshy fins. But few animals are as capable at off roading as starfish (also called ‘sea stars’ for not being actual fish).

Starfish are famous for the five way symmetry of their bodies, which they shared to varying extents with their cousins, including sea cucumbers, sea urchins and brittle stars.
All members of the invertebrate animal group Echinodermata (commonly referred to as echinoderms).

If you have ever visited an aquarium you may be aware of how starfish can climb up vertical surfaces, particularly glass windows, with ease. It’s a feat they achieve with the rows upon rows of tube feet they keep on the underside, both along each arm and around the middle. We often think of these tube feet as mere suckers, but starfish also use a chemical adhesive (i.e. glue) to strengthen their hold. That is, until powerful muscles, connected to the tubes of pressurised water that props up the whole starfish body, unsticks each tube foot and allows it to move forward.

The tube feet of a northern sea star Leptasterias polaris stretched out and presumably looking for the next surface to adhere to (left). A sunflower sea star Pycnopodia helianthoides (right), who is capable of moving at the rate of 1 metre per minute with 15,000 tube feet. A real speed demon among the starfish.
Starfish can also use their tube feet to catch their prey and, where necessary, pull its shell apart to get at the tasty animal inside, just as the common starfish Asterias rubens is doing as it tackles a mussel (left). 
Circeaster pullus (right) demonstrating how starfish can push their stomach out through their mouth to digest their prey. In this case, the starfish is attacking a coral colony.

It might seem a bit pedantic to focus on how exactly starfish hold on to a surface, but small details, like the inclusion of glue, can make a huge difference. For instance, there is evidence to suggest that a single tube foot can resist being dislodged no matter what angle it is pushed from. If starfish relied on suction alone, their predators, or competitors, may have greater success if they attacked from the side.

Sticky feet might also be more forgiving on those occasions where suckers would fail to get a good grip, whether it be due to extremely rough terrain or bad aim. And considering the number of tube feet that some species of starfish possess, mistakes are inevitable. For animals with effectively no brain, it’s amazing they can coordinate their movement at all.

Using glue to stick to surfaces is also going to have some drawbacks, otherwise everyone would do it. One possible issue could arise from the sticky ‘footprints’ they leave behind, which could betray their presence. In a previous article, I mentioned that many submerged surfaces are, over time, covered in a layer of organic material called a ‘biofilm’. The concoctions of chemicals it contains is a valuable source of information for starfish, who can read the biofilm with receptors built into their tube feet. But this can also be true of their enemies, including other starfish. 

Scanning electron image of a biofilm incubated on a steel sample under laboratory conditions. Similar biofilms in the wild would also have this kind of variety of microorganisms and associated substances. A real treasure of information for animals who can sift through it all.

Tube feet are useful pieces of kit for starfish, as well as their diverse cousins. But with such a diversity of owners, there is also a diversity in shape and structure. One source even mentioned a type of tube foot with a ‘knob’ end, as described from the suggestive shape of its tip under a microscope. It would seem that evolution’s tendency for tinkering with a core design can produce some odd results.

In crinoids, the ancient cousins of starfish, their tube feet is used exclusively for feeding as they capture pieces of food floating in the water. Given that they either flap about in the water, like the red feather star, or are rooted to the seafloor, like the white sea lily, movement by tube feet is completely unnecessary.

From a human perspective

Few sea creatures are as immediately recognisable as starfish. I have seen the joy they bring firsthand through my current job at the National Marine Aquarium in Plymouth (not affiliated with this blog). They bring joy to so many visitors, particularly those young children who run straight up to the tanks that have starfish stuck to the glass.

Their popularity can also be seen in the stories and works of literature that starfish have inspired. These include fables, poems and even non-fiction books about business management. 

A drawing of a starfish that accompanied a poem by Lydia Sigourney called ‘Hope in God’.

The influence of starfish also extends into popular culture, with the most famous example being Patrick Star, the dimwittted, but lovable best friend of Spongebob Squarepants. He fits the general appearance of a starfish (minus the many tube feet), but Patrick’s character and quirks were also inspired by the real thing. Specifically, how starfish appear to be slow and dumb, only to surprise you with their active and aggressive lifestyle.

Starfish are also important in scientific research, particularly in a laboratory where they are relatively easy to breed and care for. There is particular hope for their role in stem cell research where, one day, we might be able to replicate a starfish’s ability to regenerate almost any part of their body. I very much doubt we can push this area of science into creating real life versions of Wolverine, Deadpool or any other comic book character gifted with super fast healing. But it could help to advance the treatment of medical conditions that can only be tackled sufficiently with stem cells.

The blue bat star, Asterina pectinifera, is particularly popular as a model organism in developmental biology for various reasons. They are resilient in laboratory conditions, being happy in a wide range of temperatures and not being a fussy eater. They also live in shallow water and so can easily be collected.
Starfish missing an arm. It could have been ripped off, or the starfish could have detached willingly as a defence reaction. Either way, it will be able to grow it back again if given time to do so.

Going back to how starfish move, there have been multiple projects to create ‘starfish robots’. Like other animal inspired robots, their designs replicate the shape and omnidirectional movement of the real thing. In many cases, the bulk of the starfish robot is made out of soft and flexible materials such as silicon. These would be closer to the constitution of real starfish compared to metal, or hard plastics.

If these artificial starfish were to be let loose on the seafloor (within reason) the ‘soft robot’ approach is probably the way to go. Considering the rough and unpredictable environments that real starfish have to contend with. Although, the ‘tube feet’ that one such robot starfish is equipped with are suckers only. An oversight that is probably common in the current roster of robot starfish designs as the inclusion of an onboard reservoir of synthetic glue, and a delivery system in each replicate foot, would surely be a logistical nightmare. 

As an addendum to the whole topic of robots and starfish, a few years ago there was a project to protect coral reefs from the destructive crown of thorns starfish by dispatching them with robots armed with lethal injections. I draw your attention to this in case you want to read more about starfish robotics. You may have to trawl through a few pages of headlines about robots killing starfish, like I did when I was researching for this article. 

But what would happen if these two branches of underwater robotics were brought together? We could end up with five legged terminators long before anything that resembles Arnold Schwarzeneggar. 

Thanks for reading


NOAA. 2021. Are starfish really fish? Last accessed 11/05/2021

IFL Science. This GIF Of A Starfish “Walking” Is Legitimately Disturbing. Last accessed 31/05/2021

Wikipedia. 2020. Tube feet. Last accessed 16/05/2021

Smith. 1937. The structure and function of the tube feet in certain echinoderms

Hennebert et al. 2012. Echinoderms don’t suck: evidence against the involvement of suction in tube foot attachment


Kerkut. 1954. The mechanisms of coordination of the starfish tube feet

ChrisM. 2011. Secrets of the Starfish Sieve Plate & Madreporite Mysteries? aka What is that little circle on top?? Last accessed 29/06/2021

BS Media. 2017. WATER VASCULAR SYSTEM IN STARFISH. Last accessed 29/06/2021

Feder and Lasker. 1964. Partial purification of a substance from starfish tube feet which elicits escape responses in gastropod molluscs

Hennebert et al. 2013. Functional biology of asteroid tube feet

Kellogg, D. and D. Fautin 2001. “Crinoidea” (On-line), Animal Diversity Web. Accessed May 16, 2021 at

Wikipedia. 2021. Starfish. Last accessed 07/06/2021

McKay et al. 2001. Gadi Mirrabooka: Australian Aboriginal Tales from the Dreaming. Libraries Unlimited. ISBN. 1-56308-923-8

Alyson-Wieczorek. 2006. Poems for the sea by Sigourney, L. H. (Lydia Howard), 1791-1865. Last accessed 07/06/2021

Meier. 2010. The Starfish and the Spider: 8 Principles of Decentralization. Last accessed 07/06/2021

Luke Alexander. 2019. Case Of The Sponge ‘Bob’ (Video 2005). Last accessed 22/06/2021

UCMP. Introduction to the Deuterostomia. Last accessed 13/06/2021

Dove. 2020. The Future of Human Healing Lies in the Brain of a Starfish. Last accessed 13/06/2021

Davydov et al. 1990. The Starfish Asterina pectinifera

Jin et al. 2016. A starfish robot based on soft and smart modular structure (SMS) actuated by SMA wires

Rincon. 2011. Starfish-inspired ‘soft’ robot squeezes under obstacles. Last accessed 13/05/2021

Poungrat and Maneewarn. 2017. A starfish inspired robot with multi-directional tube feet locomotion

Yang et al. 2021. Starfish Inspired Milli Soft Robot With Omnidirectional Adaptive Locomotion Ability

MIT Computer Science & Artificial Intelligence Lab. 2021. Robo-starfish aims to enable closer study of aquatic life. Last accessed 13/05/2021

Braun. 2018. Sea-Star Murdering Robots Are Deployed in the Great Barrier Reef., Last accessed 13/05/2021

Espiner. 2015. Starfish-killing robot close to trials on Great Barrier Reef. Last accessed 13/05/2021

Image sources

Hans Hillewaert. 2000. [CC BY-SA 4.0 (].

André-Philippe D. Picard. 2011. [CC BY-SA 3.0 (].

AlexEng. 2010. [CC BY-SA 4.0 (].

M.Buschmann. 2004. [CC BY-SA 3.0 (].

NOAA. 2006. [CC BY-SA 2.0 (].

Krzysztof A. Zacharski. 2017. [CC BY 4.0 (].

NOAA Photo Library. 2013. [CC BY 2.0 (].

Miya. 2004. [CC BY-SA 3.0 (].

384. 2017. [CC BY-SA 4.0 (].

All other images are public domain and do not require attribution.

Crocodiles- The price of power

by Matthew Norton

Energy is a major currency in the natural world. Every living thing gets it by eating food, using sunlight to make sugars, or by exploiting some other natural resource. But to do anything productive, and that includes just being alive, one has to spend energy and a lot of it. This is especially true in the top predators, whose powerful hunting techniques usually incur such a high energy cost that it makes sense to maintain a budget and only unleash their full potential at the opportune moment. 

Crocodiles, such as the particularly impressive saltwater crocodile and Nile crocodile, are no exception. Though thoroughly lazy when bathing in the sun, as many reptiles are compelled to due to being cold blooded, they are not to be underestimated when they are on the hunt. As well as their formidable size and teeth, these crocs are also patient, often stalking their prey in murky water.  Once in range, the crocodile will lunge forward with incredible speed and strength. If done right, their target will have no idea what hit them until it’s too late.

The saltwater crocodile Crocodylus porosus  (left) and Nile crocodile Crocodylus niloticus  (right) are two large and powerful crocodile species who are not to be underestimated, especially when they’re in their element. This quote from the Australian Museum website about the Saltwater crocodile (also known as the estuarine crocodile) says it best.
“does not suffer foolish humans that enter its watery domain”
A Nile crocodile ambushing wildebeest as they cross the Mara river in Tanzania.

But as I said before, these bursts of power come at a cost, not just in the energy required for these strikes, but also in the speed at which this energy is needed. In turn, this requires a change in how that energy is released in the cells that make up the crocodile’s muscles.

Under normal conditions, energy is stored in an animal’s body in the form of glucose sugars. The energy is then released by a process called aerobic respiration (with oxygen), which consists of a long series of chemical reactions that includes oxygen and a great deal of cellular trickery to squeeze every last piece of energy out of each molecule of glucose.

But when that same animal needs a lot of energy very quickly, which tends to happen when you are hunting down your food, or running for your life, the aerobic method is too slow to keep up with demand. In this situation, anaerobic respiration (without oxygen) is the preferable alternative as it releases energy far quicker, though at the cost of efficiency. This version of respiration also leads to the build up of a waste product called lactic acid.

A summary diagram of the reactions of aerobic respiration.
Each glucose molecule is twisted, turned and changed to release the energy contained within. The ultimate goal is charge a bunch of ADP molecules into ATP, which for all intensive purposes are tiny flying batteries that fly off and deliver the energy where its needed. After which, they go back to being ADP until they get another recharge.
Aerobic respiration is a slow and complicated process (for a cell),  but it can squeeze over 30 ATP recharges out of each glucose molecule. Two ATPs are spent during the glycolysis stage, but there is still a sizeable net profit to be made from this process. Meanwhile, anaerobic respiration produces a pitiful 4 ATP recharges per glucose and still has to endure the two ATP investment.
A summary diagram of the reactions in anaerobic respiration, which is pretty much the first stage of aerobic respiration modified into its own self contained loop. In this version of the energy release process, the pyruvate molecules that come out are converted into lactic acid in order to reset the glycolysis cycle.  As the body continues to rely on anaerobic respiration this lactic acid builds up, which is what causes the aches and pains we get from intense exercise.
Once aerobic respiration has the chance to take over again, additional oxygen is brought to covert the excess lactic acid into less troublesome chemicals. This is often referred to as repaying an ‘oxygen debt’.

The body of any animal can only tolerate so much excess lactic acid, but it’s in this inconvenient side effect that crocodiles have a surprising advantage. For they can tolerate incredibly high levels of the acid that would kill most other animals. Perhaps this allows them to throw extra power into their attacks, or perhaps this extra resistance is linked to other behaviours that require anaerobic respiration in some capacity.

Crocodiles would have little oxygen to spare while they are submerged in water and preparing for an attack. During this time, they can minimise their energy consumption to some extent, but there are still basic life functions that cannot be switched off. To keep their body ticking over, they have to resort to anaerobic respiration and then launch at their prey with a body already brimming with lactic acid. Depending on the size of their catch, they may also have to drag it back into the water and tenderise their meal into manageable chunks. With barely a chance to catch their breath. 

This double whammy of a high energy demand during their attacks and a low oxygen supply during their underwater stalking may go some way to explain why the lactic acid tolerance of crocodiles is so high. And with the toll it would take on their existing energy reserves, it’s no wonder they have such voracious appetites. 

Even when lurking just above the water, crocodiles can be difficult to spot from a distance (most animals don’t have access to cameras with zoom functionality). Underwater, they would be practically invisible.
During which time they can minimise their energy consumption such as staying very still and slowing their heart rate right down. The heart of a crocodile even has a built in option for diverting blood away the lungs.
Normally, the right side of heart would send blood to the lungs through the pulmonary artery to pick up oxygen. That blood then comes back to the left side of the heart. The left side then pumps the oxygen rich blood through a second artery (called the aorta) to the rest of the body to deliver the oxygen.
But during a dive, sending blood to pick up oxygen that isn’t there would be pointless . So the crocodile’s heart has a third artery, the left aorta, that picks up the low oxygen blood in the right side of the heart and sends it back round the body. Skipping the lungs entirely.
The result of these extreme measures is that crocodiles can stay underwater for hours, though their dives usually only last around 10-15 minutes.

The lives of animals are full of choices that are influenced by how best to spend their energy reserves and this can lead to behaviours that would otherwise seem odd. Predators can ignore food sources that are too difficult to crack and prey can choose to ignore the first whispers of danger rather than lose valuable feeding time over a false alarm. Sometimes, parents will even abandon, or consume their current batch of young in hope that their future offspring will prove to be a better investment. Just like any human entrepreneur, animals often have to take risks by making, or not making, an energy investment for a chance to reap the best rewards in life.

From a human perspective 

Crocodiles are formidable hunters and they have been known to attack humans from time to time. Saltwater crocodiles and Nile crocodiles have racked up the highest number of attacks, both fatal and non-fatal, worldwide with a total of 1,350 and 1,005 attacks over the last ten years respectively. This sounds horrific, but these numbers average out at just over 230 attacks per year for both species combined, for a planet with billions of people. Like with sharks, the chances of being attacked by a crocodile are extremely remote and can be reduced further with little more than common sense.  

Swimmers, and anyone else in the area, would be wise to take heed of warning signs like these.
There are also other precautions you can take, such as not dangling limbs over the sides of boats, exercising caution at night, camping at least 50m away from the shore and not washing dishes, or preparing food by the water’s edge.

But let us not forget that humans exploit crocodiles for their meat and skin, often in dedicated farms. From time to time, we also capture and relocate crocodiles to zoos, for research purposes and to move problematic individuals away from a populated area. 

Meat from a Saltwater Crocodile served with rice and a creamy sauce (left).  Wallets made from crocodile skin (right).
A crocodile farm in Australia (left) and in Israel (right). The latter looks much more spacious and comfortable for the animals at face value.
But with admittedly little in the way of context  to these two photos I’d advise caution in making any quick judgements about these two farms. But I think we can all agree on the general principle that if we are to farm and eventually slaughter animals, they should be made as healthy and content with their lives as is possible.

Since it’s rare for crocodiles to be taken away without a fight, the risk to the humans involved is obvious. But as the croc itself struggles against the ropes and nets, they build up a lot of lactic acid in their blood and muscles. It attempts to restrain the animal drag on too long, this build up can surpass even their astronomical tolerance with the crocodile eventually struggling itself to death.  Sometimes, they may be captured alive only to succumb to the ordeal a short while later due to the oxygen not coming in fast enough to deal with the adverse effects on their body.

Thankfully, there have been attempts to develop new methods of capture that would make the whole experience less stressful for the crocodile. And probably less tense for their human captors as well.

A study published in 2003 trialed the use of ‘electrostunning’ on saltwater crocodiles (effectively applying a taser to the back of the neck). Compared to manual restraint alone (in this case noosing the animals with rope), the electrostunned crocodiles didn’t experience as much of a build up in lactic acid, or in other telltale chemicals in the blood. There was still a build up regardless of what method was used, as there would have been some maneuvering involved to get the crocs from the electrostunned group into position.

A later study applied a similar test, but on Nile crocodiles, stating outright that different crocodile species might react differently to electrostunning. In the end, they also concluded that electrostunning was better than manual restraint (albeit for different reasons) for welfare of the animals and the safety of their human handlers. In a dramatic twist, someone was actually bitten while grappling with one of these crocodiles from the manual restraint group.

Pitted against each other in a straightforward fight, a human being would not last long against a fully grown crocodile. But in the real world, our ingenuity has firmly shifted the balance in our favour thanks to a few millennia of technological advancements. So bolstered are we by boats, chemical formulas and electrical appliances that we don’t always need to kill everything in sight, even when they have powerful muscles and sharp teeth. A concept that would have seemed impossible to our caveman ancestors. 

Thanks for reading


Beatson. 2020. Estuarine Crocodile. Last accessed 24/02/2021

Crocodile Specialist Group. Respiration.,the%20blood%2C%20making%20it%20acidic. Last accessed 01/04/2021

Britton. How fast can a crocodile run? Last accessed 14/03/2021

NHS Gloucestershire Hospitals. 2016. Lactate (Blood). Last accessed 22/02/2021

Khan Academy. Cellular respiration review. Last accessed 07/04/2021

BioNinja. Aerobic Overview. Last accessed 07/04/2021,muscle%20and%20red%20blood%20cells.

Bennett et al. 1985. Mass-Dependence of Anaerobic Metabolism and Acid-Base Disturbance During Activity in the Salt-Water Crocodile, Crocodylus Porosus

Seymour et al. 1985. Blood gas tensions and acid-base regulation in the salt-water crocodile, Crocodylus porosus, at rest and after exhaustive exercise

Noah C. 2020. HOW DO CROCODILES STAY SUBMERGED SO LONG? Last accessed 14/03/2020

Summers. 2005. Warm-hearted crocs

Seymour et al. 2004. Evidence for endothermic ancestors of crocodiles at the stem of archosaur evolution

Viegas. 2008. Crocs divert blood to avoid busting a gut.,that%20shoots%20to%20their%20stomachs. Last accessed 22/02/2021

Farmer et al. 2008. The right-to-left shunt of crocodilians serves digestion

Baldwin et al. 1995. Scaling of anaerobic metabolism during exercise in the estuarine crocodile (Crocodylus porosus)

Campbell et al. 2010. Ecological and physiological determinants of dive duration in the freshwater crocodile

Grigg. 1991. Central cardiovascular anatomy and function in Crocodilia

Owerkowicz and Baudinette. 2008. Exercise training enhances aerobic capacity in juvenile estuarine crocodiles (Crocodylus porosus)

Crocodile Specialist Group. Crocodilian Attacks. Last accessed 25/04/2021

The State of Queensland, Department of Environment and Science. 2008. Code of Practice for the Taking, handling and transportation of crocodiles: Nature Conservation Act 1992. 

Seymourl et al. 1987. Effect of Capture on the Physiology of Crocody lus porosus

Franklin et al. 2003. Comparison of stress induced by manual restraint and immobilisation in the estuarine crocodile, crocodylus porosus

Pfitzer. 2013. Physiological parameters of farmed Nile crocodiles (Crocodylus niloticus) captured manually and by electrical immobilisation


Image sources

Gianfranco Gori. 2010 [CC BY-SA 4.0 (].–Etiopia-Omo-River-Valley-01.jpg

Lip Kee from Singapore, Republic of Singapore. 2008. [CC BY-SA 2.0 (].

Boumphreyfr. 2009. [CC BY-SA 3.0 (].

Lisawerner9. 2021. [CC BY-SA 4.0 (].

Matt from Melbourne, Australia. 2009. [CC BY 2.0 (].

Radhika Kochhar. 2012. [CC BY-SA 3.0 (].,_Mysore,_Karnataka.JPG

Kerry Raymond. 2018 [CC BY 4.0 (].,_Bramston_Beach,_2018.jpg

Alex Castro. 2010. [CC BY 2.0 (].

Kerry Raymond. 2016. [CC BY 4.0 ( .,_Slade_Point,_2016.jpg

Fritz Geller-Grimm. 2001. [CC BY-SA 2.5 (].

User:Gisling. 2006. [CC BY-SA 3.0 (].

MartinRe at the English-language Wikipedia. 2003. [CC BY-SA 3.0 (].

Zachi Evenor. 2017. [CC BY 3.0 (].

Nautiloids- The forgotten ones

by Matthew Norton

There is so much diversity in the oceans. A place where life has found millions of ways to live, build, fight and avoid dying for as long as possible. With so much out there, it’s inevitable that some species, or even groups of species, will be overlooked to the extent that we sometimes forget they exist at all. In my last article, I briefly mentioned how cuttlefish are sometimes overshadowed by their squid and octopus cousin. What I failed to mention is that there is a fourth group of cephalopod molluscs living in today’s oceans. The Nautiloids (also called Nautilus).

Nautiloids have been around for about 500 million years, long before the coeloids (the cephalopod sub-group that octopuses, squid and cuttlefish belong to) emerged. At the peak of their success, it is thought that there were around 2,500 different species of nautiloid. But over time, the world changed and new competition emerged. The mass extinction event at the end of the Cretaceous period (around 66 million years ago) was a particularly difficult time for the nautiloids. While whole animal groups were wiped out, the nautiloids pulled through, but never recaptured their former glory. Today, there are only six species left.

The last remaining nautiloid species (clockwise from top left). The chambered nautilus (Nautilus pompilius), the bellybutton nautilus (Nautilus macromphalus), the crusty Nautilus (Allonautilus scrobiculatus), the Bali chambered nautilus (Allonautilus perforates), the Palau nautilus (Nautilus belauensis) and the white-patch nautilus (Nautilus stenomphalus).
A few hundred million years ago, naming all the nautiloids in the sea would have been a much more difficult task.

Nevertheless, nautiloids are still recognisable as cephalopods thanks to some tell tale features. They have strong tentacles, sharp beaks, well developed eyes and the ability to jump around by firing jets of water. But the one obvious feature they have which other living cephalopods don’t is a shell on the outside. External shells would have been a must have for all cephalopods at one time in earth’s history, but while the coeloids chose to abandon their shells, the nautiloids were more stubborn in the ways of their ancestors.

Even so, the nautiloid shell is an impressive feat of natural engineering, particularly on the inside where it is separated into a series of chambers The outermost chamber is huge and built for housing the animal itself, while all the others are built for holding gases and liquids to keep the nautiloid afloat. To allow them to float up and sink down as needed, the system is regulated by a tube called the siphuncle. This tube passes through each shell chamber, sucking up or dishing out gases and liquids.

The inside of a nautilus shell with clearly defined shell chambers.  In the photo  on the left you can just about make out the openings in each chamber wall where the siphuncle tube was threaded through. The diagram on the right shows this arrangement in more detail.

The ocean is undoubtedly full of wonders. Some dominate this world as they have never done before while others belong to a world that faded away a long time. But that doesn’t mean these ‘living fossils’ have stopped evolving, only that they have chosen to refine existing designs rather than invent revolutionary new ones. Nautiloids have done this well enough to linger on while extinction has claimed so many others. But even for these survivors, the threats they face today are unprecedented, even for our planet. And it would be a terrible shame if we lost those last few links we have to the ancient seas.

From a human perspective

At the risk of contradicting what I was just saying about their obscurity, nautiloids have still managed to fascinate us from time to time. Their empty shells have long been moulded into jewelry and revered as natural curiosities while the very name Nautilus has inspired scientific journals, poems, science fiction aliens and Russian rock bands. In the novel Twenty thousand leagues under the sea, the Nautilus was the name of the vessel commanded by Captain Nemo as it travelled the oceans. A story that ended ironically (spoiler alert) with this particular nautilus being attacked by its most formidable cousin. A giant squid.

Nautilus shells have long been harvested for making jewelry and elaborate pieces of art including cups (left) and carvings that are engraved and decorated (right). Items that are treasured antiquities today.
The cover of The Nautilus scientific journal (top left) that specialises in mollusc research and was established in 1886 as The Conchologists Exchange. Oliver Wendall Holmes (top right) who published a number of poems including one called The Chambered Nautilus. The Russian (previously Soviet) rock band Nautilus Pompilius (bottom left) who’s popularity peaked from the late 1980s to the mid 1990s. And an illustration of Captain Nemo on top of his vessel The Nautilus (bottom right).

Returning to the real world, the ancient origins and design of nautiloids have made them very useful for understanding how life worked in the ocean long before we were around to take notes. We do also have the fossilised remains of sea creatures to work from, but often it’s only the hard shells and bones that come through the fossilisation process intact. We can work out a lot from these preserved hard parts, but we can also look to living surrogate species to fill in the blanks. In the case of those last few living nautiloids, they have been used as a surrogate species to study ammonites.

Ammonites were another group of cephalopods with outside shells who flourished at around the same time as the nautiloids. But while the nautiloids managed to just about hang on through the end of the Cretaceous period, the ammonites were completely wiped out. All we have left are the fossilised remains that have captured the attention of scientists for over 150 years. More than enough time and study to notice that the preserved shells of ammonites and the fresh shells of nautiloids are incredibly similar down to the way they’re divided into chambers on the inside.This strongly suggests that ammonites used the comparmentalised shells to float, just like our living nautiloids. 

The similarities between nautiloids (left) and ammonites (right) are uncanny. From the shell exterior (top layer) and shell interior (middle layer) to the living animal/artist’s reconstruction (bottom layer).

But despite these similarities at face value, nautiloids are not a perfect match for ammonites. They’re not even their closest cousins with a series of subtle differences revealing that ammonites are more closely related to the coeloids (octopuses, squid etc.) than they are to nautiloids. One such subtle difference is in the design of the siphuncle, the tube that carries gas and liquid between shell chambers. In nautiloids, the tube is calcified (i.e. turned to bone) on the inside and soft on the inside, while in the ammonites and early coeloids (before they went shell-less) the tube is hardened inside and out. It’s a small modification, but it could have been what saved the nautiloids while the ammonites were going extinct.

Working out how prehistoric animals lived is always going to be a tricky business. The fossil record is patchy, the use of living surrogates is far from perfect and evolution does like to give us some bizarre twists along the way. It’s a testament to the skill and patience of paleontologists, and researchers in related fields, that we are not completely clueless about earth. Furthermore, we are forever discovering more fossils and developing new techniques to probe deeper into these fossils and into the lives they once lived. 

A fossilised Baculites ammonite on display (top) and a closer look at the insides of another Baculites fossil whose outer shell layer had been dissolved away (bottom).
A study that analysed a particularly well preserved Baculites fossil (not pictured) with x-rays partially matched the structure of its jaws to that of three living molluscs. All of whom feeding on tiny planktonic animals that float around in the water.
The remains of such planktonic creatures were even found within the jaws of the Baculites. While it is possible that these tiny animals were cleaning or scavenging for food in the ammonite’s mouth, balance of probability suggests that this ancient mollusc had food stuck between its teeth when it died.
Thanks for reading


Wikipedia. 2020. Nautilus. Last accessed 06/01/2021

Wikipedia. 2021. Nautiloid.,the%20living%20Nautilus%20and%20Allonautilus. Last accessed 06/01/2021

Wikipedia. 2021. Coleoidea. Last accessed 13/01/2021

Gross. 2015. Intelligent life without bones

Ward et al. 1980. The buoyancy of the chambered Nautilus

Vendetti. 2006. The Cephalopoda. Last accessed 13/01/2021

Sedgwick Museum of Earth Sciences. Wenlock creatures: Nautiloids. Last accessed 06/01/2021

Denton and Gilping-Brown. 1966. On the buoyancy of the pearly Nautilus

NOAA. 2019. What is a nautilus?,backwards%20and%20to%20make%20turns. Last accessed 13/01/2021

Cleland. 2017. Nautilus cup. Last accessed 17/01/2021

The British Museum. Nautilus cup. Last accessed 17/01/2021

ATG Reporter. 2017. ATG letter: Here’s why we shell out for carved nautilus. Last accessed 17/01/2021

Nautiloid,” 1A Collections, accessed January 13, 2021,

Bailey-Matthews National Shell Museum. The Nautilus. Last accessed 13/01/2021

Encyclopædia Britannica. 2019. The Chambered Nautilus. Last accessed 13/01/2021

Dining with the Zarglorp. 2019. Archer. Series 10, episode 4. Written by Adam Reed and Shane Kosakowski. FXX. 19th June

Why Must I Be a Crustacean in Love? 2000. Futurama. Series 2, episode 9. Directed by Brian Sheesley and Bret Haaland. Written by Matt Groening, David X. Cohen and Eric Kaplan. Fox. 6th February

Meet Russia Online. 2019. Nautilus Pompilius – Russian rock born in USSR. Last accessed 13/01/2021

British Geological Survey. Ammonites. Last accessed 17/01/2021

Hall and Meek. 1855. Descriptions of New Species of Fossils from the Cretaceous Formations of Nebraska, with Observations upon Baculites ovatus and B. compressus, and the Progressive Development of the Septa in Baculites, Ammonites, and Scaphites

Jacobs and Landman. 1993. Nautilus—a poor model for the function and behavior of ammonoids?

Weaver. 2012. Cephalopod Awareness Days: Finally Some Fossils. Last accessed 11/01/2021

Mutvei et al. 2007. Late Cambrian plectronocerid nautiloids and their role in cephalopod evolution

Kruta et al. 2011. The role of ammonites in the Mesozoic marine food web revealed by jaw preservation

Image sources

Profberger. 2007. [CC BY-SA 3.0 (].

James St. John. 2016. [CC BY 2.0 (].,_Philippines)_2_(24078294661).jpg

Pujolle. 2007. [CC BY-SA 3.0 (].

Profberger. 2006. [CC BY-SA 3.0 (].

Teresa Martin. 2016. [CC BY-SA 4.0 (].

Sérgio Valle Duarte. 2016. [CC BY 3.0 (].

Philcha. 2009. [CC BY 3.0 (].

Nobu Tamura ( 2009. [CC BY 3.0 (].

LMCoelho. 2012. [CC BY-SA 3.0 (].

John Alan Elson. 2009. [CC BY-SA 4.0 (].

Kunstgewerbemuseum Berlin. 2011. [CC BY-SA 4.0 (].

Ian Alexander. 2017. [CC BY-SA 4.0 (].

Ninjatacoshell. 2009. [CC BY-SA 3.0 (].

Wilson44691. 2011. [CC BY-SA 3.0 (].

Cuttlefish- Where are you now?

by Matthew Norton

Communication can be a tricky business in the sea. For many animals, it is impossible to complete their life cycle without sending and receiving messages with their own kind. But they often do so at the risk of giving away their position to their enemies. Sounds can be eavsedropped, odours and chemical trails can be followed with hungry noses and visual cues can be picked up by everyone in the local vicinity. It is a necessary risk, but it can also be managed by developing communication systems that are selective in who they talk to. 

Cuttlefish are a great example of this with their ability to change their appearance at a moment’s notice. They achieve this feat with chromatophores, specialised skins cells that can be stretched and relaxed to create changes in colour and contrast. Changing a single chromatophore sounds as simple as flicking a switch, but with the incredible cuttlefish brain they can coordinate millions of these switches at once. This system makes cuttlefish highly accomplished masters of disguise who can mimic and blend in with virtually any habitat. Not bad for a group of animals who are all colourblind and only have the contrasts between light and dark to work with. 

Cuttlefish are cephalopod molluscs, a group that also includes octopus and squid. They  share a number of features with their cousins including a large brain (relative to body size), a short lifespan and the ability to move via jet propulsion.
Four ‘chameleons of the sea’ doing what they do best. Some of these cuttlefish may be easy to spot with our eyes,  particularly when we have time to intensely study these photos rather than relying on the quick glances we would often get of cuttlefish in the wild.
It is also worth mentioning that the animals in the sea see the world very differently.

When cuttlefish want to make themselves seen, they can still use those same colour changing abilities, in combination with other visual cues such as moving their tentacles and adopting certain postures. The resulting colourful displays are often used to ‘talk’ to other cuttlefish (of the same species) during reproduction to indicate their gender, readiness to mate and to threaten romantic rivals. If they sense danger approaching, they can switch all their chromatophores to stealth mode and then pick up where they left off once they get the all clear. 

But the need to reproduce can make fools out of many animals, and cuttlefish are no exception. In the Australian giant cuttlefish, the possible fools are the large brutish males who honestly display their suitably as a mate and the superiority of their genes to seduce multiple females. The smaller males in the vicinity would not stand a chance against such fierce competition and so resort to sneakier tactics. Instead, they use their own colour shifting abilities to mimic the appearance of a female to deceive their way past another male and mate with his females when he’s not looking. 

The Australian giant cuttlefish.
During the breeding season, sneaky smaller males mimic the patterns of a female to gain the confidence of a large male to gain access to his females.  Once the large male is distracted, probably fighting off another rival male, the sneaker male reveals his true identity. With a bit of luck, he can get a few matings in before he has to scarper.

For all their impressive abilities, cuttlefish are sometimes overlooked when compared to their squid and octopus cousins. The main distinguishing feature that cuttlefish possess is the cuttlebone, a shell they keep inside their body for controlling their buoyancy in the water. It may not sound like a particularly interesting namesake, but it is what makes cuttlefish unique among the cephalopods. And let us not forget that the evolution of the human race really got going with the simple breakthrough of opposable thumbs.

The cuttlebone (left) looks pretty solid, but as one of many people who has held one after finding it on the beach, I can tell you that it is surprisingly light. At the microscopic level (right), the cuttlebone is made up of hollowed out chambers.
Liquid can be filled in, or drained out of these chambers to make it light, or heavy as needed by the cuttlefish.

From a human perspective

Cuttlefish have proven to be a very useful natural resource for us and it appears that little is wasted from those we catch from the sea. For starters, we eat them in seafood dishes such as calamari and linguine where cuttlefish can be a cheap and tasty alternative to squid. This has proven to be popular in seafood cuisines in East Asia and the Mediterranean regions, but cuttlefish are also caught here in UK waters. But, as with any other seafood, it is important to buy and consume cuttlefish that have been caught sustainably with gear that causes minimal damage to the environment.

Cuttlefish is relatively easy to prepare with the exception of the ink sack, which they would tap into to create underwater smoke screens to confuse their predators.
Sometimes the ink sack is removed and discarded, but in other dishes (like the one depicted above) he ink is served with the meat as a sauce.

Throughout history we have also harvested cuttlefish for their ink to use in writing, drawing and painting. This practice is believed to date back to ancient Greece and Rome, but was probably at its most popular and refined during the 18th and 19th centuries. Only for cuttlefish ink to then become obsolete as alternatives were manufactured on an industrial scale. As for the ink itself, it typically produces a brownish colour with a hint of violet (though variations do exist) after it is extracted from the cuttlefish’s ink sac. The extraction process, depending on the exact method used, could involve drying the ink sack before subjecting the ink to a series of chemical treatments to make it more or less permanent. As demonstrated by some of the images below.

The use of cuttlefish ink, often referred to as ’sepia ink’ can also be seen in surviving paintings and sketches from the renaissance period, including the work of Leonardo DaVinci (top left) and Pieter Brugel the Elder (bottom left).
Sepia toning was also once used to treat photographs (top right) and leave them with a brownish tint (which is where the name comes from even though sepia ink wasn’t actually used. Today, sepia toning is done digitally rather than with chemical toners (bottom right). 

The previously mentioned cuttlebone has also proven to be a useful part of the cuttlefish anatomy with uses that include an ingredient in polishing powder and toothpaste, a strong mould in jewellery making and a possible use in producing building lime. Cuttlebone also has nutritional benefits due to it being rich in calcium and other minerals, and has been used as a food supplement for pet birds for over a hundred years.

Depending on the species of bird, the cuttlebone is simply hung in the cage for the bird to scrape and munch on, or it is ground into their food. The supposed health benefits include stronger bones, more effective blood clotting, a better immune system and so on. However beneficial a cuttlebone rich diet may be, there have clearly been financial incentives for supplying these supplements. During the research for this article I discovered a patent application (that was accepted) from 1885 for a new bird cage accessory for holding a piece of raw cuttlebone. 

These are the schematics that accompanied the patented designs for attaching cuttlebone to the bird cage.
Mr. Boerner had this patent enforced for the rest of his life and given that cuttlebone supplements are still  recommended for pet birds today, it was probably a wise business decision.

Talking about the impact that marine animals have had on human culture and history can be a double edged sword. On the one hand, it is important to highlight just how connected we have always been with the oceans and appreciate what they have given us over the years. But this connection is often about us exploiting the natural world, sometimes stripping it bare when it suits us. But in the case of the cuttlefish, it is somewhat reassuring that we have found ways to use the less palatable parts of the animals rather than just throwing them away. If we are to use the oceans and its inhabitants for our benefit, we can at least do so  as sustainably and efficiently as possible.

Thanks for reading


Wikipedia. 2020. Cuttlefish. Last accessed 08/11/2020

Katcla. 2017. Fact Sheet: Cuttlefish. Last accessed 14/11/2020

The Wildlife Trusts. Common cuttlefish. Last accessed 14/11/2020

Hanlon. 2007. Cephalopod dynamic camouflage

Max Planck Society. 2018. Elucidating cuttlefish camouflage. Last accessed 30/11/2020

Chiao and Hanlon. 2001. Cuttlefish camouflage: visual perception of size, contrast and number of white squares on artificial checkerboard substrata initiates disruptive coloration

Barbosa et al. 2008. Cuttlefish camouflage: the effects of substrate contrast and size in evoking uniform, mottle or disruptive body patterns

Hart. 2020. Cephalopods.,gestures%20of%20their%20ten%20tentacles. Last accessed 04/12/2020

Zylinski et al. 2011. To Be Seen or to Hide: Visual Characteristics of Body Patterns for Camouflage and Communication in the Australian Giant Cuttlefish Sepia apama

Norman et al. 1999. Female impersonation as an alternative reproductive strategy in giant cuttlefish

Denton and Gilpin-Brown. 1959. Buoyancy of the cuttlefish

AskNatureTeam. 2016. Gas-holding structure aids buoyancy: Cuttlefish. Last accessed 10/12/2020

Great British Chefs. How to cook cuttlefish. Last accessed 26/11/2020

Cornwall Good Seafoood Guide. Cuttlefish.,and%20an%20alternative%20to%20calamari. Last accessed 13/12/2020

TheeLadyG. 2018. What is a Cuttlebone, Anyway?. Last accessed 13/12/2020

Cadman et al. 2012. Cuttlebone: characterisation, application and development of biomimetic materials

惠友恒. 2013. CN104546623A – Cuttlebone-containing toothpaste. Google Patents. Patent no. CN104546623A

Ragan. 2018. How-To: Cuttlebone Casting. Last accessed 13/12/2020

Brepohl. Cuttlefish Casting Method. Last accessed 13/12/2020

Ferraz et al. 2020. Exploring the potential of cuttlebone waste to produce building lime

Great Companions. 2020. The Benefits of Cuttlebone for Birds.,trace%20minerals%20for%20your%20bird. Last accessed 21/11/2020

Boerner. 1885. Means for attaching cuttle-bone to bird-cages. UNITED STATES PATENT FMCE. Patent no. US325313A

Pets at home. Vitakraft Cuttlebone Bird Treat 2 Pack. Last accessed 26/11/2020


Encyclopaedia Britannica. 1998. Sepia. Last accessed 26/11/2020

Ana. 2019. Let’s Talk About Sepia Ink. Last accessed 26/11/2020

Image sources

Hans Hillewaert. 2008. [CC BY-SA 4.0 (].

Tongjin. 2007. [CC BY-SA 3.0 (].

Raul654. 2005. [CC BY-SA 3.0 (].

Lakshmi Sawitri. 2013. [CC BY 2.0 (].

Thomas Schoch. 2006. [CC BY-SA 3.0 (].

Sylke Rohrlach from Sydney. 2013. [CC BY-SA 2.0 (].

Mariko GODA. 2012. [CC BY-SA 3.0 (].

SecretDisc. 2011. [CC BY-SA 3.0 (].

Pieter Bruegel the Elder. 1554 (uploaded to Wikimedia in 2020). CC BY-SA 4.0 (].

David Adam Kess. 2012. [CC BY-SA 3.0 (].

Copepods- Jump for joy

by Matthew Norton

In a world with ocean giants such as sharks, dolphins and whales, it can be easy to forget about the smaller animals and how they play their own roles in the sea machine. Copepods, tiny crustaceans that can only reach lengths of 1-2 millimeters, are one such group of unsung heroes. What they lack in size, they more than make up for in sheer numbers, exceeding the head count of most, if not all animals on earth. Copepods are also found in almost any body of water and are an important food source for many animals. Often bridging the gap between phytoplankton (tiny aquatic plants) and the meat eaters.

Copepods come in a huge variety of shapes and sizes (up to around 2mm long). These are just a few examples among thousands.

But copepods are more than just a selection of light bites. One of their most impressive tricks is their ability to jump at speeds of 2-4 miles per hour to evade predators and move towards food. These speeds would be easy enough for large animals like ourselves, but if you scale it down to copepod size it would be the same as a human being reaching nearly 4,000 miles per hour in a single bound. These powerful jumps are made possible by their streamlined shape, quick reflexes and possession of two types of ‘swimming legs’. Those nearer the head are used for casual movement and feeding strategies that do not require fast jumps. The swimming legs further down the body are reserved entirely for those fast jumps and not tired out by other tasks before the copepod desperately needs them.

You can just about see the feeding/casual swimming legs around the head of the copepod on the left. The jumping legs, found further down the body, can be seen more easily in the side view image of the copepod on the right. Some copepods may even use the latter to jump out of the water if the need arises.

Such powerful jumps will inevitably leave vortex rings, spinning vacuums of water, in their wake which could leave a trail for their predators to follow. But copepods deliberately create two vortex rings per jump that spin in different directions. When combined, these vortex rings (almost) cancel each other out and cover the copepod’s tracks.

Unfortunately for our copepods, these fast jumps are mostly useless against seahorses and other predators that can approach undetected and strike with no warning. Ironically, it is the slow and gentle swimming of a seahorse, combined with their convincing camouflage, that makes them invisible to the copepods. Even their snouts are built to minimise the disturbances they cause in the water and allow the seahorse to get within striking distance.

Seahorses are such effective hunters of copepods, as well as other tiny planktonic animals, that their success rate is far higher than apex hunters such as lions and sharks.

Copepods, and their seahorse predators, are shining examples of what the littles guys and girls in the sea are capable of. If we could somehow take away the size difference, the copepods would likely outshine the ocean giants. Then again, it is just as likely that their small size is the key to their accomplishments. We all live in the same world, but the way it looks to a tiny copepod would be very different from our own experiences. A pond for example, is merely a small body of water with some interesting plants and animals for us. But to a copepod, that same pond is a vast wilderness full of dangerous beast and tempting riches. The rules they have to follow and the challenges they will face are completely different, but so are the opportunities.

From a human perspective

Not all copepods are fast jumpers. Many species have opted for a parasitic lifestyle and stay attached to a host animal for the whole of their adult lives. This can become a problem when they target marine animals that we want to eat. The aquaculture industry in particular have found copepod parasites like the dreaded ‘sea louse’ to be an expensive thorn in their side because no one wants to eat diseased fish.

Parasitic copepods as recovered from a European flounder Platichthys flesus (top left) and the deep-sea fish Pristipomoides filamentosus  as photographed in the 21st century. But copepod parasites have been recognised for some time, as evidenced by the image of an infected brook trout from 1899 (bottom left) and the two sketches of freshwater copepod parasites from the 1910s (bottom right).

Fish are also not the only possible hosts. Shellfish, worms, corals, marine mammals and many others can all be burdened with these parasites.

Direct infection of human beings is unlikely, but copepods can act as unintended carriers (so far as we know) of human diseases that can be transmitted from drinking water, such as cholera and the guinea worm parasite. Wherever there is water, there are probably copepods to make it more difficult to eradicate these diseases. But the key role that they play in transmission could also help us to predict outbreaks and put measures in place to stem the tide of infections. One such early warning system could be to monitor the environment for explosions of phytoplankton growth (called blooms). With more food in the water, there are likely to be more copepods swimming around and therefore more potential carriers of disease. 

One way to avoid contracting cholera is to use certain materials to filter out the Vibrio cholerae bacteria, and the copepods that carry them, from drinking water. Just like the woman on the right is doing.
The cycle of how the guinea worm parasite can get around when it has access to both human and copepod hosts. Presumably, any other human diseases that can be carried by copepods would work in a similar way.

Copepods can also protect us from diseases that are spread by their prey. This applies to diseases spread by insects who spend part of their lives as aquatic larvae before morphing into their adult form and flying away. The more copepods there are in a given water body to pick off the insect larvae, the fewer carriers there are available to spread disease. Using copepods in this way has had some success in controlling the spread of dengue fever and could theoretically be used against other nasty tropical diseases. 

The yellow fever mosquito Aedes aegypt (left) and the asian tiger mosquito Aedes albopictus (right) are both carriers of several nasty human diseases. If copepods are universally effective in controlling their transmission of dengue fever, they could have a similar effect against the zika and chikungunya viruses.

But we must be careful with manipulating nature to help control diseases, or we could end up getting carried away and making things worse. This was the case with the western mosquitofish (Gambusia affinis), a freshwater fish that was introduced in many countries in the early 20th century to combat the spread of malaria. As with copepods, this fish consumes insect larvae during their aquatic larval stage and using it’s natural feeding habits probably made perfect sense at the time. However, the western mosquitofish has outstayed its welcome and has been outcompeting native mosquito eating fish, species that could have done a better job of controlling the spread of malaria. I am not saying we shouldn’t ever make use of our natural resources, but in this kind of enterprise it would be wise to exercise some cautious doubt until we know the full implications of what we are doing.

Also, as helpful as copepods can be for dealing with obvious problems like disease, it is what they do in the background that is the most important and least appreciated. For example, their role in the global carbon cycle is substantial given that they are a major consumer of phytoplankton. Much of this carbon, which the phytoplankton originally extracted from the water as carbon dioxide, will travel up the food chain. But a portion will sink towards the bottom as uneaten copepod corpses and faeces. Given the rapid increases in carbon dioxide in the atmosphere, and dissolved in water, we should hope that the carbon filled remains of these copepods are locked away for a very long time.

Anyone reading this article might feel a bit uneasy about the idea of protecting our planet over protecting ourselves from disease, especially given our current circumstances. But disrupting the balance of nature will have far reaching consequences that include changes in how diseases operate and spread. The coronavirus pandemic may very well be the latest in a long line of high profile and disastrous consequences of our poor treatment of the natural world. Clearly something needs to change, but for truly effective conservation we have to show consideration to the whole environment and everything that lives within it. Even those tiny little copepods.

Thanks for reading


Wikipedia. 2020. Copepod. Last accessed 23/09/2020

Boxshall and Defaye. 2008. Global diversity of copepods (Crustacea: Copepoda) in freshwater

Dürbaum & Thorsten-D. Künnemann. 2010. Biology of Copepods: An Introduction. Last accessed 11/10/2020

Krillson. 2018. Copepod Speed and Strength. Last accessed 13/10/2020

Choi. 2020. Flea-Sized Creatures Are Fastest Jumpers Known. Last accessed 13/10/2020

Tiselius et al. 2013. Sensory capabilities and food capture of two small copepods, Paracalanus parvus and Pseudocalanus sp

Kiørboe et al. 2010. Danger of zooplankton feeding: the fluid signal generated by ambush-feeding copepods

 Jiang and Kiørboe. 2011. Propulsion efficiency and imposed flow fields of a copepod jump

Technical University of Denmark (DTU). 2010. “What makes world’s strongest animal — the tiny copepod — so successful?. Last accessed 11/10/2010

Kennedy. 2019. Special Feeding Adaptations of the Seahorse.,well%20adapted%20for%20catching%20prey. Last accessed 13/10/2020

Andrei. 2013. Seahorses are stealthy, efficient hunters.,creep%20up%20on%20their%20prey. Last accessed 13/10/2020

Morgan. 2013. Seahorses stalk their prey by stealth. Last acessed 19/10/2020

Johnson et al. 2004. A review of the impact of parasitic copepods on marine aquaculture

Feis et al. 2019. Global invasion genetics of two parasitic copepods infecting marine bivalves

Natural History Museum. Copepod colonisation and parasitism. Last accessed 13/10/2020

Leitzell. 2011. The time of cholera. Last accessed 01/10/2020

Colwell and Huq. Cholera-Carrying Copepod. Last accessed 01/10/2020

Huq et al. 1983. Ecological relationships between Vibrio cholerae and planktonic crustacean copepods.

Centers for Disease Control and Prevention. Parasites- Guinea Worm.,in%20these%20stagnant%20water%20sources. Last accessed 01/10/2020

Garrett et al. 2020. Dogs and the classic route of Guinea Worm transmission: an evaluation of copepod ingestion

Udayanga et al. 2019. Predatory efficacy of five locally available copepods on Aedes larvae under laboratory settings: An approach towards bio-control of dengue in Sri Lanka

Lazaro et al. 2015. Community effectiveness of copepods for dengue vector control: systematic review

CDC. 2018. Potential Range in US. Last accessed 24/10/2020

CABI. 2010. Gambusia affinis (western mosquitofish). Last accessed 24/10/2020

Global Invasive Species Database. 2010. Gambusia affinis. Last accessed 24/10/2020

Yorukoglu. 2019. Copepods: The unsung heroes of the ocean. Last accessed 11/10/2020

Harvard T.H. Chan. Coronavirus and Climate Change.’acceleration,pathogens%20to%20find%20new%20hosts. Last accessed 06/11/2020

Image sources

Andrei Savitsky. 2019. [CC BY-SA 4.0 (].

Uwe Kils. 2016. [CC BY-SA 3.0 (].

Florin DUMITRESCU. 2007. [CC BY-SA 3.0 (].

Hans Hillewaert. 2008. [CC BY-SA 4.0  (].

Geoff Boxshall. 2012. [CC BY 2.0 (].

Anwar Huq, University of Maryland Biotechnology Institute, Baltimore, Maryland, United States. 2003. [CC BY 2.5 (].

Muhammad Mahdi Karim. 2009. [GFDL 1.2 (].

All other images are public domain and do not require attribution

Barnacles-All together now

by Matthew Norton

It has been a long several months of limiting our interactions with each other to slow the spread of coronavirus. No doubt it has given many of us a renewed appreciation of the sociable lifestyle. But for some animals, social distancing is not an option, unless they disregard the primary objectives in life, survive, grow and reproduce. Even moving further away, or closer to each other on a temporary basis is impossible for those animals in the sea who can barely move for the majority of their lives.

Barnacles are one such group of animals. As young larvae they are free to move around and explore the oceans, but as adults they are firmly cemented to a single spot. Like other crustaceans (e.g. crabs, lobsters), barnacles have jointed legs which they have repurposed for catching food. They also reproduce by directly fertilising each other, rather than releasing clouds of eggs and sperm into the water. A task for which they are equipped with the longest male genitalia, relative to body size, in the animal kingdom.

barnacle article image 1
Rock barnacles out of water with their ‘shields’ closed to the outside world (left) and goose barnacles in water with their feeding cirri for catching food (right). Both types of these types of barnacle possess these feeding structures.

But as well endowed as barnacles are, their prospects for mating partners are severely limited by their lack of mobility. Their inability to relocate to fresh feeding grounds is also hugely restrictive and leaves little room for error when looking for property in the vast ocean. Judging by the many factors they consider, including water flow, food availability and their closest neighbours, they seem to be well aware of the importance of this one decision.

Barnacles also approach the decision making process itself very cautiously. Should a larva find a promising neighbourhood they will likely anchor themselves with a temporary sticky glue and make follow-up inquiries. As well as ‘walking’ around to investigate multiple sites, they can also make use of the thin layer of slime that covers virtually every hard surface in the ocean. A layer that contains a vast network of microorganisms, stray proteins and other nuggets of biological information that the barnacle larvae can tap into. Depending on what evidence they uncover, the barnacle larvae will either settle and transform into an adult, or detach and look elsewhere. 

barnacle article image 2
A barnacle larva looking for their first and only new home (left) and a splattering of young barnacles who have made their decision and are beginning to grow (right).

Even if they do find that perfect spot, there is a good chance that it has already been claimed by barnacles from another species who are not keen to share. These conflicts of interest can get bloody as demonstrated in one dramatic example in 1961 with two seashore species, Chthamalus stellatus and Semibalanus balanoides (called Balanus balanoides at the time). Chthamalus stellatus almost always settle on the higher shore (further from the sea), even though their young could try and settle further down to the water’s edge. But anyone foolish enough to try would be smothered, uplifted, crushed or otherwise dispatched by the larger and faster growing Semibalanus balanoides barnacles.

barnacle article image 3
The bullied, Chthamalus stellatus (left), and the bully, Semibalanus balanoides (right). Both are driven by competition for the limited space on the rocky shore. Chthamalus stellatus has the advantage of being able to thrive in the harsher conditions of the upper shore, where they are further from the sea. But Semibalanus balanoides can grow faster and bigger in the easier conditions of the lower shore and overpower Chthamalus stellatus with brute force and no mercy.  In nature there is often another species to take you down a peg long before you reach your physical survival limit.  

As careful as barnacles are when considering where to live for the rest of their lives, there are still those who may be prepared to take a leap of faith. Whether they are choosing to be bold or driven by desperation as their reserves dwindle, death or celibacy will be the likely outcomes for those barnacles who venture into unknown territory. But should these pioneers somehow succeed the rewards for their gene line could be enormous. With species of barnacle already living on whales and floating around on their own buoyancy aids, who knows what new way of living the bold barnacles will find next. 

From a human perspective

As much as barnacles would prefer to settle into ‘conventional habitats’, if push comes to shove they will settle anywhere they can. These unusual homes include boats, piers, buoys and probably any hard surface we submerge in water, causing us a fair amount of grief in the process. Boat owners in particular will find their presence to be particularly annoying and expensive from the significant drag they can cause when attached to the hull in large numbers. To maintain the same speed as if the hull was clean, they would have to burn through more fuel and incur heightened running costs as a result. The extra input of greenhouse gases is also far from ideal, though some of our methods for dealing with this problem have not done the environment any favors either.  

barnacle article image 4
These are just a few examples of how barnacles can find their way onto anything we build, dump, or use in the sea.

Removing barnacles with brute force is difficult and time consuming work thanks to their ‘cement’, one of the strongest natural adhesives we have ever come across. The best approach is to prevent them from getting that foothold in the first place and for a long time this meant coating boat hulls with tributyl-tin (TBT). After the use of TBT was banned, other toxic compounds took its place, but there is always the issue of collateral damage when using harmful chemicals to solve our problems in the water. Fortunately, we are making progress in developing effective, but non-toxic methods such as designing boat hulls to mimic the surface of animals and plants who have found success in keeping these hitchhikers away.

barnacle article image 5
A selection of animals and plants who have provided inspiration for designing anti-fouling surfaces on boat hull. Including (clockwise from top left) sharks, pilot whales, geckos, lotus flower, brown algae and pea pods.

Meanwhile, barnacles can sometimes be useful when they are attaching to surfaces in the ocean. For instance, by attaching to the body and clothing of a floating corpse they could provide useful clues for determining a cause of death. The age of the barnacles on discovery of their newfound floating home can help us to estimate how long the victim has been in the water. If this estimated floating time matches the estimated time of death then it is likely the victim died in the water due to accidental drowning. But if the evidence suggests that the victim was already dead when they entered the water then it is more likely that foul play was involved. 

Regular readers of this blog may remember that I covered a similar idea in an article about diatoms that I published last summer. The tendency of these microscopic algae to settle on floating corpses and which organs they have reached inside the body can also provide clues that might lead to establishing a cause of death. Both methods do come with flaws that need to be accounted for, particularly regarding the exact species and variation in the environment. But combining the two would surely give us a more accurate picture of what happened to the unfortunate soul they now call home.

Barnacles have managed to intrigue and infuriate us for a very long time. They even caught the attention of Charles Darwin who devoted a lot of time to studying barnacles intensively as he developed his theory of natural selection. Today, they still hold a lot of importance in advancing human technology with considerable interest in how to adapt their formidable ‘cement’ for our own ends. The secrets of this natural superglue are only just beginning to reveal themselves and who knows how big the treasure trove of possibilities is when it comes to barnacle biology.

barnacle article image 6
Part of Darwin’s collection of barnacles along with a handwritten list of the different specimens on display at the Zoological Museum of Copenhagen.

barnacle article image 7
Thanks for reading


Bemis. 2017. Five Facts: Barnacles. Last accessed 03/07/2020

Wikipedia. 2020. Barnacle. Last accessed 09/07/2020

Miron et al. 1996. Scale of observation and distribution of adult conspecifics: their influence in assessing passive and active settlement mechanisms in the barnacle Balanus crenatus (Brugiere)

Matsumura et al. 2000. Discrimination at settlement in barnacles: laboratory and field experiments on settlement behaviour in response to settlement‐inducing protein complexes

Aldred et al. 2013. Analysis of the behaviours mediating barnacle cyprid reversible adhesion

Connel. 1961. The influence of interspecific competition and other factors on the distribution of the barnacle Chthamalus stellatus

Riley, K. 2002. Chthamalus stellatus Poli’s stellate barnacle. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 12-07-2020]. Available from:

WoRMS (2020). Balanus balanoides (Linnaeus, 1767). Accessed at: on 2020-07-12

Balkenende et al. 2019. Marine-inspired polymers in medical adhesion

Marshall, C.E. 2007. Dosima fascicularis Buoy barnacle. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 08-08-2020]. Available from:

Demirel et al. 2017. Effect of barnacle fouling on ship resistance and powering

U.S. Naval Research Laboratory. 2019. Researchers develop groundbreaking process to study barnacle glue, could save Navy millions. Last accessed 14/07/2020

Salta et al. 2010. Designing biomimetic antifouling surfaces

Gittens et al. 2003. Current and emerging environmentally-friendly systems for fouling control in the marine environment

Magni et al. 2015. Evaluation of the floating time of a corpse found in a marine environment using the barnacle Lepas anatifera L.(Crustacea: Cirripedia: Pedunculata)

Shih. 2019. Barnacles Are a Clock for the Dead. Last accessed 08/08/2020

Austrian Science Fund. 2016. Researchers investigate barnacle adhesive.,to%20migrate%20to%20new%20habitats.. Last accessed 14/07/2020

Image sources

Partonez. 2018. [CC BY-SA 4.0 (].

M.Buschmann. 2007. [CC BY-SA 3.0 (].

MichaelMaggs. 2007. [CC BY-SA 3.0 (].

Auguste Le Roux. 2011. [CC BY-SA 4.0 (].

Dr Mary Gillham Archive Project. 2017. [CC BY 2.0 (].

brewbooks from near Seattle, USA. 2013. [CC BY-SA 2.0 (].

JoannaPoe. 2017. [CC BY-SA 4.0 (].

muffinn from Worcester, UK. 2014. [CC BY 2.0 (].

I, Luc Viatour. 2010. [CC BY-SA 3.0 (].

Helmut Diez. 1993. [CC BY-SA 3.0 (].

Rushenb. 2017. [CC BY-SA 4.0 (].

Ushani Thennakoon. 2020. [CC BY-SA 4.0 (].

Jerry Kirkhart from Los Osos, Calif. 2008. [CC BY 2.0 (].

FunkMonk (Michael B. H.). 2015. [CC BY-SA 3.0 (].

All other images are public domain and do not require attribution

Greenland sharks- Age before beauty

by Matthew Norton

Getting older can be a daunting prospect for some people. It would certainly explain the popularity of  beauty products that claim to make you ‘look younger’. The same can be said for some marine animals, except they can actually slow down their biological clock and extend their lifespan, rather than just delaying the effects of the ageing process.

The Greenland shark is an excellent example because they have the longest lifespan of any vertebrate animal on earth (that we know of). We don’t know exactly how long they can live for with estimates ranging between 272 and 512 years old although the halfway figure of 400 years is often the reported figure in newspapers. But even at the ‘younger’ maximum age of 272 years, Greenland sharks still outlive any other animal with a backbone, even the impressively long lived bowed whale.

greenland shark image 1
A common way to age an animal is to count the ‘growth rings’ in their hard parts  (e.g. the tooth of an orca), but the Greenland shark is too squishy for this to work. A group of scientists got around this problem by instead looking at a set of crystal-like proteins in their eyes which remain exactly the same throughout the shark’s life and provide vital clues of what the water chemistry was like when were they were born. The results can be difficult to interpret and convert into a date of birth, which is why there is a 240 year margin of error for the estimated maximum age of Greenland sharks.

Greenland sharks typically live in the cold, deep waters of the Arctic and North Atlantic Ocean. These ice cold surroundings tend to slow everything down biologically speaking, including the ageing process. But even compared to their equally chilled out neighbours, their lifespan is way too long to be fully explained by their environment. Their mysterious biology must play a part somewhere in all this.

However they manage it, the ability of Greenland sharks to stay ‘young’ for decades is nothing short of incredible.  But there are drawbacks to taking an excruciatingly long time to reach adulthood. By only gaining about 1cm in length per year it will be a long time before they can grow big enough to know no fear from predators and make baby sharks of their own. It is estimated to take them around 130-150 years old to reach sexual maturity and such a slow reproductive cycle could put the species at a serious disadvantage if their circumstances suddenly change.

Greenland sharks are also incredibly slow and sluggish in the water, but let us not forget that they are impressively large animals who can grow to over five metres in length. This puts them well within the size range of great white sharks and are just as feared among the local wildlife. Analyses of their stomach contents have confirmed their ability to hunt, or at least scavenger, on a variety of animal prey, some of whom are fierce predators in their own right. Thankfully, there is no record of a Greenland shark ever attacking a human.

greenland shark image 2
Greenland sharks (centre) have an incredibly varied diet that includes (clockwise from top left) wolfish, cod, squid, skates, harp seals, reindeer, polar bears and spider crabs.

Size is also a significant advantage for Greenland sharks against other strangely long lived creatures, such as the 500 year old Icelandic Cyprine clam and the appropriately named immortal jellyfish. In comparison, a large shark has fewer enemies to fear and a greater chance of watching the centuries pass by and making lots of baby sharks along the way. However, for reasons that will soon become apparent, this ‘slow and steady’ approach may not work as well as it once did.

From a human perspective

Extending your life by a couple of centuries may be an attractive prospect, especially after seeing so many lives tragically cut short in recent months. But as I said before, there are drawbacks to living longer and ageing more slowly. And these drawbacks are multiplied once you introduce human beings into the equation. With their fantastically slow reproduction rate it is going to be difficult for them to withstand everything we can throw at them.

One would hope that their existence in cold, deep and often isolated waters would keep them out of harm’s way, but this has not been the case. For centuries, we have hunted them for their meat which today is usually marketed as an Icelandic delicacy called ‘Hákarl’. To be fair, this is just one of many examples of a fishery targeting a large, slow growing species. The scale of this particular fishing operation is also considerably smaller and far less wasteful than the worldwide slaughter of sharks for their fins.

But, Greenland sharks must be among the least capable of replenishing their numbers to keep up with demand and it probably wouldn’t take much to put them in genuine peril. Frankly, I’m surprised that they are only classified as ‘Near Threatened’ by the International Union for Conservation of Nature. Although, the most recent assessment was in 2006 and a lot can change in 14 years.

greenland shark image 4
Hákarl is made from the meat of Greenland sharks, and some of their close relatives. Putting aside the ethical and ecological concerns for a moment, the taste of this ‘delicacy’ leaves much to be desired and has had some scathing reviews from critics. American TV chef Anthony Bourdain ranked it as one of his worst food-related experiences in the world and Gordon Ramsey spat it out into a bucket. Even so, there are probably still tourists out there looking for a culinary challenge.

Then there a potential health risks for the human consumer. The meat from any top ocean predator, a title that any large shark would qualify for, could be contaminated with the pollutants that they were exposed to before reaching the dinner plate. Any animal can be exposed to pollution in the water, but those at the top of the food chain will accumulate more of these toxic chemicals from the animals they eat in a process called ‘bioaccumulation’. 

Consuming the meat of a Greenland shark may exacerbate this risk further simply because they can live longer than any other fish and have more opportunities to accumulate a variety of different toxins. Studies have already been conducted and have reported the presence of several pollutants in their flesh including PBDE’s, SCCPs and PCBs. The use of the latter has been banned for decades, but it still persists in the sea and its inhabitants. The younger Greenland sharks in the sea today could end up acting as unwilling time capsules in 200 years time for all the crap we are currently pumping into the ocean.

There is however, one chemical that accumulates naturally in the the bodies of Greenland sharks, but is toxic to us in high doses. Trimethylamine N-oxide (TMAO) helps Greenland sharks to cope with the high water pressure they experience in the ocean depths, but if ingested by human beings it can increase the risk of certain heart conditions.

Icelandic settlers got around this problem a long time ago by devising a simple, but effective method for processing the meat and removing as much of  this chemical as possible. However, human nature often dictates that mistakes will be made on occasions and patrons may end up consuming improperly processed Hákarl. Though sometimes people consume the raw meat of a Greenland land on purpose to get  ‘shark drunk’. The experience has been compared to drinking a ridiculous amount of alcohol, or taking hallucinogenic drugs. Not something I would recommend.   

greenland shark image 5
Processing Greenland shark meat is a long and rather unpleasant process. First the head is removed and thrown away before the decapitated body is buried, or sealed in a closed container, and left to ferment in its own fluids for six-twelve weeks. Upon exhumation, the body is cut into strips and hung to dry in specially built shacks for several months (as depicted in the above photo). The exact time of the process might depends on weather, time of year and possibly on the technique of the person doing the preparation. Apparently, you know its ready for eating when it starts to smell like rancid cheese.

So why do people still eat Hákarl at all? Just like whaling, the hunting of Greenland sharks is often justified on cultural grounds and the importance of this fishery to the survival of the country’s first settlers. This argument is understandable to some degree, but history is full of traditions and practices that would not be acceptable in the 21st century. Also, given the current state of the oceans, now is as good a time as any to be asking serious questions about this practice and try to at least work out a compromise between culture and conservation.

This was actually alluded to in an article about the culinary adventures of the late TV chef Anthony Bourdain in Iceland. Björn Teitsson, a cultural critic claimed that most Icelanders would only taste Hákarl during the annual Þorrablót celebrations. If true, this could be a good example of how we can strike a balance with the needs of the creatures of the sea. By scaling down the exploitation of Greenland sharks to (hopefully) sustainable levels, the species and its important role in Icelandic tradition wil survive for many years to come.

greenland shark image 6
Thanks for reading


Best Life Editors. 2017. 20 Easy Ways to Look a Decade Younger. Last accessed 08/05/2020

Silberman and Adamiyatt. 2019. 36 Beauty Expert-Recommended Secrets to Looking Younger. Last accessed 08/05/2020

Wikipedia. 2020a. Last accessed 22/04/2020

Kyne, P.M., Sherrill-Mix, S.A. & Burgess, G.H. 2006. Somniosus microcephalus. The IUCN Red List of Threatened Species 2006: e.T60213A12321694. Downloaded on 22 April 2020.

Morelle. 2016. 400-year-old Greenland shark ‘longest-living vertebrate’. Last accessed 22/04/2020

Nielson et al. 2016. Eye lens radiocarbon reveals centuries of longevity in the Greenland shark (Somniosus microcephalus)

Eagle et al. 2019. Somniosus microcephalus. Last accessed 22/04/2020

Schrope. 2014. Mysterious giant sharks may be everywhere. Last accessed 22/04/2020

Nielson et al. 2014. Distribution and feeding ecology of the Greenland shark (Somniosus microcephalus) in Greenland waters

Howden. 2008. Clash of the fiercest predators as shark eats polar bear. Last accessed 22/04/2020

Costantini et al. 2017. The Greenland shark: a new challenge for the oxidative stress theory of ageing?

O’Sullivan. 2017. Greenland sharks beginning to shed their secrets on longevity. Last accessed 04/06/2020

BBC News. 2007. Ming the clam is oldest mollusc. Last accessed 22/04/2020

Wikipedia. 2020b. Last accessed 27/05/2020

Wikipedia. 2020c. Last accessed 22/04/2020

MacNeil et al. 2012. Biology of the Greenland shark Somniosus microcephalus

Serena. 2018. Hakarl — Iceland’s National Dish Is Basically Rancid And Toxic Shark. Last accessed 16/05/2020

Skara et al. 2015. Fermented and ripened fish products in the northern European countries

Morris. 2019. Hákarl: How Fermented Shark Became Iceland’s National Dish. Last accessed 16/05/2020

Icelandic Magazine. 2019. When Anthony Bourdain visited Iceland to eat the worst food he’d ever taste. Last accessed 08/05/2020

The F Word. 2011. James May eats Bull’s Penis and Rotten Shark – Gordon Ramsay. Last accessed 08/05/2020

Laxson et al. 2011. Decreasing urea∶ trimethylamine N-oxide ratios with depth in chondrichthyes: a physiological depth limit?

Tang et al. 2013. Intestinal microbial metabolism of phosphatidylcholine and cardiovascular risk

Qi et al. 2018. Circulating trimethylamine N‐oxide and the risk of cardiovascular diseases: a systematic review and meta‐analysis of 11 prospective cohort studies

Velasquez et al. 2016. Trimethylamine N-oxide: the good, the bad and the unknown

NOAA. 2020. What are PCBs? Last accessed 13/05/2020

Strid et al. 2007. Dioxins and PCBs in Greenland shark (Somniosus microcephalus) from the north-east Atlantic

Strid et al. 2013. Brominated and chlorinated flame retardants in liver of Greenland shark (Somniosus microcephalus)

Anthoni et al. 1991. Poisonings from flesh of the Greenland shark Somniosus microcephalus may be due to trimethylamine

Strøksnes. 2017. My Hunt for the 400-Year-Old Shark Whose Flesh Gets You High. Last accessed 29/04/2020

Fulton. 6 ÞORRABLÓT FOODS FOR THE CULINARILY BRAVE. Last accessed 16/05/2020

Image sources

Hemming1952. 2007. [CC BY-SA 4.0 (].

Hans-Petter Fjeld. 2006. [CC BY-SA 2.5 (].

Ecomare/Oscar Bos. 2016. [CC BY-SA 4.0 (].

Virginia State Parks staff. 2011. [CC BY 2.0 (].

Alexandre Buisse (Nattfodd). 2007. [CC BY-SA 3.0 (].

Arturo de Frias Marques. 2011. [CC BY-SA 4.0 (

Xfigpower. 2011. [CC BY-SA 3.0 (].

Jóhann Heiðar Árnason. 2009. [CC BY-SA 3.0 (].

Chris 73. 2007. [CC BY-SA 3.0 (].

All other images are public domain and do not require attribution