Our world under the waves

by Matthew Norton

The sea is a dangerous place to live with many of its creatures under threat from bigger animals that want to eat them. Therefore, it makes sense to have defences in place, such as a hard shell to protect their soft flesh, a flexible body to squeeze into tight hiding places, or strong, muscular fins to quickly swim away from danger. Other sea animals produce and store toxic chemicals in their organs, which will poison any predator foolish enough to eat them.

For many species of pufferfish, the weapon of choice is tetrodotoxin which is a nasty neurotoxin that disrupting the electrical signals that pass through an animal’s nervous system. Under normal circumstances these signals are transmitted by the movement of electrically charged sodium ions in and out of the nerve cells, which moves the signal along (like a Mexican wave in a crowd).

Tetrodotoxin blocks the channels through which the sodium ions re-enter the nerve cells which, depending on the dose, weakens, or completely blocks the transmission of these electrical signals. Without properly functioning nerves the brain of the poisoned animal will struggle to tell it muscles to move, resulting in symptoms such as muscle weakness, paralysis and breathing problems.

Tetrodotoxin is clearly an effective deterrent against predators, but there are ways to develop a resistance to tetrodotoxin, such as changing the structure of the sodium ion channels, otherwise pufferfish would also suffer from its effects. Even predators with limited tolerance to tetrodotoxin, or no tolerance at all, may still be able to eat pufferfish without getting poisoned for two very good reasons.

Firstly, the toxicity differs between pufferfish species, which suggests that some species need less protection, or rely more heavily on other means of protection, such as puffing their bodies up, or sudden bursts of swimming activity.

Secondly, in most species, pufferfish are not born with tetrodotoxin defences, in fact they cannot produce the toxin themselves and instead keep cultures of toxin-producing bacteria in their bodies. They get these bacteria from their prey, who in turn get them from further down the food chain, but until they can accumulate a meaningful amount of tetrodotoxin from these bacteria they will be vulnerable to predators. The only exceptions are species who can accumulate the toxin in their ovaries, such as the Grass Puffer, Takifugu niphobles, and pass it on to their larvae.

Pufferfish have adopted a strong anti-predator defence with their use of tetrodotoxin, but this strategy still has its limitations, which is probably why they have other defences in place. Some marine animals, such as cone snails and arrow worms, have even repurposed the toxin for catching and paralysing their prey, rather than defence. Nonetheless, the use of toxic chemicals for defensive purposes is still an inventive solution in the struggle to survive and not get eaten.

From a human perspective

There are a number of different animal groups that are poisonous to eat because they contain tetrodotoxin, but tetrodotoxin from pufferfish is worth paying attention to because we eat them. Human consumption of pufferfish is possible because the muscle of most species has very low concentrations of tetrodotoxin, which can be removed prior to consumption, and farmed pufferfish can be deprived of their supply of toxin-producing bacteria.

Unfortunately, there are still cases of tetrodotoxin poisoning, especially in Japan and the United States, due to pufferfish meat not being prepared properly, or a being labelled as a completely different, non-toxic, fish. This potentially fatal mistake (for the consumer) was even depicted in an episode of The Simpsons in which Homer Simpson thought he had eaten toxic pufferfish meat and only had 24 hours left to live.

Fortunately, we can also use tetrodotoxin as a painkiller by reducing the dose to a safe level (relatively speaking) that only weakens the electrical signals in affected nerves, specifically the nerves that tell the brain to feel pain from a specific body part. In modern medicine there it can be used to relieve pain in patients recovering from surgery and suffering from medical conditions that include arthritis, cancer and injuries to the spine and knee joints. Some evidence also suggests that pufferfish meat and eggs, which contained tetrodotoxin, was used in historical medical practices to treat arthritis, leprosy and muscle convulsions.

Non-fatal doses of tetrodotoxin from pufferfish may also be used insidious purposes such as voodoo zombification. Stories of this practice, where the recently deceased are believed to be magically raised from dead by a voodoo priest, or priestess, originated in Haitian folklore among slaves brought from Africa in the 18th century. These stories received more widespread attention in the western world in the early 20th century and was soon depicted in western literature and early zombie films such as “White Zombie”.

Tetrodotoxin’s role in voodoo zombification was suggested in the 1980’s by Wade Davis, an anthropologist (the study of humans) from Canada who suggested that the practice is partly based in reality. Instead of magic, he suggested that a special ‘zombie powder’ applied the by voodoo priest/priestess contained tetrodotoxin, extracted from pufferfish, caused a death-like state in the victim, who seemingly rises from the dead as the toxin wears off.

Davis’s claims were controversial at the time, and remain so to this day for a variety reasons. These include specific criticisms over how Davis conducted his research while he was in Haiti, such as confusion around the samples of zombie powder he collected and the fact he didn’t test its effects on mice. There were also personal attacks against Davis, such as claims he was involved in morally questionable activities in Haiti and that, by writing two popular books, he was exploiting the publicity around his findings. These arguments aside (I’d rather not get involved regardless of their validity) the scepticism over Davis’s claims is well founded, but if they are true it would hardly be the first radical theory to be rejected at first and then widely accepted at a later date. When Charles Darwin argued that species evolve by natural selection and Galieo Galilei argued that the Earth orbits around the Sun they were seen as radical by many people at the time.

It is bizarre to think that this one toxin can poison us, relieve us from pain and make us think we’ve risen from dead. Admittedly, the role of tetrodotoxin in voodoo zombification is questionable at best, much like the existence of the practice itself. However, even the idea that this mythical practice could exist in the real world has a powerful effect on human culture and zombies as an icon in popular culture.

Sources

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All other images are public domain and do not require attribution

by Matthew Norton

For a long, long time we have been naming and classifying every living thing around us (animals, plants etc) in an attempt understand the natural world. We often derive these names from the creature’s appearance, behaviour, where it is found as who it is related to. However, sometimes a name sticks even when later discoveries would justify the name being changed.

The Eurasian oystercatcher (Haematopus ostralegus) is one such species with a name that is not entirely suitable. This seabird, found on seashores, estuaries and some inland areas, does have a wide range that includes Northwest Europe, the Mediterranean and parts of Asia. This justifies the Eurasian part of the name, but oysters are only a small part of a varied diet which includes mussels, cockles, limpets, sand gapers, lugworms and earthworms.

Many Eurasian oystercatchers may be unable to catch oysters because they have modified their bills to the shape, size and thickness that suits a completely different diet. These modifications are achieved by simply allowing the bill to be scraped and moulded into a particular shape by the prey they eat and the habitat they live in.

Naturally, different bills for different diets would also require different feeding behaviours. Shell hammerers use blunt beaks to break open the shells of their prey, shell stabbers use chisel shaped beaks to prise open mussels (and other bivalve molluscs) and probers dig their pointed bills into mud and soil to search for buried prey. These are the three main categories of bill shape and feeding behaviour, but in some cases these birds may develop bills that combine features from two categories, allowing the bird to use two feeding behaviours. For example some birds may use pointed chisel bills to probe in soil and stab mussel shells on the shore.

You may be wondering how and why oystercatchers ‘decide’ to specialise on a particular prey type. In some cases this is simply a matter of what prey is available in the habitat. Oystercatchers on rocky shores will find mostly mussels and other shellfish while in estuaries and inland soil they will have better luck finding buried prey such as sand gapers, lugworms and earthworms. In other cases there is a clear preference in feeding strategy, even when all kinds of prey are available. This is based on what prey gets them the most food with the least cost from the prey’s defences and from competition with other birds.

For example the shell hammering strategy is typically used by the large adults who can break open the larger mussel shells, whereas younger (and smaller) birds may adopt the stabbing technique to handle smaller mussels and/or probe mud/soil for worms. These prey are less nutritious than the larger mussels, but at least these meagre meals won’t attract competition from the larger, more dominant adults. Similar differences in feeding strategy can be seen between male and female oystercatchers with the latter being smaller and more likely to adopt stabbing and probing feeding behaviours.

In summary Eurasian oystercatchers are remarkably flexible to their individual circumstances and can readily adopt the most suitable feeding strategy for where they live, what prey is available to them and what competition they have to contend with. This is a valuable skill to have in an unpredictable world that presents its inhabitants with all kinds of challenges and opportunities.

From a human perspective

Birds have a special place in human culture that spans hundreds, if not thousands of years and the Eurasian oystercatcher is no exception. In particular, this bird is the national bird of the Faroe Islands, a small archipelago located in the North Atlantic between Norway, Iceland and are symbolised as a defender of the Faroese people from oppressive Danish colonists. This is probably linked to the tendency for oystercatchers to allow smaller birds (i.e. The Faroese people) to nest close to their own nests for protection from larger birds of prey (i.e. The Danish colonists). Also, their arrival to the islands each year, almost always on the 12th March, is celebrated as part of their St Gregory’s Day celebrations and as a sign that summer is on its way.

Unfortunately, our interactions with oystercatchers are not always so positive. Sometimes there are conflicts between oystercatchers and fishermen who are after the same shellfish in the same area. These conflicts get particularly intense when supplies are low and proper fisheries management is not in place. For example, during the 1980s there were massive declines in mussel and cockle stocks in the Dutch Wadden Sea, but fishermen continued harvesting them , which is probably what led to 10,000’s of oystercatchers dying from starvation.

We also cause harm to oystercatchers by polluting their habitats, destroying their breeding and wintering grounds and even hunting them for food and feathers in some regions. However, these ‘headline threats’ make it easy to forget that even our mere presence on the seashore can be harmful if we are not careful. This is because when they see us, and they will, they see a possible predator and will fly away if we get too close. This might sound like nothing to worry about, but these birds cannot eat and fly away from predators at the same time and with repeated disturbances they lose valuable feeding time. This is especially true during the winter, when food is harder to find, and breeding season, when they have eggs to look after and chicks to feed.

I am not suggesting that we should never visit the seashore, the activity improves our health and reminds us of our connection with the sea. Fortunately, Eurasian oystercatchers are a low-medium risk species according to the RSPB and IUCN, but we should keep a respectful distance when we visit their habitat and try to keep a harmony between us and the wildlife we admire.

Sources

Wikipedia. 2018. Eurasian oystercatcher. https://en.wikipedia.org/wiki/Eurasian_oystercatcher. Last accessed 14/01/2019

Arkive. 2018. https://www.arkive.org/oystercatcher/haematopus-ostralegus/. Last accessed 14/01/2019

Pol et al. 2009. Oystercatchers’ bill shapes as a proxy for diet specialization: more differentiation than meets the eye

Durell et al. 1993. Sex-related differences in diet and feeding method in the oystercatcher Haematopus ostralegus

Cayford and Goss-Custard. 1990. Seasonal changes in the size selection of mussels, Mytilus edulis, by oystercatchers, Haematopus ostralegus: an optimality approach

Boates and Goss-Custard. 1992. Foraging behaviour of oystercatchers Haematopus ostralegus specializing on different species of prey

Safriel. 1985. Diet dimorphism’ within an Oystercatcher Haematopus ostralegus population– adaptive significance and effects on recent distribution dynamics

Goss-Custard and Durell. 1988. The effect of dominance and feeding method on the intake rates of oystercatchers, Haematopus ostralegus, feeding on mussels

Hulscher and Ens. 1992. Is the bill of the male oystercatcher a better tool for attacking mussels than the bill of the female?

Faroeislands.Fo. 2018. The Oystercatcher. https://www.faroeislands.fo/the-big-picture/national-symbols/the-oystercatcher/. Last accessed 14/01/2019

Rove.me. 2018. Grækarismessa: The Arrival of the Oyster-Catchers 2021. https://rove.me/to/faroe-islands/graekarismessa-the-arrival-of-the-oyster-catchers. Last accessed 14/01/2019

Ellis. 2018. Oystercatcher.  http://www.birdsofbritain.co.uk/bird-guide/oystercatcher.asp. Last accessed 14/01/2019

Wood. 2015. Using evidence to manage shellfisheries and their wading birds. https://marinescience.blog.gov.uk/2015/07/08/evidence-manage-shellfisheries-wading-birds/. Last accessed 14/01/2019

Atkinson et al. 2010. Impacts of shellfisheries and nutrient inputs on waterbird communities in the Wash, England

BirdLife International. 2018. Eurasian Oystercatcher Haematopus ostralegus. http://datazone.birdlife.org/species/factsheet/eurasian-oystercatcher-haematopus-ostralegus/text. Last accessed 14/01/2019

Stillman and Goss-Custard. 2002. Seasonal changes in the response of oystercatchers Haematopus ostralegus to human disturbance

Coleman et al. 2003.Sub-dispersive human disturbance of foraging oystercatchers Haematopus ostralegus

Verhulst et al. 2001. Experimental evidence for effects of human disturbance on foraging and parental care in oystercatchers

IUCN. 2017. Eurasian Oystercatcher. https://www.iucnredlist.org/species/22733462/117739875. Last accessed 14/01/2019

RSPB. 2018. Oystercatcher. https://www.rspb.org.uk/birds-and-wildlife/wildlife-guides/bird-a-z/oystercatcher/. Last accessed 14/01/2019

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All other images are public domain and do not require attribution

by Matthew Norton

Mangrove trees are one of those rare groups of plants that can survive on the seashore. Plants struggle to survive here due to the periodic flooding with seawater, limited access to fresh (non-salty) water and the waterlogged sediment (soil or mud), which makes it harder to get oxygen and nutrients.

Mangroves thrive in this hostile environment with various coping mechanisms. To control salt levels in their body they have very fine filters in their roots, to limit how much salt gets in, and expel excess salt through the leaves. At the same time, they minimise water loss by producing waxed leaves and/or (depending on the species) moving them to avoid harsh sunlight and so avoid evaporation. Some species also produce thick leaves to store water.

Finally, to overcome the limited oxygen supply in the sediment they instead get oxygen from the air through specialised pores (lenticels) in their trunk. The insides of their roots are also ‘spongy’ with open channels that allow oxygen to flow quickly throughout the root system. Some species go even further with stilted roots to prop up the trees further above the water, so they spend more time in air, while others produce specialised ‘breathing tubes’ that stick out from the sediment, away from the trunk, and are packed with lenticels.

These adaptations are impressive, but let us not forget that mangrove forests include a wide diversity of life, including bacteria, sponges, crabs, insects, birds, mammals, seaweeds and mangrove seeds and seedlings. Mangrove trees support many of these organisms with their roots providing shelter from strong waves, hiding places from predators and a hard surface for animals to hold on to. By providing shelter from the waves, mangrove roots also encourage the settlement of fine sediment (i.e. mud), which provides habitat for burrowing species. Their seedlings, dead branches and dead leaves also provides food for many of the creatures on the forest floor.

In some cases these organisms support the mangrove trees in return, supplying them with the nutrients that can be in short supply. Inside the previously mentioned ‘breathing tubes’ of black mangroves there are cultures of cyanobacteria (photosynthetic bacteria) which extract nitrogen gas from the air and convert into nitrate, an essential plant nutrient. Also, some species of giant sponge grow on mangrove roots and produce tiny rootlets to help them to extract nutrients from the surrounding soil. Animals not attached to the trees can also be helpful, for example burrowing mix up the sediment and gets oxygen deeper down in the sediment where it reacts with iron to produce ferric oxide, another essential plant nutrient.

Sometimes, we don’t appreciate how stressful the seashore environment can be for living things, despite it being the border between two very different worlds. Evolution has responded with some of the hardiest plants and animals in the world, but this process acts on all aspects of the species, including its interactions with the environment and other species it shares that environment with. In other words no-one faces the world alone.

From a human perspective

Mangrove forests, like rainforests and coral reefs, are disappearing at an alarming rate with a 1-2% loss in forest area each year. The reasons for this decline include trees being cut down for firewood and cleared away for infrastructure and shrimp aquaculture. This is a devastating loss for humanity, as we benefit from mangrove forests in various ways.

For example they can reduce the impacts of climate change by extracting carbon dioxide from the atmosphere and then locking up that carbon as dead plant matter sinks into the sediment. Mangroves also provide important nursery habitats for many commercially valuable species of fish and shellfish (e.g. rainbow parrotfish) where they can develop and grow in a relatively safe and nutrient rich environment.

Another, more direct benefit afforded to humans by mangrove trees is that they protect us from the sea’s destructive side (flooding, coastal erosion, tsunamis) by reducing the power of the waves that pass through their roots. For example in the aftermath of the 2004 Indian Ocean earthquake, and the tsunamis they caused, surveys were conducted to assess the damage caused in different regions. In the Tamil Nadu coastal region man-made structures positioned behind thick mangrove cover suffered less damage from the tsunamis. Claims were also made of fewer human deaths in these areas.

However, while mangroves can protect us from the sea, other precautions must be taken. There are also doubts in some of the findings of the Indian Ocean tsunamis surveys due to criticisms in how the data was analysed. A more recent study (2018) between two sites in New Zealand showed just how much mangrove protection against smaller waves and coastal erosion can vary. One site in the Firth of Thames, where the mangrove trees were high and tightly packed together, was very effective at resisting the flow of both tides and storm surges. At a 2nd site in Tauranga harbour the smaller mangroves offered little protection and even that was compromised by drainage channels that diverted the water around the trees. Regardless, the possibility that mangroves can protect us from nature’s wrath is reason enough for us to protect them in return.

Fortunately, there have been attempts to replant lost areas of mangrove forest, but in some cases the results have not been promising. This is partly due to lack of planning with seedlings being planted in poor growing conditions (e.g. soil pH, topography), not ideal when some species are very fussy about this. In some cases seedlings have even been planted in places where there were no mangrove trees in the first place. Worse still, these replanting programmes often attempt to create plantations of a single mangrove species, rather than an integrated ecosystem of multiple mangrove species, and their associated animals and microorganisms.

In the past when we have thought of tropical environments it is usually rainforests and coral reefs that come to mind, with mangrove forests getting much less attention. This is a shame, but in more recent years their value has become more appreciated and I am optimistic that, with the right insight and action, they can recover and thrive as part of our world.

Sources

Wikipedia, 2018. Mangrove. https://en.wikipedia.org/wiki/Mangrove. Last accessed 17/12/2018

Liang et al. 2008. Adaptation to salinity in mangroves: Implication on the evolution of salt-tolerance

Scholander et al. 1955. Gas exchange in the roots of mangroves

Gill and Tomlinson. 1977. Studies on the growth of red mangrove (Rhizophora mangle L.) 4. The adult root system

Nagelkerken et al. 2008. The habitat function of mangroves for terrestrial and marine fauna: a review

Chapman and Feller. 2011. Away‐field advantage: mangrove seedlings grow best in litter from other mangrove species

Emmerson and McGwynne. 1991. 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

Toledo et al. 1994. Cyanobacteria and black mangroves in Northwestern Mexico: colonization, and diurnal and seasonal nitrogen fixation on aerial roots

Ellison et al. 1996. Facultative mutualism between red mangroves and root‐fouling sponges in Belizean mangal

Kristensen and Alongi. 2006. Control by fiddler crabs (Uca vocans) and plant roots (Avicennia marina) on carbon, iron, and sulfur biogeochemistry in mangrove sediment

Baley. 2018. Iron For Plants: Why Do Plants Need Iron?. https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/iron-for-plants.htm. Last accessed 17/12/2018

Duke et al. 2007. A world without mangroves?

Donato et al. 2011. Mangroves among the most carbon-rich forests in the tropics

Manson et al. 2005. An evaluation of the evidence for linkages between mangroves and fisheries: a synthesis of the literature and identification of research directions

IUCN. 2017. Mangroves: nurseries for the world’s seafood supply. https://www.iucn.org/news/forests/201708/mangroves-nurseries-world’s-seafood-supply. Last accessed 17/12/2018

Othman. 1994. Value of mangroves in coastal protection

Alongi. 2008. Mangrove forests: resilience, protection from tsunamis, and responses to global climate change

Kathiresan and Rajendran. 2005. Coastal mangrove forests mitigated tsunami

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Ellison. 2000. Mangrove restoration: do we know enough?

Kodikara et al. 2017. Have mangrove restoration projects worked? An in‐depth study in Sri Lanka

Lewis III. 2014. Chapter 14.1: 14.1. Mangrove forest restoration and the preservation of mangrove biodiversity. In “The State of the World’s Forest Genetic Resources Thematic Study: Genetic Considerations in Ecosystem Restoration Using Native Tree Species” (eds. Bozzano et al.). ISBN  978-92-5-108469-4

Image sources

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All other images are public domain and do not require attribution

by Matthew Norton

Dog whelks (Nucella lapillus) are small sea snails, but they are also very ambitious predators who will attack barnacles, mussels and other shellfish that are bigger than they are. They cannot swallow their prey whole (and then spit out the inedible parts), or prise the shell apart with powerful claws. Instead they use the less common method of sliding onto the shell and then drilling into it using their radula, a conveyor belt of teeth that dog whelks have modified for drilling. To soften up the shell, making it easier to drill through, they release a chemical mixture of acids, toxins and ‘sticky’ proteins to (slightly) dissolve the shell material. Once they break through the shell dog whelks get really nasty by releasing more chemicals that turns the animal into a mushy soup that the they can suck up.

Despite these formidable weapons, dog whelks still face many problems while on the hunt, many of which are caused by the fact that it takes hours, if not days for them to break through each shell. So if they want to survive, they have to include some strategic thinking into their hunting activities. For example the long drilling time can leave dog whelks exposed to the harsh seashore environment and their own predators. When conditions are especially harsh, or their predators are close, dog whelks are more likely to hide away in rocky crevices, unless they are very hungry and the meal they desperately need is worth the risk.

Dog whelks also have to be careful in how they handle their prey. Firstly, they need the amount of food from a particular mussel (or other shellfish) to be worth the monumental effort they put into drilling through the shell. Some studies have shown that dog whelks prefer mussels of a particular size and focus drilling on the thinner parts of the shell, although this takes time to learn for inexperienced mussel hunters. Secondly, their prey sometimes fight back. In particular should a single dog whelk get too close to several mussels they may use their byssal threads to tie them down, leaving the dog whelk unable to move, or attack the mussels. If they cannot break free from this trap the dog whelk will die from starvation, or prolonged air exposure. It unsurprising that most dog whelks will hunt at the edges of dense mussel beds with only the occasional fool venturing further in.

Dog whelks are one of those creatures that show, in gruesome detail, how nasty nature can be. However, this does not make them mindless eating machines, they still consider their options and potential risks while looking for their next meal. With their brains, or what passes for a brain in this little snail, they are unlikely to ‘think’ as much as we do, but dog whelks are still effective hunters, combining powerful weaponry with strategic ‘thinking’.

From a human perspective

Unlike most creatures in the sea, animals on the seashore have been within our reach for thousands (probably millions) of years and so have had more potential to influence human history and culture. In the case of dog whelks, and other closely related species, we having been using their flesh to make purple and violet dyes for centuries. Clothing dyed in “Tyrian purple” was particularly valuable in ancient cultures, often worth its weight in silver and worn as a status symbol. Archaeological finds have dated its use to at least the 13th century B.C. in the Mediterranean region and pre-roman times for much of Europe.

Dog whelks would have been killed and crushed in vast numbers to satisfy the demand for this dye, which may sound excessively destructive, but this is nothing compared our more recent coastal activities. In particular our boats, both large and small, once had their hulls painted with a chemical called tributyltin (TBT) to prevent sea creatures from growing on it. Unfortunately, this had unexpected effects on dog whelks, causing females to grow male body parts by interfering with their hormones. This phenomenon is called ‘imposex’ and can make reproduction impossible for them by blocking up the tubes through which they eject their egg capsules, a blockage which can kill them as the capsules accumulate in their body.

Dog whelks were hit hard by TBT exposure during the 1970’s and 1980’s, especially near harbours and other boating hotspots, with populations being completely wiped out in some areas. Eventually, this issue was addressed with many countries bringing in partial bans on the use of TBT in the 1990’s, followed by a full worldwide ban in 2008. Since this action was taken TBT levels have been coming down, although many habitats will remain contaminated for a long time. To measure this decline we have been using the number of dog whelks that are still suffering from imposex as an indicator. Some have kept groups of dog whelks in cages in rivers and estuaries for several months while others have monitored natural populations on the shore for many years.

It is a sad fact that our actions have very destructive effects on sea life, whether intentional or not, and we are often slow to do anything about it. In the case of TBT its purpose was to be harmful to sea creatures, but we probably didn’t realise how effective it was, or that it could leak away from the hulls of boats. However, the actions we took (eventually) to reduce TBT contamination in the seas shows that we can learn from our mistakes.

Sources

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Rovero et al. 1999. Automatic recording of the radular activity of dogwhelks (Nucella lapillus) drilling mussels (Mytilus edulis)

Vadas et al. 1994. Foraging strategies of dogwhelks, Nucella lapillus (L.): interacting effects of age, diet and chemical cues to the threat of predation

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All other images are public domain and do not require attribution

by Matthew Norton

The short answer is yes and no. Adult green turtles (Chelonia mydas) are indeed herbivorous, feeding on seagrass, seaweed and pieces of mangrove trees, but in their younger years they are carnivores, eating a range of animals including jellyfish, worms, shellfish and fish eggs. As they grow they gradually include seaweed and plants into their diet until they become entirely reliant on them. Changes in diet during an animal’s lifetime, especially as they develop and grow towards adulthood, is common in nature, but this extreme shift between two very different types of food is less common.

For young green turtles, an animal based diet actually makes sense because they are weaker swimmers compared to the adults. Studies that have monitored green turtle diving behaviour have found that the dives of the younger turtles were shorter and shallower, which suggest they haven’t quite mastered the skill of diving, or the ability to properly control their buoyancy. This makes it difficult to reach patches of seaweed and seagrass because they grow on the seafloor, but near the ocean surface there are small, floating animals that the younger turtles can feed on.

Given that younger green turtles can survive on floating animal food, maybe we should be focussing on why they don’t continue eating animals into adulthood. This is a good question for two reasons.

Firstly, all other sea turtle species, including the green turtle’s close relatives the flatback turtle (Natator depressus) and hawksbill turtle (Eretmochelys imbricata), are either carnivores, or omnivores (they eat both plants and animals).

Secondly, plants are generally harder for animals to eat because they contain, among other things, a lot of fibre and cellulose, a complicated sugar that plants use to build walls around their cells and is hard for animals to break down into the smaller sugars that they actually need. The fruits and vegetables that we eat are only edible because plants ‘design’ them to be eaten by animals for the purposes of seed dispersal.

Herbivorous animals play an important role in the food chain by supporting larger (in most cases) carnivorous animals. However, evolution does not really work towards setting up these food chains and so a more immediate explanation is needed. One possibility is that by adapting to survive on a more difficult food, such as plants, adult green turtles avoid competing for food with other sea turtles who still rely on animal prey. Such adaptations include their serrated jaws which acts like a saw scraping up plants and seaweed from surfaces and tearing them into manageable pieces. Their stomachs also digest plants, especially cellulose, better than most other reptile herbivores thanks to a rich culture of bacteria that are particularly effective at breaking down plant cell walls.

The way that green turtles eat seagrass and seaweed also helps them to make the most of this resource. For example they mainly graze the tips of seagrass, encouraging the growth of new plant material which, if they are quick enough, they can pick off before the fibres, cellulose and other harder digest material can build up. They can also be flexible with their diet depending on what is available with some populations eating mainly seagrass, while others eat more seaweed. Some have even suggested that, in rare cases, they may still be omnivorous into adulthood.

The act of consuming other organisms is essential to the survival of any animal (with some exceptions) and regardless of what species they target there will be challenges for them to overcome. These include catching enough food, digesting, or getting rid of the hard parts and competing with other animals for a particular food source. In the case of green turtles they use different strategies depending on their age and where they live and so make good use of the most convenient and profitable food resource they can find.

From a human perspective

Sea turtles face numerous threats in today’s world as a consequence of our actions, they get tangled in our fish nets, they accidently eat our plastic rubbish and they lose their eggs, shells and lives to poachers. In other scenarios the impacts we have are more subtle, but no less devastating. In particular the rising temperatures on green turtle nesting beach, as a result of human induced climate change, can cause numerous problems for the developing embryos. These include burning through their energy reserves (contained in the egg yolk) too fast during development, as well as abnormalities and weaker swimming ability in the hatchlings. Even more worrying is how increasing temperatures can play havoc with the birth rates of males and females.

Determining whether an animal is born as a male, or a female is a more complex procedure than most of us realise, with different groups doing this in different ways. In some animals gender is determined by how many copies of their entire gene set they have, in others it’s determined by the combinations of specific chromosomes (bundles of genetic material) they inherit from their parents. In turtles, and other reptiles, genes play no role at all with gender being determined by the temperature at which the egg is incubated. Eggs incubated at 26-28oC produce male hatchlings and eggs at 30oC produce females.

Normally this would produce a healthy ratio of males and females, but with increasing temperatures, both globally and on particular nesting beaches, there is a very real risk of these ratios being skewed towards producing too many females and too few males. In some areas this has already yielded extreme results with over 99% of the population being female. The repercussions of this are pretty clear, without enough males to reproduce with the females the green turtle as a species is at risk of extinction.

There may be some nesting sites that can still produce male offspring due to the widespread range of green turtles, which exposes them to a range of temperatures despite the general trend of increasing temperature. Even within a single nesting beach, nesting temperature can still vary due to various factors, such as how high on the beach the nest is and how close it is to shade. This might be wishful thinking, but if green turtles are to survive in the future, then male producing nesting beaches should be prioritised for protection from poaching and other human threats.

Sources

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All other images are public domain and do not require attribution

by Matthew Norton

The sea is full of weird and wonderful creatures with appearances and ways of living that you do not see on land. Larvaceans, a group of transparent, swimming invertebrates are a shining example of this point with a weird tadpole-like appearance which actually resembles the larvae of its close relatives. They make a living by using their long tails (relative to their body size) to draw in water and then filter out small food particles.

Filter feeding is a very common practice among marine animals, but larvaceans are particularly effective at it and able to catch tiny particles, even down to small bacteria. They manage this by producing a ‘house’ of mucus which surrounds the animal and can be much larger than the animal itself. These house contains a complex arrangement of funnels and finely meshed, and sticky, filters which concentrate and catch the microscopic food particles.

Unfortunately, these filters are easily clogged with these particles which forces larvaceans to abandon their house and build a new one every few hours. This may sound like a lot of hard work, but with their rapid growth rate and short lifespan, only a few days in some species, larvaceans get a lot out of each house they build.

Once abandoned by their original owners larvacean houses are by no means useless. In fact they are a valuable food source because of the food still clogged in the filters and other material they accumulate as they sink, such as phytoplankton (microscopic plants), bacteria and faecal pellets (poo). Many animals take advantage of this sinking food source, such as copepods (small swimming crustaceans) and some species of eels who will actively seek out larvacean houses. Finally, these abandoned houses are especially valuable to animals on deep seafloor, who almost completely rely on food raining down from above, except around hydrothermal vents.

Larvaceans are a shining example of the ingenuity that marine animals can show so that they may survive, thrive and eventually reproduce. They are not the only group that build structures to this end, a crab’s shell is virtually separate from the animal and most of a jellyfish’s body is just jelly. However, larvacean houses are so well designed for their purpose that they take on a life of their own after being abandoned by their builder, showing that little goes to waste in the ocean.

From a human perspective

Larvaceans have proved to be a difficult group to associate with humans. They are an obscure group and there is no way (that I know of) that we directly exploit them for our benefit. Nonetheless, there are some aspects of their evolution and lifestyle which, with some work and imagination, can have future applications for humanity.
For starters larvaceans are more closely related to us than most other invertebrate life because we all belong to the phylum Chordata, which is separated into tunicates (e.g. larvaceans, sea squirts and salps), cephalochordates (e.g. lancelets) and vertebrates (all animals with a backbone). All of the above share a number of distinctive features, although in many species such features will only appear at certain times in their lives. The most notable being a notochord, a flexible rod that stretches through the length of the animal and provides them with structural support. In vertebrate animals this notochord develops into a spine made out of bone early in their development.

Because they share such features larvaceans can be used to better understand how certain features in vertebrate chordates evolved and their development at the molecular and genetic basis. Some have already studied the development of larvacean species, especially Oikopleura dioica, for this purpose. This species uses a number of genes to control the development of its own central nervous system with complicated names, such as hox1, pax6, pax2/5/8b. The important point here is that many of these genes are similar to genes found in vertebrate development and produces similar structures to the spine and parts of the vertebrate brain in larvaceans.

These similarities can be very useful for us as we can, in theory, perform experiments on larvaceans that we could not do with the embryos of vertebrate animals. In particular Oikopleura dioica are quick and easy to keep and breed in the laboratory with a lifespan of around 4 days and produce large masses of eggs. They also have a very small genomes (the complete set of genes in their cells) which makes them easier to work with.

The lives of larvaceans are also relevant, to some extent, with global environmental issues. For example the organic material (i.e. material containing carbon) contained within the sinking houses may play a role in locking up carbon dioxide away from the atmosphere. Indeed accounting for their contribution has increased the accuracy the models designed to explain how carbon is transported and consumed in the oceans.

Recently (2017), there has also been interest in how they can filter out and microplastic particles. This plastic could be locked away in houses that are buried in the deep seafloor. However, it is also likely that many of these houses, and the microplastics loaded on them, will be consumed by other sea creatures. The exact role of larvacean houses in microplastic transport will require, like many parts of their lives, further study.

I will be the first to admit that these links I have suggested between ourselves and our very, very distant cousins are limited, but there is potential and it can be explored in the years to come. Not every use we have for the sea, and the creatures that live in it, has been immediately obvious to us and even less so for species that are not well known to us. Nonetheless, it is in our nature to keep exploring the world around us and pushing to find ways to use nature to meet the challenges that nature throws at us.

Sources

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Hopcroft and Roff. 1995. Zooplankton growth rates: extraordinary production by the larvacean Oikopleura dioica in tropical waters

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King et al. 1980. Predator-prey interactions between the larvacean Oikopleura dioica and bacterioplankton in enclosed water columns

Katija et al. 2017a. New technology reveals the role of giant larvaceans in oceanic carbon cycling

Sato et al. 2011. House production by Oikopleura dioica (Tunicata, Appendicularia) under laboratory conditions

Hopcroft et al. 1998. Zooplankton growth rates: the larvaceans Appendicularia, Fritillaria and Oikopleura in tropical waters

Alldredge. 1972. Abandoned larvacean houses: a unique food source in the pelagic environment

Mochioka and Iwamizu. 1996. Diet of anguilloid larvae: leptocephali feed selectively on larvacean houses and fecal pellets

Robison et al. 2005. Giant larvacean houses: Rapid carbon transport to the deep sea floor

Hansen et al. 1996. Marine snow derived from abandoned larvacean houses: sinking rates, particle content and mechanisms of aggregate formation

Wikipedia. 2018b. Notochord. https://en.wikipedia.org/wiki/Notochord. Last accessed 08/10/2018

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Niiler. 2017. Plankton ‘Mucus Houses’ Could Pull Microplastics From the Sea. https://www.wired.com/story/plankton-mucus-houses-could-pull-microplastics-from-the-sea/. Last accessed 08/10/2018 

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All other images are public domain and do not require attribution

by Matthew Norton

Sight is a common sense in animals that live in the sea with their eyes, or equivalent visual organ, being modified in various ways to suit a particular species. These modifications can evolve in response to the light levels in their environment as well as their lifestyle, which can influence how well they need to see. This is well demonstrated in threespined sticklebacks, Gasterosteus aculeatus.

Threespined sticklebacks are small, carnivorous fish which have three spines (usually) in their dorsal fins and are found in both seawater and freshwater (rivers, lakes etc) environments. Their eyes also possess an uncommon type of light detecting cell (called cone cells) that allow them to see ultraviolet light.

Unsurprisingly, there have been a number of studies to determine why these fish need to see ultraviolet light, with the most likely explanation being that they reflect ultraviolet light with their silvery bodies and use this to communicate with each other through swimming displays. Threespined sticklebacks communicate with each other in various scenarios, such as cooperating with each other to share food and inspect potential predators, while also be aggressive to strangers and known cheaters. Breeding also involves various displays from the males to entice females to lay eggs in their nest, these include zig-zag swimming, leading females to the nest and fanning the nest with their tail.

The problem with being so expressive is that these sticklebacks risk attracting their predators. However, this is where seeing and reflecting ultraviolet light is advantageous because it does not travel far in water before it is absorbed and scattered by the water and any material and organisms suspended in it. This short range is sufficient for communication between threespined sticklebacks while minimising the risk of detection from eavesdropping predators.

However, freshwater populations of threespined sticklebacks use a different strategy, being less sensitive to ultraviolet light, apparently in favour of increased sensitivity to green light. This suggests that the range of ultraviolet light is too short in freshwater and instead rely on other wavelengths of light for communication. This shift in light sensitivity evolved since the last ice age, approximately 12,000 years ago, when the freshwater populations were separated from their seawater ancestors, a remarkable feat considering that normally such modifications would take millions of years to evolve.

How any creature sees the world is of major importance to how they interact with it, especially given that light is theoretically the fastest way that information can reach anyone in the world. This article has only touched on the sensitivity of stickleback eyes to different wavelengths of light, but there can be other variations. Some species are blind or can only ‘see’ light and dark, while others have complex eyes structures that can be use to build a detailed picture of the world and a select few can even see better than us.

From a human perspective

Sticklebacks are a popular group to use in scientific research, being small, tame and easy to keep and observe in aquariums while also possessing some interesting behaviours and evolutionary features. As a result they have played a role in some major scientific advances.

In particular the Dutch behavioural scientist Niko Tinbergen studied stickleback behaviour extensively from the 1930s, even claiming they were more reliable than popular alternatives, including bees and aquatic insects. This work, as well as that of two Austrian scientists, Konrad Lorenz and Karl von Frisch, was important during the early development of Ethology, a scientific discipline focusing on the evolution of animal behaviour.

Tinbergen himself introduced a guide for understanding an animal’s behaviour by considering all its different components, this was called “Tinbergen’s four questions”. Theoretically this can even by extended to any evolved trait, making these four questions a valuable tool for biologists.

Sticklebacks have been relevant to other fields of research, such as genetics where threespined sticklebacks were among the first species of fish to have their genome (i.e. all their genes) sequenced. This was probably because the recent separation, in geological time, between the seawater and freshwater populations provides a useful opportunity to study how an animal’s genome adapts to an environmental change. This is a topic of particular interest in evolutionary biology.

Both the areas of research I’ve mentioned can also be relevant to us. Human ethology has emerged from its ‘parent discipline’ as a way of understanding human behaviour and psychology in an evolutionary context. Meanwhile, human genomics can tell us more about certain diseases, such as heart disease, asthma and diabetes, which are influenced by multiple genes and their interactions with each other and their environment.

In scientific research there will always be some species that are better understood than others due to practicality and interest in the species. Nonetheless, these species can help us to better answer fundamental biological questions, which in some cases can be applied to better understand and help humanity.

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All other images are public domain and do not require attribution

by Matthew Norton

The seashore is a hard place for marine creatures to live with periodic air exposure, the risk of being dislodged by the waves and wide fluctuations in temperature, salinity and other environmental conditions. Many species escape these harsh conditions by burrowing into the sediment (collective term for sand, mud, muddy sand etc) where they can also hide from predators and gain access to unique food sources, such as buried microorganisms. However, as seawater trickles down its oxygen is depleted by any living thing it encounters, which can seriously restrict oxygen supply for animals buried deep in the sediment.

Some species have devised strategies to overcome this issue, including the lugworm (also known as the blow lug), Arenicola marina, which has developed a particular method of burrowing that helps it to keep a sufficient oxygen supply. These worms contract and expand their bodies in a rhythm that pumps water through their burrows and into the wall of sediment in front of them, which loosens it into smaller clumps. They consume these clumps, which incidentally contains the buried microorganisms that they eat, and then defecate them at the surface as coiled faecal casts.

The water flow generated by the lugworms constantly brings ‘new’ water into the burrow so that they don’t rely on a still body of water, within which the oxygen would be quickly exhausted. The oxygen-depleted water also needs to be drained out to make way for ‘new’ water, which is where burrow architecture comes in. In sand lugworms usually dig J-shaped burrows with an open pore in front of their head where water drains out through the tiny gaps between sand particles. In mud draining water is more difficult with much smaller gaps between the mud particles, so lugworms build U-shaped burrows with a second opening at the surface through which they can expel oxygen depleted water.

Lugworm burrowing also changes the chemistry of their surrounding environment, making it more hospitable to life. In particular the enhanced oxygen supply stimulates its reaction with ammonia, a common product of animal waste, to produce less toxic nitrogen based compounds. Both the oxygen and nitrogen recycling stimulates the growth of microorganisms on the burrow walls, some of which can remove carbon dioxide, another waste product that can be toxic at high concentrations.

However, lugworms can also have a negative impact on various species, such as other burrowing worms who may end up competing with lugworms for food, burrowing space and other resources. Lugworm burrowing may also disturb filter feeding species, who filter out tiny food particles floating in the water, lying on, or just below the sediment surface by smothering them with the sediment they excavate from their burrows.

Lugworms are shining examples of how life can adapt to any lifestyle, even in the face of real hardships. Indeed the masses of faecal casts found on sandy beaches and muddy estuaries demonstrates their status as a major force in structuring the subterranean world.

From a human perspective

Being small and living underground, it is unlikely that lugworms are a widespread attraction for anyone with a causal interest in nature, nor are they popular for human consumption. They are however, an important food source for many fish and wading seabirds, which shows that lugworms have an important supporting role in birdwatching and angling. These are both popular recreational activities and come with a variety of benefits.

Bird watching and angling can improve your physical fitness by getting you out in more natural areas of the countryside which are only accessible by foot. There are also mental health benefits from being in a natural environment, which reduces stress, and from the activity itself with increased concentration and awareness for a bite on the fishing line, or for a good view of a rare bird.

Furthermore both activities can inspire creativity to some extent. For example there was a British comic strip called “Ollie and Quentin”, which ran from 2002 to 2011, about an unlikely friendship between a seagull, Ollie, and a lugworm, Quentin. The premise was that Quentin was very adventurous and got himself into ridiculous accidents as a result and Ollie would try to protect him in a sort of older brother role. The comic’s creator Piers Baker has stated that his inspiration was the lugworms he used as angling bait during his youth.

The contact with nature and wildlife that comes with bird watching and angling can inspire a greater awareness and appreciation for the natural world. As a result anglers and bird watchers can be among the first to notice long term declines in wildlife. For example the Isle of Arran, Scotland, was once a very popular destination for recreational sea angling which drew anglers from all over the country and beyond. It was those anglers who felt the effects of decades of commercial overfishing with declines in fish catches eventually forcing the annual sea angling festival to be cancelled after 1994.

Finally, both activities bring economic benefits for local communities and conservation charities for various reasons. These include the bits of kit required, such as binoculars, identification guides, angling rods and bait, the desire for refreshments and accommodation and travel. The economic contribution can be substantial, for example the United States Fish and Wildlife Service estimate that birdwatcher make an annual contribution of over $30 billion to their economy.

When we watch and interact with wildlife it is easy to overlook small creatures like lugworms in favour of large, charismatic animals. This is a habit we need to break, because not only are the small creatures vital to the existence of more popular wildlife species, but they are also fascinating in the own way. This would be plain to see if lugworms were as large and visible as fish, seabirds and marine mammals. It is really is just a matter of scale.

Sources

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Marine Life Information Network (MarLIN). 2008. Blow lug (Arenicola marina). https://www.marlin.ac.uk/species/detail/1402. Last accessed 27/08/2018 

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All other images are public domain and require attribution

by Matthew Norton

For any living creature the world around them is a dangerous place, full of things that can inflict damage on them, which makes it crucial for their survival that they can sense and respond to the danger and minimise the damage. Virtually all animals do so through a sensory mechanism called nociception, which allows them to sense and respond to damage through unconscious reflexes. When we touch something sharp, or hot and then flinch, that is our nociceptive response kicking in before the pain.

Pain, from an evolutionary perspective, is a step up from nociception with the damage being associated with a negative sensory and emotional experience which encourages use to avoid that specific danger in the future. In non-human animals it can be difficult to distinguish pain from nociception, since we cannot ‘measure’ their emotional state. This has prompted much debate into which animals can, or cannot feel pain, especially among invertebrates. Nonetheless a series of criteria have been proposed to determine if an animal can feel pain, which have been applied in various experiments on crabs.

Some of these criteria require crabs to deviate from their usual behaviour in response to the initial damage and then modify their behaviour in the long term, based on their experience (i.e. memories) to avoid future damage. Such behavioural changes have been seen in shore crabs (Carcinus maeans) and hermit crabs in response to an electric shock. In particular hermit crabs may temporarily abandon their adopted sea snail shells, which they rely on to protect their soft abdomen, if the shock comes from within this shell. Should a new snail shell be provided for up to a day after the initial shock they may move into this shell, demonstrating the longer term avoidance behaviour and the involvement of memory.

Other criteria require some involvement of the central nervous system, including the brain, to process the initial nociceptive information and coordinate a response, a crucial step for the previously mentioned behavioural changes. Again, there is some behavioural evidence with the glass prawn (Palaemon elegans), part of the same crustacean group as crabs, and the crab (Hemigrapsus sanguineus) where they intensively groom the part of their body where they have sustained damage. This awareness of where they have been hurt would require the brain to receive information on where they have been damage and then send out information to all the other body parts involved in this grooming behaviour.

These crabs fulfil these pain criteria, but for other criteria there is more limited evidence. For example another criterion requires the crab to be stressed as it suffers damage and one study on Carcinus maenas did find that electric shocks increased the concentrations of lactate in their body. This chemical can be linked to stress, but is also linked to rapid and/or prolonged activity (build ups of lactate is what causes us to ache when we exercise) and is not enough to prove stress on its own.

An even more problematic criterion for crabs to meet is having the required level of consciousness and sentience to be able to feel pain. This is mainly because of the difficulty in defining these terms, which involves thinking philosophically, which does not mix with attempting to find ‘hard’ evidence. Therefore we must rely on imperfect evidence and make an assessment on whether animal are likely to feel pain beyond reasonable doubt. If this approach can be applied to our legal system then it can also be applied here.

From a human perspective

The issue around whether animals can feel pain is especially important given that within the last year the UK government declared in a vote that no non-human animal can feel pain. This is despite the extensive body of scientific evidence which suggests, beyond reasonable doubt, that all vertebrate animals are able to feel pain. Normally I try to keep politics out of these articles, but the complete lack of consensus between government policy and scientific evidence, compiled by researchers who have spent years studying animal pain, cannot be ignored.

One piece of legislation that may be now at risk is the Animals (Scientific Procedures) Act 1986 which protects all vertebrate animals and, after later amendments, cephalopod molluscs from unnecessary harm during scientific research. Under this Act any researcher, or researchers, that involve these animal groups have to submit an ethical assessment and then be granted the appropriate license.
Other countries have brought in similar legislation, although the invertebrate animals groups they protect can vary, especially for decapod crustaceans (crabs, lobsters, hermit crabs). For example UK, EU and Canadian legislation only covers cephalopod molluscs, whereas New Zealand extends this protection to crabs, lobsters and crayfish and Norway and Switzerland protect all decapods.

For fish and cephalopods the debate around whether they can feel pain is very similar to that around decapod crustaceans. For example some have argued that because fish show learned avoidance behaviours towards a source of damage, which is suppressed with morphine, they can experience pain. Yet others argue that, like in crabs, they lack the required brain function and level of consciousness to feel true pain.

So why is protection more consistent in fish and cephalopods compared to decapod crustaceans? One possible explanation is that because fish and cephalopods have brains and nervous systems that more closely resemble ours we instinctively think that they feel pain like we do. This is part of a particular way of thinking called anthropomorphism, where we assume non-human animals think, act and generally experience the world in the same way we do. In some cases this can be a positive thing, inspiring empathy for animals, but it can become less influential with animals that lack similarities to us.

Still, the difficulty we have in recognising pain in animals makes the whole issue very difficult to resolve to everyone’s satisfaction and so we have to resort to debatable assessments based on the evidence available. Considering the human perspective raises another issue, which is that our idea of pain does not necessarily apply to other animals, although we often assume it does.

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Dyuizen et al. 2012. Changes in the nitric oxide system in the shore crab Hemigrapsus sanguineus (Crustacea, Decapoda) CNS induced by a nociceptive stimulus

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All other images are public domain and do not require attribution

by Matthew Norton

Sperm whales are large toothed whales and are known for diving into the ocean depths to feed on squid, forming close social groups and featuring in the famous novel Moby Dick. However, its most distinctive characteristic is its enlarged forehead, which contains a reservoir of spermaceti, an oily mixture of fats and waxes, around an upper ‘spermaceti organ’ and lower ‘junk’ compartment. Various theories have been proposed to explain their function, especially the spermaceti organ. These include a role in controlling air movement within the whale’s body, for using their head as a ‘battering ram’ and to control their buoyancy at depth.

The most prevailing theory is that both the spermaceti organ and junk compartment are involved in echolocation, to explain how requires some context. Echolocation in toothed whales involves sending out sounds, ‘clicks’, which when they hit an object in the water causes the sound to be bounced back as an echo. These echoes cause vibrations in a fatty mass, in the case of sperm whales this is the junk compartment, which can then be interpreted to locate the object, which could be their prey.

For sperm whales there are added challenges that come with hunting by echolocation in the deep sea, such as the total darkness and the sparse distribution of life. Squid, the primary prey of sperm whales, also give off weak echoes which makes it difficult to get a precise location with the usual echo locating clicks. Sperm whales overcome this by using a different type of sound called ‘creaks’, a rapid series of clicks, to home in on their targets.

Producing the echolocating creaks is where the spermaceti organ comes in. Around the spermaceti organ there are air sacs which are believed to reflect sound waves as they are bounced around the organ. This splits the sound wave into multiple pulses with each pulse travelling a different path within the organ depending on how far away the air sac, from which the pulse is reflected, is from the source of the sound wave. Therefore each pulse is released into the water at different time intervals, separated by milliseconds, creating the series of sounds that make up the creaks. Meanwhile the junk compartment is primary receiver of the returning sound waves in the water.

The enlarged head may seem an extreme feature for the sperm whale to evolved, just so they can produce this extra type of sound. But as previously mentioned, the deep sea is an extreme environment for any species to try and make a living, and this is even more true for sperm whales who must make regular migrations to and from the ocean surface in search of both air and food.

From a human perspective

Sperm whales were prime targets for the whaling industry, especially in the 19th and mid-20th century. As in other targeted whale species, their blubber was a valuable commodity, but the real prize was the spermaceti oil, which was an effective lubricant and burnt bright in oil lamps.

Fortunately, whaling is nowhere near as widespread as it used to be thanks to the 1972 United States Endangered Species Act and 1987 International Whaling Commission moratorium. Some small scale sperm whale hunting continues in Japan and Indonesia, but global populations overall have been given a much needed chance to recover. In the 21st century they are doing just that with numbers estimated to be in the hundreds of thousands of sperm whales, compared to the pre-whaling figure of 1.1 million. Recovering is the key word as they are still classed as vulnerable by the IUCN and endangered under the United States Endangered Species Act. These figures and assessments are based on data by various research methods and it is interesting to see how such methods have evolved over the years.

Historic whaling practices may be responsible for the decimation of global population in the first place, but their records provides us with a useful baseline from which to work out how sperm whale populations have changed during and post-whaling. However, this whaling data is recorded as the rate at which sperm whales are caught, which is not influenced by just how many whales are in the water. For example technological advancements, which make whaling vessels more effective at catching their targets, can be a contributing factor that needs to be accounted for.

Whaling records can also give us a good idea of where to start looking for sperm whales using alternative research methods. One such method involves regular whale spotting trips along particular stretches of ocean, looking out for sperm whales as they surface and recording how many they see. Photo-identification techniques may also be used to keep track of individual whales, using unique marks and patterns on their tail. Over time these markings may change and risk a previously recorded whale from being confused for a new individual, but if the surveys are done often enough these changes may be noticed and accounted for.

Another common method is to use sound recording equipment to estimate how many sperm whales there are from their clicks, creaks and other vocal signals. The multi-pulse structure of their creaks also makes it easier to distinguish sperm whales from other species, and the different types of vocalisations they produce allow us to study their behaviour. There has been some suggestion that variations in the size of the spermaceti organ, produced by variation in head size, can help us to identify individuals from their creaks, but I think the method needs to be refined and tested.

The data from all these techniques give us only a glimpse into how sperm whale populations are recovering and all the figures that are reported are educated guesses based on this limited information. Nonetheless the evidence overall points to the recovery of sperm whale populations and it is the balance of evidence, while keeping in mind its limitations, is essentially how, although some will say this is an oversimplification, scientific study works.

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