Our world under the waves

by Matthew Norton

Few ocean predators have quite the reputation of the White shark, Carcharodon carcharias. They are accomplished hunters of a variety of prey, including tuna, sea turtles and other sharks, but we have been especially interested in how they pursue various seals, such as cape fur and elephant seals.

As with most active predators, they first need to find their prey and White sharks have numerous sensory tools at their disposal. In particular they have an especially acute sense of smell, able to detect 1 drop of blood in 1 million drops of water, and are able to detect the weak electrical fields produced by critical biological processes, such as beating hearts and moving muscles. However, in the vast oceans even the most effective sensory tools have their limits.

Fortunately for White sharks, seals live in colonies for part of the year, allowing them to maximise their chances in coming into contact with their prey by simply patrolling the waters around these colonies. These areas are so ideal that the larger White sharks will even drive away smaller sharks that try to access these hunting grounds.

Having found their prey, the next step is for them to approach the seal so that they can attack. This will not be easy as seals are agile in the water, and if given enough warning will outmaneuver their attacker. White sharks therefore, adopt a hunting strategy that minimises the chance of being detected by the seal, stalking them from below and mostly attacking at sunrise and sunset, when their approach is well hidden by the poorly lit water below.

Attack speed is also important for taking the seal by surprise, and as evidence by their ability to launch themselves out of the water they clearly have phenomenal swimming power. The torpedo-like shape of their body gives them the aerodynamic edge, but the real power comes from their internal physiology. The circulation system of White sharks, and some of their close relatives, is structured in such a way that it conserves their internal body heat, effectively making them warm blooded. This increased body temperature raises their metabolic rate, which gives them the energy bursts needed to attack seals at speed.

Clearly white sharks are an intimidating predators, but this does not necessarily make the seals they target helpless. Where possible, seals can swim along the seabed or among structurally complex habitats, such as kelp forests, where their agility gives them the advantage over the shark’s straight line speed. In open water they often rely on safety in numbers, the larger the group is, the lower the probability is for each seal that they will be the one targeted. On some occasions a group of seals will also make aggressive displays towards a shark to drive it away.

In the end white sharks and seals are stuck in an evolutionary arms race, as the hunting behaviour of white shark improves through natural selection, so does the ability of seals to avoid being eaten by the sharks, which is why the white shark is such an impressive predator today.

From a human perspective

White sharks, as well as many other sharks, are large, powerful animals with intimidating jaws and teeth, so the fear that some people feel towards them is understandable, but not really justified given the actual threat they pose to humans. Sharks cause far fewer fatalities than rare natural disasters, such as tornadoes and lightning strikes, or seemingly low risk activities, such as using a toaster. We are not even remotely desirable as a food source for sharks compared to their natural prey, due to our low fat content.

Yet, attacks do occur and many theories have been put forward to explain why. One of the oldest put forward is the ‘rogue shark’ theory, which suggested that attacks along any stretch of shoreline are perpetrated by a single shark that has moved away from its natural prey. The theory has been considered outdated for decades, but it was very influential for its time, having been loosely used in the film ‘Jaws’. More recently numerous other theories have been proposed, such as mistaken identity between human and a shark’s natural prey, sharks being inquisitive about an unusual animal in the water, and that a human victim unknowingly invaded a shark’s personal space, or territory. What most of these theories have in common is that sharks do not intentionally target humans and this is supported by both the low likelihood of attack and that in most attacks the shark only bites once and then swims away.

Yet the sharks’ undeserved reputation as mindless killers has lingered, which sadly is a key reason why shark finning is such a widespread practice that is driving sharks to extinction. For anyone who isn’t aware shark finning is the practice of catching sharks at sea, cutting off their fins while they are still alive, and then chucking them overboard to suffer slow and painful deaths. The market for these fins, for shark fin soup, is so great that an estimated 63-273 million sharks are killed each year. With such a large scale slaughter (few other words feel more appropriate) it is not surprising that 181 species, around ¼ of all sharks and rays, are at risk of extinction, being classified as vulnerable, endangered or critically endangered.

Fortunately, attitudes towards sharks are changing for the better, so there may be hope for them. Today there are numerous shark and ocean conservation charities and NGOs (non-government organisations) which are working to spread the word that sharks are worth protecting so that their population may recover. There is one such organisation that I want to acknowledge, Fin Fighters. I have seen their efforts first hand, having attended talks and their first Sharkfest, and although I was unable to attend their second Sharkfest I wish them luck in the future.

Sources

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Goldman. 1997. Regulation of body temperature in the white shark, Carcharodon carcharias

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Milman. 2015. ‘Sharks don’t like to eat people’: attack statistics contradict untested theories. https://www.theguardian.com/environment/2015/jul/20/sharks-dont-like-to-eat-people-attack-statistics-contradict-untested-theories. Last accessed 13/05/2018

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All other images are public domain and do not require attribution

by Matthew Norton

The ability to move through one’s environment is key the existence of virtually all life on earth. For any multicellular organism many of these movements need to be coordinated to benefit the organism. Most animals achieve this through a system of nerves, which transmit information as electrical signals, and a ‘central processor’ (e.g. a brain) to interpret this information. Some animals groups achieve coordinated movements with very simple nervous systems, but one group, the sponges (Porifera) has no ‘true’ nervous system at all.

Sponges, so called for the pores around their bodies, consist of two cell layers with a middle gelatinous layer which may contain ‘loose’ cells and pieces of carbonate, spongin (protein fibres), or silica (glass), depending on the class of sponge, for structural support. Water flows in through the pores, where specialised feeding cells, choanocytes, filter out particles of food, and then back out through a collective opening, the osculum, along with any waste. They lack any tissues or organs (e.g. mouth, muscles, heart), instead functioning through the individual actions of specialised cell types. These do not include nerve cells, yet many species appear to overcome this limitation and perform behaviours only achievable through cell to cell coordination.

Sponges can contract specific body parts, such as to close their osculum (see above image), and their entire bodies, dramatically shrinking their size in the process, in response to certain stimuli. These stimuli include electric and chemical stimulation, changes in light levels (i.e. changes from day to night and vice versa) and touch, although only the last two have been observed in the wild. Some species can also crawl along surfaces with the use of ‘crawling’ cells on their underside. This movement is slow, but still allows them to perform some remarkable feats, for example the carnivorous sponge, Asbestopluma hypogea, can surround and engulf their prey in a matter of hours.

How can sponges move like this without even a basic nervous system? Apparently by relying on other mechanisms for cells to communicate with each other, the use of chemical messages, such as hormones, remains a viable alternative for all sponge species. These are used in all animals (e. g. the control of blood sugar with the release of insulin), but the nervous systems are still used for quick and coordinated movements as chemical messages are too slow to take effect. However, in sponges their slow movement and simple body structure, with fewer cells to communicate between, could mean that fast acting electrical signals are not needed.

Glass sponges, which possess silica shards for structural support, may be an exception, possessing a primitive system for transmitting electrical signals between cells. Normally nerve cells rely on the movement of sodium (Na+), and to a lesser extent potassium (K+) ions to move the electrical signal through the nerve cell and specialised junctions, called syanpses, to transfer this signal to the next cell. In glass sponges each signal appears to be transmitted through direct channels between cells by changes in calcium ion (Ca2+) concentrations.

There has been a lot of interest in sponges from an evolutionary perspective as they are thought to closely resemble the common ancestor of all animal life, and so can provide unique insights into how certain features evolved. Nervous systems are no exception, with the coordinated behaviours and primitive signaling systems that facilitate them giving us an idea of the starting position from which ‘true’ nervous systems later evolved.

From a human perspective

Two groups of sponges (Hippospongia and Spongia species), commonly called ‘soft sponges’ have been used for centuries for various purposes, such as helmet padding, paint applicators and bathing sponges.

More recently there has been interest in many of the chemical compounds produced by sponges, and their associated microorganisms (bacteria, fungi etc). Sponges use these compounds to protect themselves from predators, fouling organisms and disease, but they have been effective in treating a number of human ailments. These include viral infections, such as HIV, certain cancers, especially leukaemia and breast cancer, and could also be a valuable source of antibiotics.

Unfortunately extracting these valuable chemical compounds from the sponges has proven difficult. Harvesting wild populations is not feasible due to low population densities, which has been exacerbated by previous overfishing and disease outbreaks. Furthermore each individual sponge contains these chemicals is relatively low concentrations. Cultivating sponges in captivity would appear to be the most logical alternative, but this comes with its own issues to overcome. ‘Open’ cultivation systems, which involve keeping captive animals within their natural(ish) environment, are unsuitable due to variable environmental conditions. In ‘closed’ cultivation systems environmental conditions can be tightly controlled, yet there have been few occasions where sponges have been successfully cultivated in such condition.

A third option would be to acquire the chemical compounds without the sponges. In theory, once we know the molecular structure we could then recreate these compounds from their core components. Unfortunately, once again, this is easier said than done as the structure of many of these compounds are complex and thus difficult to recreate.

We have used sponges, as well as other marine organisms, for a variety of purposes, and we continue to find new ways to exploit them as our technological capabilities advance. I would however, ask that we remain cautious, it is a sad truth that we often learn how to exploit the environment far quicker than we how to do so sustainably.

Sources

Margulis and Chapman. 2010. Kingdoms and Domains. ISBN. 978-0-12-373621-5

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Leys and Meech. 2006. Physiology of coordination in sponges

Weissenfels. 1990. Condensation rhythm of fresh-water sponges (Spongillidae, Porifera).

Vacelet and Duport. 2004. Prey capture and digestion in the carnivorous sponge Asbestopluma hypogea (Porifera: Demospongiae)

Nickel. 2010. Evolutionary emergence of synaptic nervous systems: what can we learn from the non‐synaptic, nerveless Porifera?

Leys et al. 1999. Impulse conduction in a sponge

Elliott and Leys. 2010. Evidence for glutamate, GABA and NO in coordinating behaviour in the sponge, Ephydatia muelleri (Demospongiae, Spongillidae)

Müller. 2003. The origin of metazoan complexity: Porifera as integrated animals

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Duckworth. 2009. Farming sponges to supply bioactive metabolites and bath sponges: A review

Chelossi et al. 2004. Characterisation and antimicrobial activity of epibiotic bacteria from Petrosia ficiformis (Porifera, Demospongiae)

Thakur and Müller. 2004. Biotechnological potential of marine sponges

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All other images are public domain and do not require attribution

by Matthew Norton

The ultimate goal of every living organism is to pass on their genes to the next generation through reproduction, the individual itself is only a temporary and short lived holder of these genes. Sexual reproduction within a species (usually) requires the combination of genes from a male and female, which produce sperm and egg cells respectively. However, in some species each individual can be both male and female, a phenomenon called hermaphroditism, of which there are two types. In simultaneous hermaphroditism the individual is both genders at the same time, whereas in sequential hermaphroditism the individual shifts from one gender to the other at a key point in their life.

The latter occurs in clownfish, a subfamily of fish (Amphiprioninae) containing around 30 fish species which live among anemones, protecting by the stinging tentacles for which they have developed a tolerance. Within each occupied anemone there is one breeding pair, the female being the larger, and a series of gradually smaller juvenile (sub-adult) males. When the female is lost the breeding male is gradually reformed into the new female.

Within the first 24 hours the male, due to changes in brain activity, changes its behaviour to better suit its new role, becoming increasingly aggressive to juvenile males and potential intruders. Later on, further changes in brain activity triggers the change in their reproductive organs from testes to ovaries, which produce sperm and egg cells respectively. Even in fully male clownfish there is a small portion of immature ovarian tissue within their testes, containing the bare essentials to make the sex change possible. During the transition this ovarian tissue grows and develops as the testicular tissue is reabsorbed. Meanwhile each juvenile male moves up in the pecking order with the largest juvenile male becoming the new breeding male.

It is remarkable that clownfish can make this transition so quickly and smoothly, but you may be wondering why they undergo such an extreme change instead of sticking to one gender for their entire lives. Some have suggested the energy requirements for sperm and egg cell production may be involved, egg cells are far larger, and more expensive to produce, than sperm cells. As a result only egg cell production is limited by body size, which directly controls the energy reserves they can invest in reproduction.

To maximise the number of offspring they produce it makes sense that they are male when they are young and small, when they don’t have a lot of energy reserves, and then female when they are older, larger and thus have the energy to invest in egg cells. There is also the lifestyle of clownfish to consider. Due to their reliance on anemones for protection they need to avoid straying too far, or they’ll be easy prey. Therefore among the surviving male clownfish the breeding female can be easily replaced without having to move to another anemone.

From a human perspective

Clownfish have become a famous group of fish thanks to the popular animated film “Finding Nemo”. Unfortunately, despite the positive conservation message, there was a surge in the aquarium trade of clownfish for years after the film’s release with many wanting their own Nemo. As a result millions are taken from the wild which is causing major population declines and, in some areas of Southeast Asia, local extinctions. Worse still, some of the fishing methods are very destructive, causing considerable damage to the surrounding coral reefs. For example the use of cyanide as a fish anaesthetic, while not as destructive as blast fishing, can cause corals to expel their symbiotic algae, a phenomenon called coral bleaching, on which they rely for food.

So what can be done to stop this destructive trend from continuing? The most commonly discussed solution is for most of the clownfish demand to be satisfied from aquaculture, fish bred in captivity, relieving the fishing pressure on wild populations. This approach can also benefit those involved in the aquarium trade, ensuring a consistent supply of clownfish and the opportunity to create new ‘designer’ varieties.

There are however, some issues that need to be addressed. A common issue with aquaculture is how to feed the fish, catching other marine organisms, especially other fish species, would defeat the purpose of breeding fish in captivity in the first place fortunately, the clownfish diet mainly consists of invertebrate larvae, which makes using cultured insects as a food source is a viable alternative.

Also, while their methods are destructive, the trade in wild caught clownfish does provide a source of income for fishermen in developing countries, which aquaculture could take away. One possible solution is to introduce captive bred clownfish into the wild, safeguarding both wild populations and fishermen’s incomes. However, for this to be effective fishermen need to be persuaded to use less destructive methods and captive bred clownfish need to be in contact with their particular anemone species, otherwise they won’t develop their tolerance to the stinging tentacles.

The most important aspect is that we as the potential consumers buy clownfish from a sustainable source. On a more general point we need to be careful that popular media (e.g. films, books, TV) about charismatic marine creatures does not cause us take actions that, while with good intentions, ultimately cause more harm than good. The impact that we have had on clownfish after the release of Finding Nemo is a sobering example, but I am hopeful that we can learn from our mistakes so that history does not repeat itself in the years following the sequel “Finding Dory”.

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Godwin. 1994. Behavioural aspects of protandrous sex change in the anemonefish, Amphiprion melanopus, and endocrine correlates

Warner. 1975. The adaptive significance of sequential hermaphroditism in animals

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All other images are public domain and do not require attribution

by Matthew Norton

Sea lettuce is a collective name used for green seaweeds of the genus Ulva, although it is more commonly associated with the ‘type’ species Ulva lactuca and Ulva linza, due to the their flat bodies that resemble a lettuce. Normally we see them as fully formed seaweeds, washed up on the shore or attached to the seabed, but to get to this point they need to settle on the seabed and then properly develop and grow. However, they heavily rely on the influence of marine bacteria, which may come as a surprise as bacteria are often thought of as organisms that cause disease, but in fact they are vital to the existence of all organisms larger than themselves.

As floating spores, the seaweed equivalent of plant seeds (sort of), Ulva seaweeds need some sort of signal to guide them to a suitable surface to settle on. Fortunately the bacteria found in microbial biofilms, which cover virtually all submerged surfaces, release chemical signals which draws these spores to the biofilm and encourages them to germinate. Once they settle they begin the development process which ultimately (all being well) produces a fully formed seaweed.

Key to this process is their ability to produce cells that are specialised for specific functions, a process called cell differentiation. A number of experiments suggest that Ulva seaweeds cannot achieve this without the influence of some bacteria, with Cytopghaga and Flavobacterium species being especially important [6]. This could not be more clearly than in Ulva seaweeds grown in their natural environment, or in cultures where suitable bacteria are present, compared to cultures where bacteria are excluded altogether, which reduces the seaweed to disorganised clumps.

As they develop, and once fully developed, the Ulva seaweeds can themselves become an attractive surface for other organisms to settle on, which could put some serious strain on them if this influx is not kept in check. Fortunately some bacteria, particularly Pseudoalteromonas, Phaeobacter and Alteromonas species, have demonstrated their antifouling ability, repelling algal spores, invertebrate larvae, fungi and other bacteria. However, they may not be 100% effective against other bacteria as the threat of pathogenic (disease causing) bacteria appears sufficient for the Ulva seaweed to invest in their own antimicrobial measures. Also, the inhibition of algal spore settlement by antifouling bacteria can be problematic for Ulva seaweeds if these bacteria are present in the biofilm.

All the above mentioned bacteria are clearly influential in the lives of these seaweeds, and we should take notice given the influence that seaweeds have on other marine organisms as a source of food, oxygen (due to photosynthesis) and shelter.

From a human perspective

Sea lettuce is eaten as seafood, raw and cooked in salads and soups respectively, in a number of countries, including Great Britain, Scandinavia, China and Japan, and is high in proteins, minerals and vitamins.

However, if not properly stored this seaweed can become a health hazard, releasing hydrogen sulphide (H2S) as it decomposes. At low concentrations this gas is just unpleasant, producing a vile ‘eggy’ smell, but larger concentrations can cause health problems such as nausea, fatigue and respiratory problems.

An infamous example is the accumulation of Ulva seaweeds on the coast of Brittany in the summer of 2009. Residents became alarmed when a horse rider was left unconscious after being exposed to the H2S fumes, sadly the horse was not so lucky, but efforts to remove the seaweed lead to the death of a lorry driver, who passed out at the wheel while transporting a load of decaying seaweed. A jogger was also believed to have died from exposure to seaweed fumes in 2017, again in Brittany.

There is also a risk that the sea lettuce we eat could be contaminated from the pollutants we release into the sea, in particular a number of studies have demonstrated their capacity to ‘soak up’ toxic heavy metals. This issue is not unique to seaweeds, with every source of seafood there is a risk of contamination, although seaweeds may be relatively ‘low risk’ due to their position at the bottom of food chains, which eliminates the risk of pollutants bioaccumulating through the food chain to us.

We could also use Ulva seaweeds to filter out pollutants from closed off water sources before being released into the environment. One study looked into using Ulva lactuca to filter out nitrogen based compounds, such as ammonia, from intensive fish farms, but this could be extended to farmlands, where the nitrates used in fertilisers can wash off into adjacent streams. This could help solve a major problem as waters saturated in nitrogen (and phosphorous) can cause microalgae populations to explode into blooms at the surface, suffocating all life below it as dead algae sink and decompose.

However, Ulva seaweeds also have the potential to bloom, as demonstrated by the outbreaks off the Brittany coast, and cause damage the environment and human health as a result.

By whatever means we exploit Ulva seaweeds there are risks involved, but they are risks we can manage. With appropriate monitoring we can ensure we only eat seaweed from locations that are (relatively) clean and by containing the seaweed we use to control pollution we minimise the risk of it getting out into environment. In the end it really comes down to common sense.

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All other images are public domain and do not require attribution

by Matthew Norton

Sea urchins belong to the phylum Echinodermata, which includes starfish and sea cucumbers, and have interested scientists and natural historians for centuries. This is best demonstrated by the structure of their mouths, which is still described as an ‘Aristotle’s lantern’ after the ancient Greek philosopher, and natural historian. Recently it has been debated whether Aristotle meant that the mouth itself resembles a lantern, or that the mouth’s position makes the whole internal body resembles a lantern.

Sea urchin lanterns consists of five symmetrically arranged ‘teeth’ around a mouth opening, demonstrating the definitive fivefold symmetry of all echinoderms, on the underside of the sea urchin body. Despite its apparent simplicity, the sea urchin’s lantern is a powerful and multipurpose tool used to exploit various prey, such as algae, mussels and sea cucumbers, and participate in various other activities such as burrowing into sand and boring into rock.

The teeth themselves have a self-sharpening mechanism built into its microstructure to prevent them from getting dull from intense use. The boundaries between the bony plates of each tooth act as fault lines so that when the outermost plates are chipped away the newly exposed plates remain in the same sharp arrangement. However, the continuous growth at the base of the teeth to replace the lost bony plates requires a high material and energy investment. This can be problematic when food is scarce, on the one hand they should be more frugal with the resources they use up, but on the other hand if they don’t properly maintain their lanterns they would struggle even more to feed themselves. The solution that many species seemed to have arrived at is keeping up the maintenance of their lanterns at the expense of other structures, such as their spines and gonads (reproductive organs), and their overall body size.

Their feeding activities can have a considerable impact on their surroundings, especially when they feed on, or damage habitat forming species. In particular I would like to draw attention to how seaweed eating urchin species, such as Strongylocentrotus droebachiensis, can maintain ‘urchin barrens’, now dominated by small coral like algae, in kelp forests. They don’t create these barren areas, normally they would feed on ‘drift algae’ rather than living kelp, but once they form the heavy urchin grazing on the seafloor makes it especially hard for kelp seedlings to settle in without being quickly consumed.

In other circumstances sea urchins can protect habitat forming species. For example many coral reefs would be overgrown by the faster growing seaweeds if the latter are not kept under control by grazing sea urchins. Admittedly sea urchins can also damage reefs, but the shift from coral to seaweed dominated reefs after mass mortalities of urchins suggests their influence, at least in some species (e.g. Diadema anrillarum) , is beneficial.

In either case it is clear that, in sufficient numbers, sea urchins have the potential to influence the structure of underwater habitats.

From a human perspective

Sea urchins, especially their gonads (otherwise called roe), are popular seafood in many regions, including Chile, Japan, Pacific North America and Mediterranean, and are sometimes kept in aquarium tanks to keep them clean from algae build up. As with effectively everything we take from the sea there is a risk of overfishing, a risk that has already been realised in many cases of local urchin fish stocks declining, or collapsing. For example in 1998 Chile dominated the urchin fishery, taking half the worldwide catch, but by 2002 a series of major depletions had occurred.

However, the Chilean fishery still held its position among the worldwide sea urchin fisheries, which suggests the other countries were not doing much better.

Along with the danger to the urchins themselves and the loss of income to the fishermen, the loss of the sea urchins’ influence on the surrounding habitat can cause ‘unnatural’ shifts in its structure. For example due to their previously mentioned suppression of seaweed growth on coral reefs, removing the vast majority of sea urchins could put them in danger of being overrun. On the flip side major increases in urchin populations, due to declines in their predators, which normally keep them at manageable levels, can be just as damaging to kelp forests and other seaweed based habitats.

Many of these predators are also in danger from human activities, for example the sea otter (Enhydra lutris) is currently classed as endangered, due to historic hunting and vulnerability to recent impacts (eg. oil spills and disease), and the American lobster (Homarus americanus), while not endangered, is under increasing fishing pressure.

All of this can be avoided by keeping sea urchin fishing sustainable. However, with many fisheries this is not easy as the short term effects on fishermen’s’ livelihoods can inspire some resistance to the regulations aimed at keeping fishing sustainable. It doesn’t help when the issues around fishery regulation are exploited for political gain. A recent and sobering example is the use of EU fishing regulations to demonise the EU during the UK Brexit referendum.

I want to make it clear I am not a fishing industry expert, nor am I attacking the men and women they employ, any concern they have is understandable, and there can be cases where regulations are poorly handled. However, without proper management it is the fisheries themselves that have the most to lose in the long term.

Sources 

Wikipedia. 2018. Sea Urchin. https://en.wikipedia.org/wiki/Sea_urchin. Last accessed 05/02/2018

Carnevali et al. 1993. The Aristotle’s lantern of the sea-urchin Stylocidaris affinis (Echinoida, Cidaridae): functional morphology of the musculo-skeletal system

Killian et al. 2011. Self‐Sharpening Mechanism of the Sea Urchin Tooth

Ma et al. 2009. The grinding tip of the sea urchin tooth exhibits exquisite control over calcite crystal orientation and Mg distribution

Voultsiadou and Chinntiroglou. 2008. Aristotle’s lantern in echinoderms: an ancient riddle

Ebert. 1980. Relative growth of sea urchin jaws: an example of plastic resource allocation

Levitan. 1991. Skeletal changes in the test and jaws of the sea urchinDiadema antillarum in response to food limitation

Black et al. 1984. The functional significance of the relative size of Aristotle’s lantern in the sea urchin Echinometramathaei (de Blainville)

Heflin et al. 2012. Effect of diet quality on nutrient allocation to the test and Aristotle’s lantern in the sea urchin Lytechinus variegatus (Lamarck, 1816)

Harrold and Reed. 1985. Food availability, sea urchin grazing, and kelp forest community structure

Scheibling et al. 1999. Destructive grazing, epiphytism, and disease: the dynamics of sea urchin-kelp interactions in Nova Scotia

Hughes et al. 1987. Herbivory on coral reefs: community structure following mass mortalities of sea urchins

Bak. 1994. Sea urchin bioerosion on coral reefs: place in the carbonate budget and relevant variables

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Wahle et al. 2011. Homarus americanus. http://www.iucnredlist.org/details/170009/0. Last accessed 05/02/2018

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Carpenter. 2016. The EU Common Fisheries Policy has helped, not harmed, UK fisheries. https://www.opendemocracy.net/can-europe-make-it/griffin-carpenter/eu-common-fisheries-policy-has-helped-not-harmed-uk-fisheries-0. Last accessed 05/02/2018

Booker. 2014. The Telegraph. No end to the EU’s crazy fishing policy. http://www.telegraph.co.uk/comment/11305123/No-end-to-the-EUs-crazy-fishing-policy.html. Last accessed 05/02/2018

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pelican from Tokyo, Japan. 2013. [CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0)]. https://commons.wikimedia.org/wiki/File:Sea_urchin_and_fish_cake_(8706153434).jpg

All other images are public domain and do not require attribution

by Matthew Norton

For anyone who has seen the northern or southern elephant seal, Mirounga angustirostris and Mirounga leonine respectively, gather on beaches during the breeding season may have noticed that there are many females accompanied by a single male, or at least a very small group of males, which can outsize the females several times over. This is a classic example of a polygamous mating system, where the male attempt to father as many offspring as possible by dominating a harem of females and excluding the smaller males. Every male wants dominance over this harem, which causes contests between the dominant male and potential challengers. These contests can turn in violent and bloody fights.

The most famous of these contests is the violent and bloody fights which ends in one male ending up dead or seriously injured (https://www.youtube.com/watch?v=bpb7Oks5kWI), but in most cases these disputes over access to females are settled without bloodshed. For the loser it makes no sense to risk death, or serious injury, in a fight they cannot win, especially as they can still force an occasional mating with a female by sneaking around the harem. While preferable to trying to usurp the dominant male, it is still a risky strategy as the resistance from the female, who themselves may be injured or lose pups, may attract the attention of the dominant male. For the winner, even if they could easily crush their opponent, they would be better off saving as much time and energy for future contests.

For both males, deciding how to settle their dispute depends on how clear the difference in size and fighting ability between them is. If one male is clearly smaller he may simply back down and his larger opponent may not even need to assert his dominance. However, if the difference is less conspicuous then each male may need more information about their opponent’s size and fighting ability relative to their own before they can determine if they are the weaker opponent and that they should back down. They do this by making visual threats and roars, which indicates their age and size, at each other, if that fails then a bloody fight to the death, or serious injury, will make it abundantly clear who is the stronger male.

However, avoiding the bloody fights is only a viable option if they still have future mating opportunities available to them. From an evolutionary perspective death is just as bad as living through their last breeding season without fathering any offspring. For any male that is large and strong enough to hold a harem there is a good chance that they are close to the end of their lives and so they cannot afford to back down from an opponent.

From a human perspective

Humanity has had a long, and bloody, history with elephant seals with both northern and southern species being hunted to near extinction for the oil in their blubber. Fortunately since the 20th century sealing activities have become heavily regulated and both species are now protected under various pieces of legislation and their current conservation status is least concern. However, they are still at risk from other human impacts, such as climate change and overfishing of prey species, which have been attributed to some population declines in the southern elephant since the 1950s.

Both elephant seal species are also less well equipped to withstand these impacts as they would have been in pre-sealing years. While their numbers have recovered, they are descended from the few individuals that survived the intense sealing period, with a considerable loss in genetic diversity as a result. Genetic diversity refers to the number of alternative versions, alleles, of each gene in the population, which can generate variations in features, from body weight to body chemistry and behaviour, between individuals.

Natural selection works by altering the abundance of these alleles in the population in response to environmental conditions, with the alleles most suitable for these conditions increasing in abundance. However, under low genetic diversity it is unlikely that a meaningful number of individuals will possess the alleles suitable to respond to a change in environmental conditions.

All around the world we are making great strides in protecting endangered species and helping them come back from the brink of extinction, for example in the last two years the conservation status of the snow leopard and giant panda has been changed from endangered to vulnerable. However, if such species are to recover and thrive in the long term there needs to be a substantial recovery in their genetic diversity, but this can take far longer than a recovery in population size. Therefore extensive protection would be needed for a long time after they are no longer classed as endangered, especially with all the threats that they face because of human activities, such as invasive species, plastic pollution and climate change.

Sources

Wikipedia. 2020. Elephant seal. https://en.wikipedia.org/wiki/Elephant_seal. Last accessed 16/01/2018

Marine Bio. 2017. Northern Elephant Seals. 2017. http://marinebio.org/species.asp?id=295. Last accessed 16/01/2018

Marine Bio. 2017. Southern Elephant Seals. 2017. http://marinebio.org/species.asp?id=296. Last accessed 16/01/2018

Cox. 1981. Agonistic encounters among male elephant seals: frequency, context, and the role of female preference

Clutton-Brock et al. 1979. The logical stag: adaptive aspects of fighting in red deer (Cervus elaphus L.)

Haley et al. 1994.Size, dominance and copulatory success in male northern elephant seals, Mirounga angustirostris

LeBoeuf and Mesnick. 1990. Sexual behavior of male northern elephant seals: I. Lethal injuries to adult females

LeBoeuf. 1974. Male-male competition and reproductive success in elephant seals

Sanvito et al. 2007. Vocal signalling of male southern elephant seals is honest but imprecise

Shipley et al. 1981. Individual differences in threat calls of northern elephant seal bulls

Enquist and Leimar. 1990. The evolution of fatal fighting

Hückstädt. 2015. The IUCN Red List of Threatened Species 2015. http://www.iucnredlist.org/details/13581/0-. Last accessed 16/01/2018

Hofmeyr. 2015. The IUCN Red List of Threatened Species 2015. http://www.iucnredlist.org/details/13583/0. Last accessed 16/01/2018

McMahon et al. 2005. Population status, trends and a re‐examination of the hypotheses explaining the recent declines of the southern elephant seal Mirounga leonina

Pistorius et al. 1999. Survivorship of a declining population of southern elephant seals, Mirounga leonina, in relation to age, sex and cohort

Weber et al. 2000. An empirical genetic assessment of the severity of the northern elephant seal population bottleneck

BBC News. 2017. Snow leopard no longer ‘endangered’. www.bbc.co.uk/news/world-asia-41270646. Last accessed 16/01/2018

World Wildlife Fund. 2016. Giant panda no longer Endangered. https://www.worldwildlife.org/stories/giant-panda-no-longer-endangered. Last accessed 16/01/2018

Daves. 2018. Southern elephant seal. https://travelwild.com/resources/antarctica-wildlife/southern-elephant-seal/. Last accessed 16/01/2018 

Hofman. 2013. Would you get this close to an elephant seal? Don’t worry he’s safe – just taking a rest after mating with THIRTY females. http://www.dailymail.co.uk/news/article-2289006/Would-close-elephant-seal-Dont-worry-hes-safe–just-taking-rest-mating-THIRTY-females.html. Last accessed 16/01/2018

Kernaleguen. 2013. How Elephant Seals Know Who’s Boss. https://www.livescience.com/41751-seals-use-unique-calls.html. Last accessed 16/01/2018

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All other images are public domain and do not require attribution

by Matthew Norton

Christmas Island is home to a strange group of crabs that live in the island’s rainforests, completely out of water, for most of their lives. The red land crab, Gecarcoidea natalis, is the most well-known, and abundant species, with their mass movements through human settlements being captured on video (https://www.youtube.com/watch?v=gR02_MFpOYY).

These are unusual surroundings for what is usually an aquatic animal and so they have unique difficulties to overcome. For example crabs would normally struggle to breathe in air as the individual sections of their gills, called lamellae, are very thin and kept close together to maximise the surface space over which dissolved oxygen and carbon dioxide can be exchanged with the surrounding water. This works in aquatic animals because the buoyancy of the water keeps the gills upright, but in air this delicate structure collapses and the lamellae fold in on each other, reducing the surface space for gas exchange. The Christmas Island land crabs thus have stiffer gills and thicker lamellae so that they remain upright in air, presumably the greater availability of oxygen in air compare to water compensates for any hindrance on gas exchange.

The gills also provide aquatic crabs with the means to exchange dissolved salts, which they need to maintain a favorable balance of in their bodies. Land crabs cannot get these salts from air and so they rely on the salts in their food and the redirection of their urine over their gills from which additional salts can be recovered. They balance the salts in their body by releasing, or suppressing the release of hormones to control the volume of urine they produce and the intensity of the salt recovery process over their gills.

Another unique problem for land crabs is that they need to keep their bodies from drying out. They manage this by simply retreating into their burrows when they are at risk, so much so that the humidity at the forest floor virtually controls how active these crabs are.

However, they must leave the safety of their burrow and make an arduous journey to the coast in order to breed, so that their young can develop and grow in seawater. Gecarcoidea natalis carefully time this migration with the start of the wet season, so that they don’t dry out on the way, and in sync with the lunar cycle so that they all get there at roughly the same time.

Still, walking such a distance is tiring for them, and with a high rate of oxygen consumption, even at their average walking speed, they often resort to anaerobic (non-oxygen) means of getting the energy they need. This in turn causes lactic acid to build up in their muscles, as it does in our muscles when we are aching from exercise, and so they have to take regular breaks so that this lactic acid can clear.

From a human perspective

The various species of land crab do not directly affect anyone living Christmas Island, but they are critical to the island’s rainforests. They make a substantial contribution to recycling nutrients into the soil by feeding on plant material in general, and by selectively feeding on the seeds of trees and vines they affect which species dominate the rainforest. This arrangement keeps the rainforest ecosystem healthy and stable, which in turn supports the charismatic wildlife that draws tourists to the island. Also the vast numbers of young land crabs, which develop in the sea, are a source of food for charismatic marine wildlife, such as whale sharks.

It is therefore unfortunate that these crabs are under siege from invasive species that humans have introduced into their environment, albeit accidently, which could then thrive at their expense. They can resist some of these invaders, for example most snails of the giant African species Anoplolepis gracilipes are killed by Gecarcoidea natalis, the exception being areas of rainforest that are too damaged to support large numbers of land crabs. Unfortunately, the yellow crazy ant (Anoplolepis gracilipes), is a far more aggressive invader, overpowering these land crabs with sheer numbers and killing them in their millions.

The loss of these crabs, and the direct consumption of plant material by the invasive ants, will have a knock on effect on the rainforests, altering which plant species dominate the invaded rainforest areas. The benefits and drawbacks of these changes to the rest of the plants and animals are debatable, but there is little doubt that it will be an extreme shift in the structure of the rainforest as the influence of the land crabs is weakened.

Sources

Farrelly and Greenaway. 1992. Morphology and ultrastructure of the gills of terrestrial crabs (Crustacea, Gecarcinidae and Grapsidae): adaptations for air-breathing

Morris. 2001. Neuroendocrine regulation of osmoregulation and the evolution of air-breathing in decapod crustaceans

Morris and Ahern. 2003. Regulation of urine reprocessing in the maintenance of sodium and water balance in the terrestrial Christmas Island red crab Gecarcoidea natalis investigated under field conditions

Green. 1997. Red crabs in rain forest on Christmas Island, Indian Ocean: activity patterns, density and biomass

Dowd and Lake. 1990. Red crabs in rain forest, Christmas Island: differential herbivory of seedlings

Dowd and Lake. 1991. Red crabs in rain forest, Christmas Island: removal and fate of fruits and seeds

Lake and Dowd. 1991. Red crabs in rain forest, Christmas Island: biotic resistance to invasion by an exotic snail

Lowe et al. 2000. https://s3.amazonaws.com/academia.edu.documents/33655728/100_world_worst_invasive_alien_species_English.pdf?AWSAccessKeyId=AKIAIWOWYYGZ2Y53UL3A&Expires=1513282478&Signature=zzccjlhxRfJ9vtk7TO09RTnMZVc%3D&response-content-disposition=inline%3B%20filename%3D00_OF_THE_WORLDS_WORST_INVASIVE_ALIEN_SP.pdf . Last accessed 25/12/2017

Abbott. 2005. Supercolonies of the invasive yellow crazy ant, Anoplolepis gracilipes, on an oceanic island: Forager activity patterns, density and biomass

Australian Government Parks Australia. 2017. Christmas Island National Park. https://parksaustralia.gov.au/christmas/. Last accessed 25/12/2017

Adamczewska and Morris. 2001. Ecology and Behavior of Gecarcoidea natalis, the Christmas Island Red Crab, During the Annual Breeding Migration

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Meekan et al. 2009. DNA evidence of whale sharks (Rhincodon typus) feeding on red crab (Gecarcoidea natalis) larvae at Christmas Island, Australia

Google Maps. 2017. https://www.google.co.uk/maps/place/6798,+Christmas+Island/@-13.8464888,110.6047478,1627255m/data=!3m1!1e3!4m5!3m4!1s0x2ef59a27e3c0a7cf:0x15e7d6090475ea16!8m2!3d-10.447525!4d105.690449?hl=en. Last accessed 25/12/2017

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Dowd et al. 2003. Invasional ‘meltdown’on an oceanic island

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All other images are public domain and do not require attribution

by Matthew Norton

Mantis shrimp are remarkable creatures for a number of reasons. They can see ultraviolet light, something that even human eyes cannot do, and for crustaceans they can be unusually social with their fellow mantis shrimps (within the same species). However it is the modification of their limbs into powerful clubs or sharp spears, depending on the species, which draws the most attention.

‘Puncher’ species use their club shaped limbs to strike, with extreme force and speed, the shells of prey that are slow moving, if not fixed in place. For example in the peacock mantis shrimp, Odontodactylus scyllarus, the impact force when the club meets it target ranges from 400 to 1500N at maximum speeds of 31-51 miles per hour. These strikes also generate vapor bubbles (cavitation bubbles) which, when they collapse, release energy in the form of shockwaves and sudden changes in temperature that are damaging to any solid surface. Even when the club fails to impact with its target the shockwave from the cavitation bubbles can be enough to stun, and even kill the prey of mantis shrimp.

‘Spearer’ species often lie in wait in their burrows and launch their spears to pierce the bodies of their prey, which are softer, but faster than the prey that puncher species typically attack. However these strikes appear slower, for example the maximum strike speeds of Lysiosquillina maculata and Alachosquilla vicina are 5.1 and 13 miles per hour respectively. This may simply be because these speeds may be sufficient to catch prey with their spears, whereas punchers may need the faster strikes to generate the force required to damage the shells of their prey.

These limbs function with such deadly speed and power from the build-up and rapid release of energy, from elastic strain. They achieve this by a latch system which only releases when the muscles and tendons are fully contracted and by a number of power amplification mechanisms. These include a specialised spring for storing additional elastic energy and a leverage system to minimise the loss of that energy from rotation at the limb joints. The limb itself could be thought of as merely as the system that integrates all these components.

However, given that there are so many species of mantis shrimp, possibly around 400 species, the mechanics of their modified limbs could vary greatly, with varying speed and power.

From a human perspective

The powerful strikes of puncher mantis shrimp, the larger species at least, has proven to be a problem for those who try to keep them in aquariums as there have been instances where they have inflicted damage onto aquarium glass. However, for these clubs to be effective weapons they also have to be able to withstand the power of their own strikes with little damage, which in itself is a remarkable feat.

The secret to how this is achieved is in the microstructure of the shell around these clubs which is designed to minimise the spread of cracks, which would cause it to fall apart. The spiral arrangement of mineral chitin fibres forces the cracks to constantly shift in different directions to progress through the microstructure. Also the helical arrangements are positioned perpendicular to the impact surface, minimising the surface area of each helix hit by the impact, and the chitin concentration is especially high concentration at the surface, both of which minimise the cracks that generated by the impact.

Given what this structure can withstand it is hardly surprising that it has been used to design materials used in sports equipment, lightweight body armour and airplane frames, all of which need to withstand impacts with considerable force. This is one of many examples of how materials built by marine animals can inspire new designs for synthetic materials that better fulfil our needs in a modern world.

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deVries et al. 2012. Strike mechanics of an ambush predator: the spearing mantis shrimp

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McHenry et al. 2012. Gearing for speed slows the predatory strike of a mantis shrimp

University of California. 2014. Mantis shrimp stronger than airplanes: Composite material inspired by shrimp stronger than standard used in airplane frames. https://www.sciencedaily.com/releases/2014/04/140422130944.htm?utm_medium=cpc&utm_campaign=ScienceDaily_TMD_1&utm_source=TMDage sources. Last accessed 06/12/2017

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https://video.nationalgeographic.com/video/worlds-deadliest/deadliest-mantis-shrimp. Last accessed 06/12/2017

Marshall and Oberwinkler. 1999. The colourful world of the mantis shrimp

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Bernard DUPONT from FRANCE. 2009. [CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0)]. https://commons.wikimedia.org/wiki/File:Peacock_Mantis-Shrimp_(Odontodactylus_scyllarus)_(6059032349).jpg

Daniel Yudi Miyahara Nakamura. 2019. [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]. https://commons.wikimedia.org/wiki/File:Mantis_shrimp_muscle.png

All other images are public domain and do not require attribution

by Matthew Norton

The vampire squid, Vampyroteuthis infernalis, is the only living species within the order Vampyromorpha. Their Latin name translates as the “vampire squid from hell”, but in reality this small squid does not inspire the dread that the name would suggest, with the exception of the smaller planktonic animals they feed on.

However, these are still curious animals. They are often regarded as living fossils among the cephalopods, meaning they closely resemble the ancestor species of squid, cuttlefish and octopuses, although they are a sister group to the octopuses. This is reflected in certain features which appear to combine the features of octopuses and squids and cuttlefish.

A clear example is their feeding apparatus. Like squids they have 10 arms in total, 2 of which have been modified into extendible filaments, and like octopuses there is webbing between their arms, which is believed to the remnant of their shell. These components work together to bring food to the mouth, the extendible filaments detect and capture food which is then passed to the arms, which wrap the food in mucus. Finally the food is carried to the mouth along the web. Alternatively this web could be used to engulf larger prey, similar to how octopuses catch their prey.

Other features which are unique, or at least more developed, in vampire squid are a consequence of the depth at which they live. At 600-1200m down there is a lack of light and oxygen, and so arises a need to compensate, and even make the most of these challenging conditions. They have the largest eyes, relative to body mass, of any animal, which no doubt allows them to see what little light is available. They can also produce light from bioluminescent organs at the tips of their arms and the release of bioluminescent particles to distract predators.

To conserve their limited oxygen supply, vampire squid have resorted to a number of extreme measures. Similar to other squid that visit these oxygen poor depths, such as the Jumbo squid, they suppress their metabolism and use a stronger oxygen binding protein, haemocyanin instead of haemoglobin. However, in vampire squid their metabolic rate is especially low and the affinity of haemocyanin to oxygen is especially strong. They further reduce their energy expenditure from swimming through the neutral buoyancy of their bodies and, in adults, their use of fin propulsion, instead of jet propulsion . Females also invest relatively little in the production of their eggs.

From a human perspective

Vampire squid do not provide us with any material gains, we do not fish them for food, nor do we harvest them for any other valuable substances. However this species has proven to be a source of artistic inspiration and features in an illustration, called “fight the vampire squid”, of the Occupy Wall Street protest movement against economic inequality. Specifically this illustration is targeted against individuals and corporations that participate in greed, corruption and undue influence on government.

However, this illustrations portrays the vampire squid in a negative light that is disproportional to its nature, and has little in common with those that the Occupy movement targets in their protests. True, they do prey on smaller and weaker marine creatures, but this essential to their survival. Sadly this is not the only case of a marine animal being portrayed negatively to define a certain persons, or persons. A common example is of the use of the term “loan shark” to portray disreputable money lenders that exploit their ‘customers’, which exacerbates the undeserved reputation that sharks get.

Still, it is a testament to the creativity that the marine environment, and its inhabitants, can inspire, even for those who are unaware of most of its diversity. It goes without saying that this is especially true in coastal cities and communities, but some of the oceans’ influence can to nationwide, if not worldwide scales.

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Robison et al. 2003. Light Production by the Arm Tips of the Deep-Sea Cephalopod Vampyroteuthis infernalis

Seibel et al. 1999. Vampire blood: respiratory physiology of the vampire squid (Cephalopoda: Vampyromorpha) in relation to the oxygen minimum layer

Rosa and Seibel. 2010.  Metabolic physiology of the Humboldt squid, Dosidicus gigas: implications for vertical migration in a pronounced oxygen minimum zone

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Wikipedia. 2020. Occupy Wall Street. https://en.wikipedia.org/wiki/Occupy_Wall_Street. Last accessed 15/11/2017

Mason. 2012. Does Occupy signal the death of contemporary art? http://www.bbc.com/news/magazine-17872666. Last accessed 15/11/2017

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Citron. 2010. [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/)]. https://commons.wikimedia.org/wiki/File:Vampire_des_abysses.jpg

Bruno Sanchez-Andrade Nuño from Washington, DC, USA. 2011. [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)]. https://commons.wikimedia.org/wiki/File:Occupy_Wall_Street_(6174072544).jpg

All other images are public domain and do not require attribution

by Matthew Norton

When we think of box jellyfish it is their deadly sting, easily fatal to humans if not treated quickly, that comes to mind. What is less well known is their remarkably complex visual system, for such a simple organism, with a grand total of 24 eyes arranged among four sensory clubs, called rhopalia.

Many organisms are able to detect light, even when they can’t actually ‘see’ the world around them, when it is absorbed by light sensitive pigments embedded. The chemical reaction that is triggered produces an electrical signal that tells the organism that light has been detected.

But within each rhopalium of a box jellyfish, there are distinct eye types that include a pair of eye pits and a pair of eye slits. Both of which contain light sensitive pigments, and an upper and lower camera lens eye. The simple structure of the pit and slit eyes suggest that their primary function is detecting and monitoring the level of light in their surroundings. However the discovery of a lens-like structure in the slit eyes may suggest that they may have some limited ability to interpret images of the world around them.

Both camera lens eyes are similar in structure to the eyes of many vertebrate animals (including humans). Theoretically we could say that box jellyfish can ‘see’ the world around them, although the ‘quality’ of their sight is likely to be very limited compared to our own, for two very good reasons. Firstly, the lens is too close to the retina, thus inhibiting their ability to focus images, and secondly, it is doubtful that the box jellyfish’s simple nervous system has the processing power to produce a coherent image.

Despite these limitations the complex eye arrangement is still put to good use, helping box jellyfish perform visually guided behaviours such as avoiding obstacles and altering swimming speed and direction in response to changes in light intensity. Each rhopalium helps to guide these behaviours by transmitting electrical signals, nicknamed “swim pacemaker signals”, which directly control swimming contractions.

Furthermore observations from some species suggest that box jellyfish could also see in colour. For example the camera lens eyes of Tripedalia cystophora are especially sensitive to blue-green light, and Chironex fleckeri has shown a curious ‘figure eight’ swimming pattern in response to blue light. In theory colour vision would also be very useful to box jellyfish in shallow water as glare from the water surface could confuse any creature that relies on changes in light intensity to see, whereas the contrast between different colours would be unaffected.

However, there are convincing counterarguments. In most species the camera lens eyes only possess one type of light sensitive pigment which, despite being sensitive to a particular colour of light, still leaves them colour blind. Also, the idea that it helps to overcome surface water glare is a very general theory on the evolution of colour vision and may be more applicable to other animals. Furthermore, the close proximity of the lens to the retina in the camera lens eyes has been speculated as a possible alternative to overcoming surface water glare.

From a human perspective

The arrangement and capabilities of the many eyes that box jellyfish possess is definitely fascinating, but there are at least two ways in which an understanding of how box jellyfish see the world may benefit us.

Firstly, existing measures to reduce contact with human swimmers (eg. protective clothing, beach warning signs) could be improved by exploiting the visually guided behaviours of box jellyfish. For example the use of dark coloured material in protective wetsuits may increase the likelihood of a box jellyfish perceiving the wearer as an obstacle, and actively avoid them.

Secondly the similarities between the human eye and the camera lens eyes of box jellyfish could, in theory, be used to develop treatments for malformations in the former. Some such malformations, such as displaced pupils, have been linked to mutations in a ‘master control gene’ in eye development, Pax6. PaxB, a gene which performs similar functions to Pax6, has been found in the camera lens eye of Tripedalia cystophora, which makes it a potentially useful ‘model species’ to test possible treatments. However, there are still some structural and developmental differences between human and box jellyfish eyes. Therefore Tripedalia cystophora should probably only be used in the early testing stages of such treatments.

Sources

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