The secret life of starfish

In every sea, in every ocean,
Beasts of freakish locomotion
Prowl the substrate, seeking prey
To feast on in a monstrous way.

Dinner is served. On a plate before you, there is a delicious roast chicken. However, the bird is larger than your head and you have no hands or teeth you can break it up with, let alone a knife and fork to use. How are you going to eat it? Are you going to push one half of your stomach out through your mouth, smothering the chicken in digestive juices to dissolve it, then haul your stomach back into place, slurping up the nutritious broth as you go? No? Well you are obviously not a starfish!

edward_forbes
Edward Forbes. © Wikimedia Commons image.

Members of the class Asteroidea, to give them their proper name, are among the most familiar of all sea creatures, the five-fingered favourites of many a seaside publicity brochure. Yet, even a cursory investigation of their biology, ecology and evolutionary history reveals the familiarity to be a deception. These icons of the intertidal are about as strange as life on Earth gets. If their feeding habits weren’t weird enough, asteroids have a skeleton made of crystals, possess extraordinary powers of regeneration and move around on a system of tiny hydraulic tentacles. And they don’t even have a brain.

What they do have is membership of an exclusive club: the Echinodermata or ‘hedgehog-skins’. If you have ever seen footage of crown-of-thorns sea stars chomping their way across the Great Barrier Reef, you will know how they get their name. Echinoderms aren’t just spiny, though. They also share a fondness for the number five: the five living groups (starfish, sea urchins, sea lilies, brittle stars and sea cucumbers) all have five-fold body symmetry. Sea lilies can have scores of feathery arms, but always in multiples of five and, although sea cucumbers and some sea urchins are, at first glance, bilaterally symmetrical, the quintupled body segments are still there if you know where to look. This pentametry is unique in the animal kingdom, though five has not always been the magic number. There were no starfish, sea urchins or brittle stars in the Cambrian, but their relatives were plentiful and, while most looked pretty familiar, there were some with only three-fold symmetry and some without symmetry at all. However, for reasons unclear, only the pentametrists persisted.

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Corn-on-the-cob arms and an explosive body: Gray’s scaly star (Lepidaster grayi) from the Silurian of England. M is for madreporite, the asteroid’s own water pressure regulation device.

A third feature that links all echinoderms is the structure of their skeleton. In sea urchins, the elements (or ossicles – ‘little bones’), have fused to form a rigid shell known as a test. In sea cucumbers they are often reduced to microscopic specks in a gelatinous bag of soft tissue. However, all are formed of the mineral calcite with a uniquely porous structure, called stereom. Optically, each ossicle functions like a single crystal, so some brittle stars – echinoderms with rigid, disc-shaped bodies and writhing, snake-like arms – can literally see through their skin, while there are sea urchins that can peer through their spines. The image resolution of each ossicle may not be of the highest quality – biologist, Adam Summers, likens it to “looking through a peephole covered with tissue paper” – but the compound effect at least enables the echinoderm to distinguish dangerous daylight from the cover of darkness. In avoiding predation, every little bit helps, although it must make echinoderm optometry a complicated business.

Not as optical as a brittle star’s, nor as flexible as a sea cucumber’s, an asteroid body consists of myriad tiny ossicles held together by collagen and other soft tissues. Therefore, the preservation potential of the average dead starfish is about that of the cake left out in the MacArthur Park rain (if you’ve never listened to Richard Harris’s epic take on psychedelic pop, I suggest you remedy that oversight soon). However, they do get fossilised. Indeed, thanks to the intermittent benevolence of the fossil record, we know that asteroids have been shuffling about the oceans for just shy of half a billion years.


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