Secret life of starfish

Dr Liam Herringshaw (Canada)

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!

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.

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.

For the first 70-odd million of these, starfish were small and five-rayed, rarely much larger than a strawberry. However, around 425Ma, in the Silurian period, something changed. One of the most catastrophic mass extinctions in Earth history had occurred at the end of the preceding geological period, the Ordovician, when an estimated 86% of marine species disappeared. However, it wasn’t the end of the world and the Silurian survivors had room to experiment. It seems starfish seized the opportunity. Long-armed asteroids appeared and really short-rayed forms (more biscuit than animal) excelled. However, most bizarre of all was the evolution of species that broke the five arm mould.

The first of these deviants was discovered in 1850 by the Manx naturalist, Edward Forbes, in the Much Wenlock Limestone Formation of the Welsh borderlands.

With its wide body, an explosion of bony shards and its arms like corns-on-the-cob, Gray’s scaly star (Lepidaster grayi) was defiantly at odds with its contemporaries.

It wasn’t just the arms’ appearance either, it was their number. Not a quintet, nor a sextet, or even a septet. No, Lepidaster had 13. As author-illustrator of the definitive British guide, Forbes knew his living starfish, but this fossil baffled him. At first, it reminded him of the common sun star, Crossaster papposus, individuals of which often have 13 arms, but Forbes began to have his doubts.

Was this palaeontological oddball even a starfish at all? Could it be the missing link between starfish and sea lilies, their many-armed cousins?

Sadly, Edward Forbes died before he resolved the mystery and Gray’s scaly star drifted into obscurity. More then 60 years passed before anyone properly examined it again. The scientist who finally took up the challenge was William Kingdon Spencer, a school inspector who claimed to make the best omelettes in England. Spencer was fascinated by fossil starfish and, on the cusp of the Great War, began a monograph that would take another half-century to be published completely. He’d be dead by then, too, but only after he’d brought primeval asteroids to life.

Spencer was able to prove Lepidaster was a starfish, his detective work turning up some exquisitely preserved specimens unknown to Forbes. What he could not figure out was why the fossils always seemed to be found upside-down. Had Gray’s scaly star flipped itself over and taken to living with its mouth pointed towards the surface, its extra arms helping to gather whatever morsels of food drifted its way? It was an unprecedented concept, but then so was the starfish.

In the end, Spencer’s hypothesis gained no followers. Palaeontologists are fairly parsimonious and there was no compelling evidence that Gray’s scaly star had not functioned like living forms, with arms for locomotion and its mouth against the substrate. The broader puzzle remained, though, of why a lineage of starfish ended up with not one or two extra arms, but eight. Then a second triskaidekaphile turned up, in slightly younger rocks from Australia, and the waters were muddied further. Among Silurian starfish, 13 was clearly lucky for some, but how had they evolved?

To decode this enigma, we need to look at modern bodies. Bilaterally symmetrical humans acquiring a third copy of everything would be pretty freaky – An extra eye in your forehead, sir? A third arm protruding from your rib cage, madam? – but multiradiate starfish are common at the present day. So, if the biological present is the key to the palaeontological past, living asteroids can help us unlock the mysteries of their ancient ancestors.

Having many extra arms might seem to undermine the hypothesis that five-fold body symmetry is a defining characteristic of living echinoderms. Indeed, in his book Climbing Mount Improbable, Richard Dawkins argued just that. However, marine biologist and palaeontologist, Fred Hotchkiss, has demonstrated that even multiradiate mutants begin life with five body segments, adding extra rays later in development. As a result of his work, it is now generally accepted that living echinoderms have five-part body subdivision programmed into them at a basic level, even if it gets obscured subsequently. So, why do only some starfish go strange?

It might be a corruption of their regenerative powers. Imagine you lost your arm in an accident and it grew back. You’d be pretty pleased. But imagine how you’d feel if not only your arm reappeared, but your severed arm grew a whole new you. Suddenly you’d be a twin. I wouldn’t recommend trying it, but many starfish can do exactly this – sacrificing a limb they will grow back later and, in some species, reproducing themselves from a cast-off. With tiny new rays budding from a nucleus, and a streaming one-armed ‘tail’, such asteroids are known colloquially as comets. Regeneration is another echinoderm eccentricity, taken to the greatest extreme by the sea cucumbers.

When faced with a potential predator, these garish mobile sausages can self-eviscerate, spewing out all their internal organs before wriggling off to grow them back in safety. It’s a long way from being certain, but multiradiate mutants might just be starfish that added extra arms without having lost any in the first place. Switch a gene on at a different point in development and five rays might become six, seven, thirteen or even fifty, the record set by the Antarctic sun star Labidiaster annulatus.

The next question that arises is: does having extra arms help? If the ratio of body size to arm length is kept consistent, but eight extra arms are added, the volume (or biomass) of a multiradiate starfish is roughly double that of a ‘normal’ form. This means a marked increase in the energy required to keep the animal functioning. If you need more food, there are two ways you can go about getting it. You can out-muscle your competitors for existing food sources or you can start exploiting new ones. Many starfish with supernumerary rays seem to have taken the latter course of action, from consuming coral, to eating other echinoderms, even to scoffing fellow starfish. Perhaps Gray’s scaly star was the pioneer. With its mouth in the middle of a much larger and more flexible body than that of its contemporaries, it would certainly have been capable of doing something different.

My, grandma starfish, what a lot of arms you have!
All the better for grabbing you with!
Why grandma starfish, what a flexible mouth you have!
All the better for extruding my stomach and smothering you with!

It won’t come from the sky, but, if you are a coastal fisherman, the threat of an asteroid impact is very real. Oyster (and mussel) slurping starfish are a nightmare for most commercial shellfish harvesters. The sucker-covered rays wrap round the shells of the oyster and exert an opposing force to pull them apart. Oysters can pull their shells together incredibly tightly, but starfish suction needs only to outlast the muscle power of its opponent. Once the bivalve tires and its shells part ever-so-slightly, the asteroid has won.

Its stomach can squeeze through the sliver of an opening and liquefy the shellfish inside. It is said, perhaps apocryphally, that fishermen used to solve this problem by dredging up the starfish, chopping them in half with a knife, and then chucking the bodies back into the sea. Of course, they didn’t know that this was simply helping their opponents double their numbers. Therefore, Oystermen in the eastern US developed alternative strategies to keep their beds asteroid-free: wagon-loads of starfish were carried off for use as farm fertiliser or the pests were mopped up from the seabed with special cotton-tipped poles and boiled alive in on-board pots. Asteroids may have many abilities, but they aren’t invulnerable.

Don’t write asteroids off as a soft touch, though, certainly not if you are a small sea creature. The surface of various starfish is covered by unpronounceable weapons called pedicellariae. These snapping pincers are so independently minded that they were thought by many early researchers to be separate organisms entirely, rather than part of the asteroid. They resemble miniature tulips, but pedicellariae are certainly not flowery. The valves bite at anything that touches them, deterring small organisms and parasites from setting up home or hitching a ride, sometimes with a dash of poison as extra discouragement. In one species – the Velcro star, Stylasterias forreri – this has evolved to the level of lazy but very sophisticated predation. Grabbing hold of fish that mistake the asteroid for a rather spiky rock, the pedicellariae skewer the stricken animal and, like rock fans moving a crowd-surfer, pass it across the surface of the starfish and round to the jaws.

Since we’ve come full circle, re-imagine before you the dinner I began with. If you are thinking that a bird-eating starfish is a bit preposterous, I offer you this. In the early 1980s, scientists in British Columbia were studying the dietary preferences of the Pacific sunflower star, Pycnopodia helianthoides. With 24 arms and a body up to a metre wide, this is a sea star supernova. Not only is Pycnopodia big, it is also fast and flexible, having almost nothing in the way of a skeleton to slow it down. This enables it to scuttle over the seabed at speeds of up to ten feet a minute, chasing down crabs, hoovering up sand dollars and panicking molluscs into taking up swimming. Some escape, but many don’t – one sunflower star was found with 110 individuals of the orb clam, Diplodonta, in its stomach.

However, impressive as it was, this gluttony soon paled into comparison, for another Pycnopodia was caught in the act of consuming an alcid, a member of the bird family that includes puffins and guillemots. The sunflower star moves relatively fast, but how could it have captured a nimble diving seabird? Had the starfish lain in wait until one came within reach, then reared up off the seabed, stretching its suckered arms auk-ward and grappling it into submission? Sadly not. It was apparently just scavenging a carcass it had come across. Nonetheless, if you do find yourself on the Pacific coast, you might want to keep any small animals you own away from the water. After almost half-a-billion years acquiring an astonishing array of skills, there is no predicting what starfish might master next.