The secret lifetime of starfish

In each sea, in each ocean,
Beasts of freakish locomotion
Prowl the substrate, looking for prey
To feast on in a monstrous method.

Ainternal is served. On a plate earlier than you, there’s a scrumptious roast hen. However, the fowl is bigger than your head and you haven’t any palms or tooth you possibly can break it up with, not to mention a knife and fork to make use of. How are you going to eat it? Are you going to push one half of your abdomen out via your mouth, smothering the hen in digestive juices to dissolve it, then haul your abdomen again into place, slurping up the nutritious broth as you go? No? Well you’re clearly not a starfish!

Members of the category Asteroidea, to offer them their correct identify, are among the many most acquainted of all sea creatures, the 5-fingered favourites of many a seaside publicity brochure. Yet, even a cursory investigation of their biology, ecology and evolutionary historical past reveals the familiarity to be a deception. These icons of the intertidal are about as unusual as life on Earth will get. If their feeding habits weren’t bizarre sufficient, asteroids have a skeleton product of crystals, possess extraordinary powers of regeneration and transfer round on a system of tiny hydraulic tentacles. And they don’t also have a mind.

What they do have is membership of an unique membership: the Echinodermata or ‘hedgehog-skins’. If you have got ever seen footage of crown-of-thorns sea stars chomping their approach throughout the Great Barrier Reef, you’ll understand how they get their identify. Echinoderms aren’t simply spiny, although. They additionally share a passion for the quantity 5: the 5 dwelling teams (starfish, sea urchins, sea lilies, brittle stars and sea cucumbers) all have 5-fold physique symmetry. Sea lilies can have scores of feathery arms, however all the time in multiples of 5 and, though sea cucumbers and a few sea urchins are, at first look, bilaterally symmetrical, the quintupled physique segments are nonetheless there if you realize the place to look. This pentametry is exclusive within the animal kingdom, although 5 has not all the time been the magic quantity. There have been no starfish, sea urchins or brittle stars within the Cambrian, however their family members have been plentiful and, whereas most appeared fairly acquainted, there have been some with solely three-fold symmetry and a few with out symmetry in any respect. However, for causes unclear, solely the pentametrists continued.

A third function that hyperlinks all echinoderms is the construction of their skeleton. In sea urchins, the weather (or ossicles – ‘little bones’), have fused to type a inflexible shell often known as a check. In sea cucumbers they’re typically decreased to microscopic specks in a gelatinous bag of sentimental tissue. However, all are shaped of the mineral calcite with a uniquely porous construction, referred to as stereom. Optically, every ossicle features like a single crystal, so some brittle stars – echinoderms with inflexible, disc-formed our bodies and writhing, snake-like arms – can actually see by way of their pores and skin, whereas there are sea urchins that may peer by means of their spines. The picture decision of every ossicle will not be of the very best high quality – biologist, Adam Summers, likens it to “wanting by means of a peephole coated with tissue paper” – however the compound impact a minimum of allows the echinoderm to differentiate harmful daylight from the duvet of darkness. In avoiding predation, each little bit helps, though it should make echinoderm optometry a sophisticated enterprise.

Not as optical as a brittle star’s, nor as versatile as a sea cucumber’s, an asteroid physique consists of myriad tiny ossicles held collectively by collagen and different smooth tissues. Therefore, the preservation potential of the typical lifeless starfish is about that of the cake neglected within the MacArthur Park rain (should you’ve by no means listened to Richard Harris’s epic tackle psychedelic pop, I recommend you treatment that oversight quickly). However, they do get fossilised. Indeed, because of the intermittent benevolence of the fossil report, we all know that asteroids have been shuffling concerning the oceans for simply shy of half a billion years.

For the primary 70-odd million of those, starfish have been small and 5-rayed, not often a lot bigger than a strawberry. However, round 425mya, within the Silurian interval, one thing modified. One of probably the most catastrophic mass extinctions in Earth historical past had occurred on the finish of the previous geological interval, the Ordovician, when an estimated 86% of marine species disappeared. However, it wasn’t the top of the world and the Silurian survivors had room to experiment. It appears starfish seized the chance. Long-armed asteroids appeared and actually brief-rayed varieties (extra biscuit than animal) excelled. However, most weird of all was the evolution of species that broke the 5 arm mould.

The first of those deviants was found in 1850 by the Manx naturalist, Edward Forbes, within the Much Wenlock Limestone Formation of the Welsh borderlands. With its extensive physique, 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 simply the arms’ look both, it was their quantity. Not a quintet, nor a sextet, or perhaps a septet. No, Lepidaster had thirteen. As writer-illustrator of the definitive British information, Forbes knew his dwelling starfish, however this fossil baffled him. At first, it reminded him of the widespread solar star, Crossaster papposus, people of which frequently have thirteen arms, however Forbes started to have his doubts. Was this palaeontological oddball even a starfish in any respect? Could it’s the lacking hyperlink between starfish and sea lilies, their many-armed cousins?

Sadly, Edward Forbes died earlier than he resolved the thriller and Gray’s scaly star drifted into obscurity. More then 60 years handed earlier than anybody correctly examined it once more. The scientist who lastly took up the problem was William Kingdon Spencer, a faculty inspector who claimed to make one of the best omelettes in England. Spencer was fascinated by fossil starfish and, on the cusp of the Great War, started a monograph that may take one other half-century to be revealed utterly. He’d be lifeless by then, too, however solely after he’d introduced primeval asteroids to life. Spencer was capable of show Lepidaster was a starfish, his detective work turning up some exquisitely preserved specimens unknown to Forbes. What he couldn’t work out was why the fossils all the time appeared to be discovered upside-down. Had Gray’s scaly star flipped itself over and brought to dwelling with its mouth pointed in the direction of the floor, its additional arms serving to to collect no matter morsels of meals drifted its means? It was an unprecedented idea, however then so was the starfish.

In the top, Spencer’s speculation gained no followers. Palaeontologists are pretty parsimonious and there was no compelling proof that Gray’s scaly star had not functioned like dwelling types, with arms for locomotion and its mouth towards the substrate. The broader puzzle remained, although, of why a lineage of starfish ended up with not one or two additional arms, however eight. Then a second triskaidekaphile turned up, in barely youthful rocks from Australia, and the waters have been muddied additional. Among Silurian starfish, thirteen was clearly fortunate for some, however how had they advanced?

Caption: Corn-on-the-cob arms and an explosive physique: Gray’s scaly star (Lepidaster grayi) from the Silurian of England. M is for madreporite, the asteroid’s personal water strain regulation system.

To decode this enigma, we have to take a look at trendy our bodies. Bilaterally symmetrical people buying a 3rd copy of the whole lot can be fairly freaky – An additional eye in your brow, sir? A third arm protruding out of your rib cage, madam? – however multiradiate starfish are widespread at the moment day. So, if the organic current is the important thing to the palaeontological previous, dwelling asteroids may help us unlock the mysteries of their historic ancestors.

Having many additional arms might sound to undermine the speculation that 5-fold physique symmetry is a defining attribute of dwelling echinoderms. Indeed, in his guide Climbing Mount Improbable, Richard Dawkins argued simply that. However, marine biologist and palaeontologist, Fred Hotchkiss, has demonstrated that even multiradiate mutants start life with 5 physique segments, including additional rays later in improvement. As a results of his work, it’s now usually accepted that dwelling echinoderms have 5-half physique subdivision programmed into them at a primary degree, even when it will get obscured subsequently. So, why do just some starfish go unusual?

It may be a corruption of their regenerative powers. Imagine you misplaced your arm in an accident and it grew again. You’d be fairly happy. But think about the way you’d really feel if not solely your arm reappeared, however your severed arm grew an entire new you. Suddenly you’d be a twin. I wouldn’t advocate making an attempt it, however many starfish can do precisely this – sacrificing a limb they’ll develop again later and, in some species, reproducing themselves from a forged-off. With tiny new rays budding from a nucleus, and a streaming one-armed ‘tail’, such asteroids are recognized colloquially as comets. Regeneration is one other echinoderm eccentricity, taken to the best excessive by the ocean cucumbers. When confronted with a possible predator, these garish cellular sausages can self-eviscerate, spewing out all their inner organs earlier than wriggling off to develop them again in security. It’s a great distance from being sure, however multiradiate mutants may simply be starfish that added additional arms with out having misplaced any within the first place. Switch a gene on at a unique level in improvement and 5 rays may grow to be six, seven, 13 and even fifty, the document set by the Antarctic solar star Labidiaster annulatus.

The subsequent query that arises is: does having additional arms assist? If the ratio of physique measurement to arm size is stored constant, however eight additional arms are added, the quantity (or biomass) of a multiradiate starfish is roughly double that of a ‘regular’ type. This means a marked improve within the power required to maintain the animal functioning. If you want extra meals, there are two methods you possibly can go about getting it. You can out-muscle your rivals for present meals sources or you can begin exploiting new ones. Many starfish with supernumerary rays appear to have taken the latter plan of action, from consuming coral, to consuming different echinoderms, even to scoffing fellow starfish. Perhaps Gray’s scaly star was the pioneer. With its mouth in the midst of a a lot bigger and extra versatile physique than that of its contemporaries, it will definitely have been able to doing one thing totally different.

My, grandma starfish, what lots of arms you’ve!
All the higher for grabbing you with!
Why grandma starfish, what a versatile mouth you’ve got!
All the higher for extruding my abdomen and smothering you with!

It gained’t come from the sky, however, in case you are a coastal fisherman, the specter of an asteroid impression could be very actual. Oyster (and mussel) slurping starfish are a nightmare for many business shellfish harvesters. The sucker-coated rays wrap around the shells of the oyster and exert an opposing pressure to tug them aside. Oysters can pull their shells collectively extremely tightly, however starfish suction wants solely to outlast the muscle energy of its opponent. Once the bivalve tires and its shells half ever-so-barely, the asteroid has gained. Its abdomen can squeeze by way of the sliver of a gap and liquefy the shellfish inside. It is claimed, maybe apocryphally, that fishermen used to unravel this drawback by dredging up the starfish, chopping them in half with a knife, after which chucking the our bodies again into the ocean. Of course, they didn’t know that this was merely serving to their opponents double their numbers. Therefore, Oystermen within the japanese US developed various methods to maintain their beds asteroid-free: wagon-a great deal of starfish have been carried off to be used as farm fertiliser or the pests have been mopped up from the seabed with particular cotton-tipped poles and boiled alive in on-board pots. Asteroids might have many talents, however they aren’t invulnerable.

Don’t write asteroids off as a mushy contact, although, definitely not in case you are a small sea creature. The floor of varied starfish is roofed by unpronounceable weapons referred to as pedicellariae. These snapping pincers are so independently minded that they have been thought by many early researchers to be separate organisms solely, fairly than a part of the asteroid. They resemble miniature tulips, however pedicellariae are definitely not flowery. The valves chew at something that touches them, deterring small organisms and parasites from establishing house or hitching a journey, typically with a touch of poison as additional discouragement. In one species – the Velcro star, Stylasterias forreri – this has advanced to the extent of lazy however very refined predation. Grabbing maintain of fish that mistake the asteroid for a moderately spiky rock, the pedicellariae skewer the stricken animal and, like rock followers shifting a crowd-surfer, cross it throughout the floor of the starfish and spherical to the jaws.

Since we’ve come full circle, re-think about earlier than you the dinner I started with. If you’re considering that a chook-consuming starfish is a bit preposterous, I give you this. In the early Nineteen Eighties, scientists in British Columbia have been learning the dietary preferences of the Pacific sunflower star, Pycnopodia helianthoides. With 24 arms and a physique as much as a metre broad, this can be a sea star supernova. Not solely is Pycnopodia huge, additionally it is quick and versatile, having virtually nothing in the best way of a skeleton to sluggish it down. This allows it to scuttle over the seabed at speeds of as much as ten ft a minute, chasing down crabs, hoovering up sand dollars and panicking molluscs into taking over swimming. Some escape, however many don’t – one sunflower star was discovered with one hundred ten people of the orb clam, Diplodonta, in its abdomen.

However, spectacular because it was, this gluttony quickly paled into comparability, for an additional Pycnopodia was caught within the act of consuming an alcid, a member of the chook household that features puffins and guillemots. The sunflower star strikes comparatively quick, however how might it have captured a nimble diving seabird? Had the starfish lain in wait till one got here inside attain, then reared up off the seabed, stretching its suckered arms auk-ward and grappling it into submission? Sadly not. It was apparently simply scavenging a carcass it had come throughout. Nonetheless, in the event you do end up on the Pacific coast, you may need to hold any small animals you personal away from the water. After virtually half-a-billion years buying an astonishing array of expertise, there isn’t any predicting what starfish may grasp subsequent.

♦ Buy Fossils, Crystals, Tools
♦ Subscribe to Deposits
♦ Join Fossil Hunts
♦ UK Fossil Locations

Filed underneath: fossils Tagged: Edward Forbes, Fossil, fossil starfish, fossils, Gotland, Lepidaster grayi, Much Wenlock Limestone, Naturhistoriska Riksmuseet, Starfish, Wenlock
Deposits Magazine