Spirals in Time: The Secret Life and Curious Afterlife of Seashells Read online

  Using his finely tuned sense of touch, Vermeij studies shells by turning them over and over in his hands, feeling their intricate shape and noticing details that other people miss. In his book A Natural History of Shells, he writes about how his hands have allowed him to explore the way shells from different places vary in appearance: the geography of shape.

  He describes how the shells he finds on tropical shores are radically different from those on Dutch beaches. For starters, they are much more carefully made. Individuals from the same species of tropical mollusc will make shells that are identical copies of each other. They stick closely to a set of hidden rules, imposed perhaps by the presence of so many predators and competitors. Slightly wonky shells just won’t cut it in the race for survival in these crowded, species-rich waters; they might not be strong enough, or well protected enough from attack. In cooler and deeper waters, where life in many ways is more relaxed and less extreme, molluscs can get away with being less finicky about their shells. On the whole, away from the tropics, molluscs are built relatively sloppily.

  Vermeij also writes in his book about another key moment in his life, when a big idea hit him. He spent the summer of 1970 in the western Pacific Ocean, on the island of Guam, on a field trip with his friend Lucius G. Eldredge. On one particular day they were searching for shells in the falling tide at Togcha Bay on the windy side of the island when Eldredge (known as Lu) handed Vermeij the shell of a Money Cowrie with its top sliced clean off. Lu made an offhand remark that he often saw crabs cutting open cowries in his aquarium tanks.

  Until then, Vermeij hadn’t paid much attention to the fact that he often found masses of broken shell pieces on tropical beaches and he suddenly got to thinking about predation. He realised that tropical seashells have a really hard time with so many predators trying their best to crack, smash, peel open and drill into them. He began to wonder how their shells have evolved to ward off these attacks, and soon realised there are many reasons why shape matters.

  An obvious way a mollusc can avoid getting eaten is by making a very big, thick shell, but that comes at the cost of having to make and then drag around a massive, heavy lump. A more economical way to make a shell more difficult to handle and swallow is to give it a covering of spines and bumps. Realising this, Vermeij finally understood why Mrs Colberg’s Floridian shells, and so many other tropical species, have fancy ornaments. In the crowded tropics, molluscs are doing their best to survive. As they grow, they can add embellishments to their shells; prongs can be added at regular intervals, or they can form a dense tangle like the quills of a porcupine. Spondylus, for example, the thorny oysters, are industrious spine-makers, expertly producing new ones and fixing any that have broken at a rate of a few millimetres every day.

  Vermeij also figured that the pleats and corrugations on many tropical shells are a cost-effective way of creating a strong body armour that’s difficult to break into while keeping the weight down. Thickening and flaring out the aperture of shells is another way of deterring predators, as in the Malaysian microsnails with their trumpet-shaped mouths.

  Shape can also help shells to hide. Sleekly shaped molluscs can slip silently through the water without sending out telltale ripples that predators detect; being more hydrodynamic also allows for a quicker getaway. We can surmise that parts of Raup’s imaginary museum may remain empty of real shells simply because they are not streamlined enough.

  For shells that live in sandy, muddy places, shape can mean the difference between resting on top and sinking in. Epifaunal species are ones that have adapted to a life of lying on the surface of the seabed; their shells are often wide and flat, acting like snow shoes. They include species like the Big Ear Radix, a gastropod that lives in lakes across Europe; throughout their lives they continually expand a winglike flap on their shells that prevents them from sinking into silty mud. Another strategy used by epifaunal species is what Vermeij describes as the ‘iceberg habit’. Instead of lying on the surface they allow themselves to sink in slightly so that most, but not all, of the shell is submerged. Scallops commonly have a curved lower shell that sticks a short way into the mud.

  Shape also matters for infaunal species, those that spend their lives burrowed down into mud and sand. Among the sea snails and bivalves there are champion diggers that use their feet as spades to bury themselves completely in under a second. Some have tiny ratchets on their shells to prevent them slipping backwards, and others have smooth whorls to make sure sand and mud don’t stick to them and increase the load.

  Burrowing shells face the additional problem of being unearthed. If you’ve ever stood barefoot in lapping waves on a sandy beach, you may have noticed the sand being scoured from around your toes. When waves and currents flow around a solid object they stir sand grains into suspension and whisk them off elsewhere. To overcome this, burrowing shells evolved spines and ribs that trap sand particles and stabilise the sediments around them. A group of typical diggers are tower shells, which look like little unicorn horns; their sculpted whorls help to hold them in place in their sandy, muddy homes and reduce the chances of being swept away.

  Back inside Raup’s imaginary museum of all shells, there is another perplexing detail that needs explaining: all the coiling shells twirl in the same direction. Suspended from their wires, the glass models have their tips pointing downwards and their apertures all open to the right. Or, seen from the top, they coil in a clockwise direction. Raup could easily have filled his museum with shells that twist the other way, or perhaps made two giant rooms that were mirror images of each other. But he didn’t, and for good reason.

  Take a look at any real, spiralling shell and see which way it turns. Go and find that seashell sitting on a bookcase, or pick up a snail from your garden or local park; your shell almost certainly coils to the right. There is a smattering of species that always coil to the left, and occasionally sinistral oddities will occur in a right-coiling species, but currently the natural world favours righties over lefties. More than nine out of ten coiled shells today are dextral (curiously, a similar proportion of people are right-handed).

  Shell collectors go crazy for rare sinistral specimens, so much so that over the years clandestine trades have prospered in fake lefties. Some are right-coiling shells that have undergone a bizarre molluscan version of plastic surgery, with some bits cut off and others glued back on; X-rays show their insides are in fact dextral. There are also true left-coiling shells that masquerade as something more special. Around the world, Hindus and Buddhists are summoned to prayer by the call of sacred conch-shell trumpets, known as shankh in Sanskrit. These are made from a large species of Indian Ocean gastropod, known in English as a chank shell, which normally coils to the right. Rare left-coiling specimens are highly revered, and are referred to variously as dakshinavarti shankh or sri lakshmi shankh. Their anticlockwise whorls are said to mirror the passage of the stars and sun across the heavens, and the curly hair and twisting bellybutton of the Buddha. Unscrupulous shell-traders make counterfeit sri lakshmi shankh shells from a different species, the Lightning Whelk, which lives in the Gulf of Mexico and normally coils to the left.

  A famous left-handed shell was drawn by Rembrandt. He portrayed a Marbled Cone Snail which, like most of the poisonous cone snails, naturally coils to the right. Art historians speculate that Rembrandt hadn’t made a mistake, as many early shell illustrators did. Failing to appreciate the significance of coiling direction, artists would commonly etch what they saw into metal plates; their shells would then become reversed as mirror images in the printing process. In Rembrandt’s case, though, it’s thought he reversed his shell on purpose, for aesthetic reasons: he just felt it looked better that way. Pleasingly, other artists who copied Rembrandt’s cone did so directly and faithfully, without thinking to reverse the etching, so these printed shells were restored to their rightful place as right-coilers.

  The abundance of right-coiling shells in the natural world, and lack of left-coilers, comes down to
one simple but inescapable truth: if right- and left-coiling snails try to mate, their genitals don’t match. Not only are shells coiled one way or another but the rest of the snail’s body is also asymmetrical. Female snails have a genital pore offset to one side into which a male will inject sperm through his penis. Most gastropods in the oceans have separate sexes – they are shes and hes; land snails are commonly hermaphrodites, each one with both bits of equipment, but they will pair up and take turns being male and female. Face-to-face is a popular position for snail sex, and for this to work it’s crucial for the female pore and male penis to overlap: this only happens if both snails coil in the same direction (a little like when you go to shake someone’s hand – it only works if you both offer the same hand). The shells and bodies of left- and right-coiling species are mirror images of each other. Even the corkscrew-shaped penis of the Asian Trampsnail twists the other way in lefties, and the choreography of their circular mating dances is reversed. In a tryst between right- and left-coiling snails, everyone is confused, and everything is in the wrong place.

  To gauge just how much of a problem coiling direction is in mating molluscs, researchers place pairs of mismatched snails together in cosy containers. Roman Snails, known and eaten in France as escargots (and highly protected in England), are often used in these sorts of sex studies because most of them are right-coiling, but once in a while a lefty shows up. No matter how much the left-right partners are feeling in the mood, the slurp of a baby snail’s feet never issues from the mating cubicles.

  An alternative mating tactic adopted by some snails is for one to clamber up from behind on the shell of the other. Similar snail-in-a-box studies show that shell climbers have more success in crossing the left-right divide than face-to-facers, but things are still rather awkward. Far fewer offspring will result from a right-left union than from snails paired up with same-shelled partners.

  All of this means that for sinistral snails in a mostly dextral world, life can be lonely. It’s not that right-coiling shells are inherently any better than their left-coiling brethren, it’s really just a matter of chance. Whichever form is less abundant within a species will be less likely to find a matching mate and therefore not as successful at passing on its genes; this pushes a population towards one dominant coiling direction. It just happens that at the moment right-handed shells are most abundant and get the best chances to mate. But that hasn’t always been the case, and the fossil record shows that fashions can change, although exactly why this happens remains a mystery. In The Natural History of Shells, Vermeij describes the eight or nine ancient groups of cephalopods that, through time, evolved right- and left-coiling shells, with no particular inclination towards twisting one way or the other.

  It is tempting to link the coiling of gastropod shells to the fact that when they are very young, their soft bodies also undergo a major twist. This process, torsion, is unique to the gastropods and involves all the major organs spinning around 180 degrees (clockwise in sinistral and anticlockwise in dextral shells). Among many things that move, the anus shifts to a new position above the mollusc’s head. Torsion is genetically determined, but a separate gene deals with shell coiling. It is an ancient gene, known as a nodal, that evolved long ago and today governs the asymmetry of many animals, including humans: we wear our hearts on the left thanks to the same gene that makes snails twist one way or the other.

  Looking back into the fossil record, there are lineages of gastropods that over time have untwisted their shells, like limpets, until they look like conical Asian hats. In at least one group, molluscs have unwound their shells, then around 100 million years later, against all the odds, their descendants have coiled themselves back up again. These changes would have been driven by mutations in the coiling gene.

  Given that a single mutation in an inherited nodal gene can switch a snail from being dextral to sinistral, all in one go, it raises the interesting possibility that a new species could instantly evolve. The mating struggles that take place between mismatched shells create exactly the kind of barrier that can subdivide populations and allow new species to split off, in this case leading to separate right- and left-coilers that can’t interbreed. And there are a few spots on the planet where having a rare sinistral shell can put a snail at a distinct advantage.

  Satsuma snails live in the Ryukyu archipelago in southern Japan and a surprising number of them are left-coilers. It just so happens that these islands are also the realm of Iwasaki’s Snail-eating Snakes. A land-snail expert from Kyoto University, Masaki Hoso, studies these snails and has spent many hours watching what happens when a snake sneaks up on a target, sliding up silently and swiftly striking from behind. Because of the way their mouths are shaped, the snakes can grasp a shell with the upper jaw while plunging their teeth through the aperture and into the soft flesh inside – but only in right-coiling snails. When they try the same thing on left-coiling snails, the snake can’t get enough purchase and the shell pings off to safety. Snakes pose such a terrible threat for satsuma snails that when young dextral snails are attacked, they voluntarily amputate their feet (geckos do a similar thing, dropping their tails to confuse predators while they dash off and make their escape). Hoso has never spotted a sinistral satsuma resorting to such a risky escape strategy; they always hold on to their feet.

  Mapping out the distribution of snails and snakes, Hoso found that left-coiling species of satsuma snails only occur in or near areas where there are also these fearsome reptilian predators. So it seems that avoiding the chomp of lopsided snake jaws gives the left-coiling snails the edge over right-coilers and as a consequence sinistral snails have flourished. Although it will probably be only a matter of time before the snakes likewise evolve to become left-handed.

  When nature is allowed to play

  The final flourish in the process of shell-making is where molluscs are at their most creative. As well as forming intricate shapes, shells are also decorated in elaborate patterns. There are few other animals that paint themselves in such a profusion of complex markings. With their spots, stripes, waves, zigzags and triangles you could perhaps assume molluscs are simply playing with their shells.

  There are two strange things about the shell patterns. First, no one knows which pigments molluscs use to paint their shells. So far, only a broad group of organic molecules has been detected, including porphyrins and polyenes. The closest anyone has come to pinpointing an actual shell pigment is a carotenoid in the yellow rings of Money Cowries.

  The second peculiar thing about seashell patterns is that often they go completely unseen. Many ornately painted bivalves and gastropods spend their lives hidden out of sight, burrowed in sand or mud. And there are some that grow a layer of protein (the periostracum) over the outside of their shell, often making them look like weedy rocks. What purpose, then, can there be for these shrouded shell patterns? Why should these highly decorated shells get all dressed up with nowhere to go?

  For a long time, biologists assumed that shell patterns don’t really matter, one way or another. The assumption was that since their output is never seen, the processes that lay down intricate patterns in a snail’s shell had become unshackled from the strict forces of natural selection, and were essentially neutral – they had been left to wander around an art gallery of all possible patterns, without any rules telling them what they were allowed to do.

  Exactly how and why such elaborate patterns evolve, with apparently no purpose, does seem at first to be a bizarre and inconvenient mystery, the sort of thing that creationists leap on as proof that it was God who made it so. But as scientists have unpicked the process that leads to these patterns, an explanation comes to light that makes sense without our having to wave a magic wand.

  Shell patterns are so very diverse and complex that the idea of searching for a theory to explain how they’re all made seems foolhardy, to say the least. Undeterred, however, that’s exactly what some researchers have been trying to do for the last few decades. Just as mat
hematicians and palaeontologists have set out to describe shell shape, others have done the same for shell patterns.

  Their general approach has been to think of these patterns as a form of space-time plot in two dimensions, rather like an inkjet printer. The printer nozzle squirts drops of ink onto a sheet of paper along a straight line and, likewise, the outer rim of a mollusc’s mantle secretes pigment into the growing edge of the shell. In both printing and shells, patterns are built up, line by line, as the paper passes through the printer or, much more slowly, as the shell is secreted. Running a finger from the top to the bottom of an inkjet-printed picture, or the pattern on a shell, you’re moving through time, from the part laid down first, and hence the oldest, down to the newest. For the printer, digital instructions come down a cable, or through the air, telling it which colours of ink to lay down and when. The question is, what form of instructions do molluscs have to guide them in laying down colours in their shells?

  From the start, people tinkering with this question assumed that unlike a computerised printer, molluscs don’t carry an image of their complete patterns in their mind which they then break down and reconstruct line by line. Instead, the shell’s patterns could be assembled spontaneously at the mantle edge based on a series of relatively simple rules.

  In the 1980s, Hans Meinhardt from the Max Planck Institute formulated a computer model that produced astonishing mimics of real shell patterns. Unlike David Raup, Meinhardt didn’t spend time thinking about all possible patterns, but was kept busy enough trying to recreate reality. He published a paper in 1987, followed by a book in 1995, The Algorithmic Beauty of Sea Shells, which comes with a CD of the MS-DOS program he developed so readers could have a go at decorating their own shells.