Spirals in Time: The Secret Life and Curious Afterlife of Seashells (8 page)

Other parts of the museum were empty, Raup suggested, simply because the process of natural selection hasn’t got around to filling them yet. He thought that as soon as a situation arises in which those theoretical shells become useful and confer an advantage on their owners, then, sure enough, they will evolve. Other researchers disagree. They think the vacant spaces of the museum will never be filled, because the necessary genetic mutations to make those shells haven’t happened and maybe never will. Their view is that natural selection doesn’t have at its disposal all the genetic variation that is necessary to fill every part of the imaginary museum. Debates still rage over who is right.

Following on from Raup’s original concept, many other museums of possible creatures have been built (they are now known technically as theoretical morphospaces). There are museums for beetles, aquarium tanks for fish, sea urchins and phytoplankton, even a herbarium for plants and aviaries for birds and pterosaurs. Just like the shell museum, these
rambling spaces are filled with both real and imaginary beasts, and they are encouraging biologists to think about which forms and shapes in nature are possible and popular, and which are impossible or for some other reason have never occurred or will never occur.

Throughout his papers, Raup was always careful to point out that his model isn’t perfect, and it doesn’t account for all the things we see in the real world. For one thing, he confessed to being overly simplistic about fixing the various shell dimensions throughout a mollusc’s lifespan; there are real shells that seem to shift the values of T, D and W over time, so they hop around the imaginary museum as they get older. And, as
Clements and Liew
found with their strange microsnails, there are some molluscs that break all the rules. One of the tiny snail species from the limestone hills of Malaysia makes a shell that spins around not just a single axis but four: the most of any known shell.

For the sake of simplicity, other features seen on many real shells are also omitted from the museum of all possible shells. For example, Raup left out the ornaments – spikes, knobbles, ribs and spines – that molluscs use to decorate their shells.

Why shape matters

Geerat ‘Gary’ Vermeij has probably spent more time than anyone else thinking about the shapes of seashells. Born and raised in the Netherlands, the first shells he encountered were what he describes as ‘drab chalky clams’ on windswept North Sea beaches. Then, in 1955, his family moved to Dover, New Jersey, where Vermeij experienced something of an epiphany. His fourth-grade teacher, Mrs Colberg, decorated the classroom windowsills with dozens of shells she had gathered during holidays to southern Florida’s tropical shores. They were nothing like the shells Vermeij had got to know in Europe, being elegantly sculpted and covered in prickles and bumps. Her cowrie and olive shells
were so shiny he was sure someone had varnished them. When a classmate brought shells from the Philippines to ‘show and tell’, Vermeij saw these were even more exotic and enthralling. He resolved to begin collecting his own shells and to find out as much as he could about them.

A decade or so later, Vermeij graduated with a Ph.D from Yale University, and since the 1980s has been Professor of Paleoecology at the University of California, Davis. It became his lifelong passion to understand how and why shells grow in so many different forms throughout space and time. He has travelled the world exploring the coasts and seashells of nearly every continent, and published more than a hundred scientific papers and four books about shells and evolution. And, since the age of three, Gary Vermeij has been blind.

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.