Spirals in Time: The Secret Life and Curious Afterlife of Seashells (9 page)

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 mathematicians 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.

Meinhardt’s idea was that there could be substances wafting through the mantle that trigger cells to produce pigment. It doesn’t so much matter what those substances actually are (they could be hormones or some other form of messenger molecule). What mattered to Meinhardt was their
effects; imagine that instead of pumping out drops of coloured ink, a desktop printer produces colourless substances that react with the paper – and each other – in different ways, creating colours and patterns. One of these substances is an activator that switches on pigment production. The activator also triggers the production of more of itself as well as another substance that acts as an inhibitor. Meinhardt predicted that there are antagonistic waves of these activators and inhibitors, chasing each other across the mollusc’s mantle edge, stimulating colourful patterns as the shell grows.

At the heart of Meinhardt’s model are two differential equations that define how these activator and inhibitor molecules move and interact (and if you like numbers you can find them in his book). By tweaking those equations, he was able to simulate the basic patterns seen in real shells, including all manner of stripes, spots and zigzags.

Stripes parallel to the shell opening are made when pigment production is turned on and off periodically. At first all the pigment cells are stimulated to produce a line of colour, then they are switched off; keep repeating this and stripes unfurl on the growing shell. For bands in the other direction, perpendicular to the shell opening, some pigment cells are switched permanently on and others are permanently off. Meinhardt simulated both of these stripes by altering the relative speeds of the activators and inhibitors in his model.

Diagonal stripes are formed by a process similar to the movement of an epidemic through a human population. A cell loaded with activator can infect neighbouring cells, which after a delay then go on to infect the next-door cells, and so on. This triggers a travelling wave across the array of cells. Interesting things begin to happen when pairs of travelling waves collide. One possibility is they will mutually annihilate each other, drawing a ‘V’. Or one wave can annihilate the other, then carry on as a single stripe. Alternatively, they
bounce off each other and continue in the opposite direction, drawing an ‘X’ (although the waves actually cancel each other out, then immediately reignite and continue on their way).

Some travelling waves veer off in different directions while keeping their tails in touch, until suddenly both waves stop in their tracks, creating empty triangles. Waves rushing at each other can also either speed up or slow down, producing spots and teardrops. More involved adjustments to Meinhardt’s basic equations lead to more complex patterns, including undulating waves, empty triangles on a dark background and fractal patterns of triangles within triangles, known as the Sierpinski Sieve. All of these shapes and patterns are seen on real shells.

There is, however, one major problem with Meinhardt’s ideas: there is no evidence to show that any of this
actually
happens in mollusc shells. No one has ever found a single diffusing substance, no activator or inhibitor, to prove that his ideas are correct. As Meinhardt himself admits in his book, ‘Theory can only provide a shopping list of possible mechanisms.’

At around the same time that Meinhardt first published his diffusion model, another research group wrote a paper with an alternative explanation for shell patterns. Bard Ermentrout from the University of Pittsburgh, along with his colleagues George Oster from University of California, Berkeley and John Campbell from UCLA, showed that similar patterns could be created not by unseen substances diffusing around the mantle but via the firing of nerves.

It was Campbell who, in 1982, suggested that the pigment-producing cells in the mollusc’s mantle might be stimulated by nerve impulses, just like secretory cells in other animals. The team’s model was in effect very similar to Meinhardt’s; both simulate a process known as Local Activation with Lateral Inhibition, or LALI. In the 1950s, the great mathematician Alan Turing showed how LALI could work with diffusing molecules, the concept on which Meinhardt
based his models. A neural version of this was originally described back in 1865 by Ernst Mach, to explain the optical illusion now known as Mach bands. This occurs when a row of stripes in different shades of the same colour appears to curve inwards from a flat page. This happens because nerves in the back of the eye are activated by the edge of a stripe and will inhibit neighbouring nerves, accentuating the boundary between two stripes. And in a similar way to Meinhardt’s diffusing substances, nerve signals can also activate or inhibit the production of pigments and their effect can sweep along, creating travelling waves and various other intricate patterns. Ermentrout and the team implied a very different mechanism to the diffusion model, but made very similar patterns.

The neural and diffusion models had something else in common: Ermentrout, Oster and Campbell also had no proof that their model was correct. Back then no one knew whether nerves do in fact control pigment production in mollusc shells. ‘At the time there was no evidence for it, it was just a good idea,’ George Oster told me when we chatted on the phone about making shell patterns. After their original paper came out, it would be another 20 years before Ermentrout and Oster published again on shells. When they did, they came closer than anyone ever has to formulating a unified theory that explains not only how seashells get their patterns, but also why they do it.