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

  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.

  Decoding the mollusc diaries

  If you could listen in on a mollusc’s thoughts, the chances are you wouldn’t hear anything especially profound because, strictly speaking, they are brainless (unless you’re eavesdropping on one of the super-intelligent octopuses, the smartest of all invertebrates). Nevertheless, their simple nervous systems could be responsible for creating the complex decorations that seep across their shells as they grow. The latest computer models that reconstruct shell patterns involve an intriguing new idea: molluscs have the ability to read the patterns on their shells, like the pages of a diary. In this way, patterns become memories etched across their shells.

  Shell-making is an expensive business, in terms of getting both raw materials to build them and the energy to lay down new shell. As such, molluscs don’t make their shells continually, but in bursts, when they can afford to. Because of the stop-start nature of shell-making, it’s vital that molluscs continue construction in the correct orientation, otherwise they’d be all over the place. In their most recent studies, Ermentrout and Oster put forward the idea that shell patterns are a way for molluscs to remind themselves where they left off. This allows them to line up their mantle and continue sculpting their shell in the right places, keeping their intricate shape on track. If this idea is right, then it could be that shell patterns are not quite so useless after all.

  Over the last few decades, evidence has been mounting to support the idea that shell-making in molluscs is under neural control. Electron microscopes reveal that mollusc mantles are filled with nerves. These connect back to paired clusters of densely tangled nerves, known as ganglia, that come as close as you will ever get to a general mollusc brain (the ganglia fuse to form a ring through which the oesophagus passes, which means that when a snail swallows, its food goes right through its mind). Nerves stimulate cells in the mantle to secrete new shell layers and, by controlling the amount and direction of material made, different shapes emerge. The mantle also has sensory nerves that seem capable of detecting existing patterns of pigment in the shell. It’s possible that each time a mollusc prepares to make more shell, it begins by licking its mantle over the edge of its shell to ‘taste’ the pattern already laid down. At the same time, nerves in the mantle could also be responsible for switching pigment production on and off.

  Based on these ideas and using a revamped version of their 1980s equations, Ermentrout and Oster set out to build a new shell-making program, this time with the help of UC Berkeley grad student Alistair Boettiger. This model not only churned out complex two-dimensional patterns, but it wrapped them around three-dimensional models of shells. For the first time, a realistic mechanism h
ad been formulated for growing shells and decorating them, using a single model.

  Whether or not mollusc mantles can actually sense the colours on their shells remains unclear, and it’s a major challenge to study real shell-making because it happens so very slowly. A hint that this idea is right, though, comes from the way molluscs repair their shells. As Gary Vermeij knows only too well, in the real, dangerous world it’s easy for shells to get whacked, pinched by a crab claw or hurled against a rock. If they survive, molluscs will fix their shells and keep on growing. When a shell becomes damaged or part of it is chipped away, the pattern can get messed up with stripes knocked sideways or stopped in their tracks. But a short way down the line, the pattern usually recovers and continues as before. This suggests that molluscs can detect damage but take a little time to correct themselves. Boettiger’s computerised shells do exactly the same thing when inflicted with simulated injuries.

  Chaos also features in this latest shell-making program – not that it is all a jumbled mess, but in the mathematical sense. Small changes in initial conditions produce different versions of the same pattern; a little bit of noise here and there makes a real difference. Ermentrout and Oster think this could be why shell patterns in nature can vary substantially between individuals of the same species. Rather than being identical, markings are commonly like human fingerprints, unique to each shell while still sharing similarities in overall pattern.

  In 2012, Ermentrout and Oster used their neural model to look at how shell patterns evolve: if they could show that the way patterns change over time isn’t completely random, it would support their idea of patterns being useful to molluscs as a way to mark and read their shells. They gathered together a larger team of cell biologists and computer scientists, including Zhenqiang Gong from University of California, Berkeley, who constructed an even fancier computer programme, duplicating 19 species of cone snails that have complex patterns on their shells. The team mapped out a family tree based on the different shell patterns of living species, and used the model to reconstruct what the patterns would have looked like in ancestors further back in the cone snail lineage. They tracked how patterns may have changed over time, as species diverged and split apart. This suggested that some elements of the patterns remained relatively stable over long periods, while others have shifted quickly here and there.

  To test the accuracy of their model, the team drew a second family tree, this time using DNA sequences from the cone snails. The match-up between the DNA and pattern-based family trees was striking, far closer than would be expected by chance alone.

  All this backs up Ermentrout and Oster’s theory that shell patterns aren’t frivolous playthings but important registration markers for shell-making that have been subject to the forces of natural selection, and have evolved over time. It may not matter exactly what kind of patterns are made, as long as there is some way for a mollusc to figure out where to put its mantle before continuing to make more shell.

  These latest models have undoubtedly taken us a major step closer to understanding how and why molluscs decorate their shells. At the same time, this area of research has cracked open a new window that could have a profound effect on broader reaches of science. The notion that molluscs may leave themselves messages across their shells, allowing them to track the past and make decisions about the future, could give neuroscientists vital clues about how more sophisticated nervous systems work. With this in mind, Ermentrout and Oster are moving on from gastropods and bivalves to work with brainier molluscs, the cephalopods, and in particular cuttlefish. At least that’s what the press release from their 2012 paper said. Both Ermentrout and Oster chuckle when I ask them about this. ‘We’ve talked about it a lot,’ Oster says. But the reality is that working with cuttlefish and the stunning patterns they display across their bodies is much more difficult than working with shells. Not only is funding for this sort of research hard to come by, but cuttlefish coloration is much more complex than shell-patterning. Ermentrout and Oster would have to turn their attention from patterns laid down over months to ones triggered in milliseconds. Cuttlefish (and octopuses too) are draped in a mantle that doesn’t secrete an external shell, but changes colour to camouflage them or to shout sexy messages to potential mates. These patterns are controlled by a similar network of nerves to those in shell-making molluscs, and, as George Oster points out, ‘there’s a lot of speculation, but nobody actually knows what the neural circuitry in cuttlefish skin is.’

  Nevertheless, I get a strong sense that both of them would love to work with cuttlefish. ‘They can flash their colours like gang signs,’ Bard Ermentrout tells me. He spends each summer in Woods Hole on Cape Cod in Massachusetts, and clearly enjoys paying a visit to the Oceanography Institution to see the cuttlefish. ‘You’re not supposed to do this,’ he admits, ‘but if you put your hand in and touch them, the image of your finger remains on their skin for a few seconds. It’s really cool.’

  If Ermentrout and Oster can find a way of working with cuttlefish, then perhaps by understanding how their networks of nerves create patterns of ‘thoughts’ across their skin, it could ultimately help reveal how human brains form memories, deep down where no one can see them.

  CHAPTER THREE

  Sex, Death and Gems

  Forty years ago, in the city of Varna on the Bulgarian coast of the Black Sea, workmen were digging a trench to lay a power cable when they stumbled on something unexpected: human remains – very old human remains – and a great hoard of gold treasure. Archaeologists quickly stepped in and uncovered the rest of a vast necropolis, a prehistoric city of the dead, comprising at least 300 graves that had been dug more than six and a half thousand years ago.

  The glittering gold that first caught the workmen’s eyes turned out to be part of the oldest haul of buried gold known in Europe. But the gold jewellery and ornaments were not the only treasures left in these graves. In the finest of them all, the resting place of the most powerful man of this ancient community, was a circular bracelet carved from a single seashell that came from far away. It was carried hundreds of miles overland and given to a skilled artisan, who spent many hours carefully polishing and carving it. When it was finished, the shell bracelet was snapped in two, then fixed back together with strips of gold plate hammered with rows of fine dimples.

  No one knows exactly why this shell bracelet was broken and then mended. There is no written record from this time, only a series of objects to tell us about these people of the past. Yet there’s little doubt that for the person who made it, and the person who was buried with it, the bracelet held great meaning. The shell had perhaps been just as precious as the gold that was used to fix it – maybe even more so.

  Just like the molluscs that use their shells to hunt and dig and move, so people have also fashioned shells into all sorts of objects. Some are practical tools. Archaeologists have found shells made into anvils, choppers, knives, fish-hooks and weights for fishing nets. There are shells that lent themselves to particular uses, based on their size and shape, like the bailer shells (of the genus Melo) that seafaring cultures have used for centuries to scoop out water from canoes and sailboats. Ground down into powder, shells are added to animal feeds as a source of calcium. The powder can also be combined with ceramics; pottery made a thousand years ago in the Mississippian culture of North America was commonly made stronger by mixing burnt, crushed shells in with the clay.

  Besides the usefulness of shells, people have also admired their elegant shapes, dazzling patterns and gleaming iridescence. It’s no great surprise that cultures worldwide have used shells to decorate people and places. What is astonishing, though, is how universally shells have come to hold great meaning. Far from being just pretty things to look at, shells have been embraced as powerful emblems of sex and power, of birth and of death.

  For millennia, people in distant corners of the globe have placed whole shells in graves alongside the bodies of their loved ones. Even a long way inland, thousands of mil
es from the sea, piles of shells lie in ancient burial sites. The dead are interred, sometimes clutching shells in their hands or with cowries placed over their eyes (perhaps because the shells themselves look like eyes). The Scythians, a group of ancient Iranian nomads, roamed the central Asian steppes on horseback, yet made burial mounds decorated with cowrie shells. The Seneca people of New York State believed shells placed in the grave could purify decaying flesh and allow the soul entry to the spirit world. They also made masks with shells for eyes, believing that to look through a shell is to gaze back to the beginning of time. The Winnebago tribe of Nebraska considered shells to be the stars of the sea and the apparitions of dead children, women who died in childbirth and men who died in battle; shells were placed inside sacred caves to honour these dead.

  One reason, it’s thought, that shells have turned up in so many graves is their colour; in many cultures white represents purity and peace, and, accordingly, it is the colour of birth or death. There is also the notion that shells come from an unseen, watery underworld. Empty shells that wash up on beaches are messengers from the deep. Beachcombers pick them up from the strand line, while pondering the hidden realm they came from, or divers bravely visit this dangerous place themselves and return bearing exotic objects.

  Around the world, shells are ancient symbols of sexuality, fertility and renewal, perhaps in part because of their shape. So many people have picked up a cowrie shell, turned it over and seen a lengthwise, dark opening like a corrugated smile that reminded them of female genitalia. Even the cowrie’s rounded bump is reminiscent of a pregnant belly. Shells are associated with the life-giving properties of water, and they’ve come to represent the protective womb, a place of conception and the generation of life.