Spirals in Time: The Secret Life and Curious Afterlife of Seashells (6 page)

The mantle and waterproof shell played key roles when molluscs first clambered out of the water and adapted to life on land. A cavity underneath the shell acts as a water reservoir, to see them through parched times, and part of the mantle forms a simple lung that draws oxygen from the air. Various other things go on within the safe confines of the shell, including fertilising eggs and rearing babies; instead of laying eggs and leaving them unguarded, some snails keep hold of their young until tiny, fully formed infant snails crawl out.

Plenty of molluscs use their shells not just as a place to live but as a weapon. In particular, molluscs have evolved ingenious strategies for breaking into each others’ shells. There are whelks that jam the lip of their spiralling shell between the gaping shells of cockles, preventing them from closing shut, then slurp out their soft insides. Tulip shells use their tough shells as battering rams to smash their way into other molluscs. And there are gastropods that have become expert at shucking oysters; they use a prong sticking out of their shells to jemmy open hapless bivalves.

Shells have also helped molluscs adopt different modes of moving around. Chambered nautiluses use their shells as flotation devices. They’re divided into gas-filled chambers, which boost buoyancy and allow the nautiluses to hover effortlessly in the water column, saving energy. Cuttlefish do a similar thing, only they grow their shells internally, not on the outside. Cuttlebones commonly wash up on beaches and are offered to pet birds (and snails) to nibble; they’re not really bones but are actually the cuttlefishes’ modified shells, and are light, spongy and filled with air pockets.

Burrowing molluscs often use their shells to dig, most notoriously the shipworm. Admittedly, they are wormlike in appearance, but at one end they have an unmistakable pair of shells revealing their true identity – a type of clam. Using their shells to grind wood into a network of holes, battalions
of shipworms have sunk entire shipping fleets, and left piers and wharves crumbling.

There are even molluscs that use their shells as greenhouses. Heart Cockles are small, heart-shaped and pink, and can be found lying on sandy seabeds near coral reefs. Like other bivalves they sift nourishment from the water, but they also grow food inside their bodies. Colonies of photosynthetic microbes in their tissues harness sunlight to make sugars. In return for a free feed, the shells give the microbes, known as zooxanthellae, somewhere safe to live and a ready supply of light; the shells have small, transparent windows that let the sunshine in.

Perhaps the most startling example of the seashell’s versatility comes from the Clusterwink Snail of Australia and New Zealand. During the daytime, these denizens of rocky shores are fairly unremarkable, small yellow shells. However, if you wait until nightfall and give one a gentle prod it will glow with a greeny-blue light. Two small spots on the snail’s body shine brightly, and their shells act as highly efficient diffusers, spreading the light out and making the entire shell glow.

Why go to the effort of lighting up? It’s thought the clusterwinks’ beaming displays surprise intruders, which will either scuttle quickly away or fall victim to other predators that have been alerted to their presence. The glowing shells essentially act as burglar alarms.

From spades and light bulbs to life rafts, battering rams and drills, the catalogue of things molluscs do with their shells is rambling and eclectic, helping these creatures live incredibly diverse lives in many different places. Mollusc shells may come in a huge variety of shapes and sizes, but all of them are made according to the same set of basic shell-making rules for turning seawater into ceramic spirals.

CHAPTER TWO

How to Build a Shell

O
n the banks of the Kinta River, at the furthest navigable point inland from Peninsular Malaysia’s western coast, stands the former mining town of Ipoh. Behind the bustling Chinese shophouses, white colonial town hall and railway station lies a backdrop of some seventy limestone hills, clad in forest. As visitors climb steps to the Buddhist temples perched in these green humps, or descend into the caves beneath them, they are surrounded by biological treasures, including some of the world’s smallest and strangest shells.

Karst limestone formations, like the ones in Ipoh, can be seen throughout South-east Asia, from northern Vietnam through Cambodia and Thailand to the Philippines and Indonesia; they rise from the sea as idyllic islands, and poke through rainforest canopies. The limestones were formed millions of years ago by the remains of ancient sea creatures,
including corals and shells. Since then, their calcium carbonate skeletons have been uplifted, then eroded by wind and rain into jagged silhouettes with giant caves inside them and underground rivers running through them.

A riot of unusual wildlife lives in these limestone landscapes. Bumblebee Bats, the world’s smallest mammals, flit through the caves; blind fish crawl from subterranean ponds and out onto rocks; beetles and millipedes prosper in huge piles of bat dung; and out on the rugged hilltops roam troops of leaf monkeys, including such incredibly rare species as the Delacour’s Langur with its striking black and white fur (the Vietnamese name for it,
vooc mong trang
, means ‘the langur with white trousers’). The chalky soils are also a haven for molluscs that find a plentiful supply of the principal raw material to make their shells.

A single Malaysian limestone hill can be home to between 40 and 60 species of tiny microsnails, each one a millimetre tall, and all of them with highly ornate shells. Of those, two or three species could be unique to that individual hill. As well as snails, there are heaps of other endemic species here, ones that are found nowhere else on the planet: geckos, crickets, orchids, begonias and spiders. Just like oceanic islands, the limestone outcrops are isolated dots of habitat where evolution dances to a different beat, generating new and peculiar species.

When biologist
Reuben Clements
went snail-hunting in the hills of Ipoh, he discovered a shell like no other. To find it, all he had to do was take a few scoops of soil, place them in a bucket of water and wait for the empty shells to rise to the surface (for a long time only recently dead specimens were found, and no living snails). Seen under a microscope, these tiny shells reveal their curious physique. They look like the corrugated pipe of a vacuum cleaner that’s been left tangled on the floor, with the end flared out like a tiny trumpet. These shells twist and turn, this way and that, as if they can’t decide which way to grow.

A few years later, Clements’ colleague Thor-Seng Liew finally tracked down live specimens of this tiny snail and set about studying them for his Ph.D, devising a theory to explain their bizarre coiling shapes. Liew suggested that the snails are doing their best to avoid getting eaten by predatory slugs. Retreating into their shells, the snails force their attackers to reach into an empty, bendy tube while their prospective dinner cowers at the end. The slugs’ proboscis simply can’t reach into such a deep and convoluted recess.

Meanwhile, Clements and other limestone enthusiasts have been campaigning to protect the remarkable but often overlooked places these snails come from. Being useless for agriculture or development, limestone hills were left more or less alone for a long time, but now cement companies are getting in on the act, razing them to the ground for the limestone inside. These are imperilled arks of biodiversity that few people have heard of. Year on year, hundreds of species are going extinct, most of them before they are discovered, when the hills they once lived on are taken away.

Compared to Clements and Liew’s bizarre find in the Malaysian hills, most shells are far less erratic in the way they grow, and indeed they are often quite predictable. For centuries, many great minds have contemplated the elegant sculptures and patterning of shells and wondered what might govern their construction. They have hunted for clues to explain the amazing realities and tempting possibilities of shells; they have probed ideas of what makes a shell work and which shapes may ultimately never show up; and they imagined that if they could find ways of drawing shells, if they could mimic what nature has been doing for eons, it would not only bring them closer to understanding how molluscs make their intricate homes, but they might also catch a glimpse of the origins of beauty itself. What many generations of mathematicians, artists, biologists and palaeontologists have found is unexpected and elegant: to
construct an elaborate seashell – and decorate it – requires only a handful of rules.

Of all shell shapes, one of the simplest and most pleasing is the spiral of the chambered nautilus. The internal twist of these ocean-wanderers is revealed when their empty shells are sliced in two, from top to bottom. Trace the outer edge of a nautilus shell and you’ll see that it spins inwards in a very particular way. This graceful curve was among the first shapes in nature to be granted its own mathematical formula.

In the seventeenth century, French philosopher René Descartes composed a simple piece of mathematics for drawing a shape called the logarithmic spiral. Unlike an Archimedean spiral, which has whorls that are always spaced the same width apart, like a coiled snake, the gaps between successive whorls on a logarithmic spiral get increasingly wide. Logarithmic spirals flare open as they get bigger, just like a nautilus shell.

A chambered nautilus shell cut in two, revealing its logarithmic spiral.

It was the cleric and mathematician Reverend Henry Moseley who, in 1838, first pointed out that many coiled shells are versions of the logarithmic spiral. Take a photograph of a nautilus shell cut in two, overlay the outline of a logarithmic spiral and, given the right dimensions, it should be a good fit.

These expanding spirals pop up all over the natural world; you can spot them in patterns of seeds in a sunflower, in spiralling galaxies, in the bands of rain and thunderstorms that swirl around the eye of a tropical cyclone, and in the path taken by a doomed moth as it flies mesmerised towards a candle. All these spirals are subtly different; what unites them is the fact that they all get bigger at a constant rate. In other words, the gaps between successive coils get wider by the same amount each time the spiral makes a complete turn around its central point. This means that no matter how big the spiral becomes, its overall shape doesn’t change – and that is one of the key rules for making a seashell.

The way molluscs make shells is reminiscent of the ancient practice of coiling pottery. For thousands of years, people around the world have rolled strips of clay between their hands and coiled them into simple pots. In a similar way, a mollusc creates its shell as a hollow tube. The mantle (the fleshy cloak that spreads across a mollusc’s body) lays down new shell only at this open end, known as the aperture. It does so by first secreting a scaffold of protein, which is then shored up with calcium carbonate in one of two varieties (and sometimes both): aragonite or calcite, the latter being a more stable form. The main building blocks for the shell are carbonate ions, consumed in the mollusc’s diet or absorbed from seawater, and squeezed into a small gap between the mantle and the growing edge of the shell. Finally a layer of nacre, or mother-of-pearl, is added on the inside, creating a smooth layer that protects the mollusc’s soft body. As this composite tube grows it becomes wider at the open end, transforming it into a cone. The mollusc
scrolls this cone round and round, forming, in cross-section, a logarithmic spiral.