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

  Other carnivorous molluscs have evolved more elaborate means of hunting. Cone snails, augers and turrids spit their teeth at their prey. Their highly adapted fangs are hollowed-out harpoons, which they load with a complex cocktail of deadly toxins to instantly paralyse unsuspecting worms and fish. These shells can be so toxic that they occasionally kill a full-grown human (a baffling ability that we will come back to later). There are also plenty of molluscs that have turned their radulas on their own kind. Their modified mouthparts drill neat, circular holes in shells; they then squeeze digestive enzymes into the hole and slurp out the contents. These ones are known, perhaps a little unfairly, as boring molluscs.

  Not all molluscs have radulas. Bivalves lost theirs, and instead feed using their feathery gills. Most of them, including oysters and mussels, have adopted an idle approach to life. Instead of dashing after prey or crawling around looking for weeds to munch, they settle down on the seabed and stay put (more or less), and let food come to them. Tiny hairs called cilia cover their gills and beat rhythmically, creating a current. This draws oxygen-rich water inside the shell for the bivalves to breathe, and also brings in floating particles that stick to the gills in a layer of mucus. A gentle trickle of nourishment – mostly in the form of plankton – gets wafted along by the cilia, towards the bivalve’s mouth. Most of them have evolved enormous gills, folded up inside their shells in a W-shape, offering a large surface area to filter food from the water around them.

  Multi-tasking like this is another important factor behind the molluscs’ great success. Different organs have been put to various different uses, depending on the circumstances. Gills are used to breathe and to gather food; the heart can both pump blood around the body and filter impurities from it, acting like a kidney. There are also a whole host of different uses for their singular feet.

  Best foot forward

  Wide sandy beaches on the Pacific coast of Costa Rica are home to sea snails that have learnt how to surf. Legions of olive snails swash-ride the waves that lap up and down the beach, using their feet as underwater surfboards; it’s a more energy-efficient way of getting around compared to crawling. Once it’s landed at the top of the beach, a surfing snail will put its broad, muscly foot to another use, turning it into a pouch to trap prey. Like a cat burglar with a stripy top and a sack, it engulfs its target, then quickly smuggles it away, burrowing down in the sand. And these olive snails are not choosy eaters; pretty much whatever they bump into, they will try and shove into their foot pouch. Usually it’s another olive snail, because there are a lot of them about, but sometimes they find something else. Winfried Peters, from Indiana University-Purdue University Fort Wayne, has studied olive snails and offered them a variety of potential foodstuffs. He has filmed one of these thumbnail-sized snails trying its very best to swallow a pencil.

  Some molluscs, including limpets and chitons, use their feet to clamp themselves tightly to rocks and stay put (creep up and gently prod a limpet and you’ll see it quickly clench down; then it becomes almost impossible to shift). Usually, though, the molluscan foot is a means of getting from A to B, often accompanied by lashings of gummy slime. So how exactly does an animal with one foot walk through glue?

  The tiniest gastropod molluscs move around on hairy feet. Minute estuarine snails, called Hydrobia, have feet covered in masses of cilia similar to bivalve gills. These beat like a thousand tiny oars, propelling these little gastropods around their muddy homes. This method isn’t powerful enough to shift larger snails and slugs, so instead they move around on waves of muscular contraction that ripple along their feet. The waves generate just enough force to slowly pull or push them along, at a speed of generally between a millimetre and a centimetre per second, in one direction only; for the most part, slugs and snails can’t go backwards.

  The silvery trail molluscs leave behind them as they crawl along is made of sticky stuff that doesn’t play by the rules. Scientists discovered 30 years ago that molluscan slime changes its behaviour, depending on how firmly a snail or slug pushes against it. A blob of slime is indeed very sticky, but give it a squeeze – as when a wave of contraction passes by – and it turns into a free-flowing liquid. This reduces the friction on part of the foot and allows the mollusc to push forwards. Sliding through slime is an effective way for molluscs to move, to climb walls, trees and rocks and hang upside down, but it comes at a great cost; some species use up 60 per cent of their energy on making protein-rich mucus. To try and save energy, many slime-sliders including periwinkles will sniff out and follow the fresh trails laid down by other molluscs.

  Clams, scallops and other bivalves don’t glide around on their feet; instead they shuffle, hop, jump and dig. When it feels the need, a cockle can poke its foot out and shove itself along, hopefully out of harm’s way. And while scallops clap their shells together and swim through open water for short bursts, they will also use their feet to dig down into the seabed and bury themselves. Burrowing opened up a whole swathe of new habitats for the molluscs, as did their ability to swim.

  Cephalopods have highly adapted feet. Part of them has evolved into a hollow tube that squirts out water and thrusts them through the sea, by jet propulsion. And somewhere along the line, the cephalopod ancestors reshaped their feet to sprout clusters of arms and tentacles, making them the most dextrous of all the molluscs (and you can easily tell octopuses from squid by counting their arms and tentacles: octopuses have eight arms, with suckers all the way along; squid have eight arms plus two tentacles, that only have suckers at the end).

  And there’s little doubt that the most charming adaptation of the mollusc foot is in the gastropods that fly through the open ocean. Sea butterflies and sea angels are gastropods that bade farewell to the seabed, split their feet into two tiny wings and flitted off into the big blue yonder.

  A thousand and one uses for a shell

  Pulling one final piece from the mixed bag of molluscan body parts, we are left contemplating the shell. And, as it turns out, there’s a lot you can do with one of these wonders of calcium carbonate.

  Sculpted and moulded by natural selection, the mollusc shell has proven to be an extremely useful piece of kit. Molluscs use their hard shells, and the soft mantles that make them, to move, to eat, to hide and to fight, plus a few other surprises along the way.

  Starting with the mantle, this draping cloak of tissue has various uses other than shell-making (which we will come to later). Often, mollusc mantles are quite beautiful. Cowries stick out their mantles and flap them over the tops of their shells (it’s because of the mantle that cowrie shells are so shiny and smooth). In some species, the mantle offers a disguise, matching the shell brilliantly to its surroundings; some spindle cowries have bright red mantles, covered in knobbles, camouflaging them against the soft corals they live on. The shell-less nudibranchs harbour a variety of noxious compounds in their bodies, and the ostentatious colours of their mantles shout ‘move along, nothing to eat here’ – predators soon learn to steer clear. Cephalopods have the most sophisticated mantles of all; squid and octopuses can change colour in the blink of an eye, to communicate messages to each other or instantly blend with their surroundings, camouflaging themselves with cloaks of invisibility.

  Many bivalves have rolled part of their mantle into a hollow tube, called the siphon, which they use like a snorkel. Burrowed in sand and mud, they reach up into the water to breathe and feed. Native to the Pacific Northwest, in Canada and the US, clams called Geoducks (pronounced ‘gooey-ducks’) have colossal siphons, up to a metre (three feet) in length. They allow the clams to live deep down in soft mud, and are so big they no longer fit inside the shells, but remain permanently stuck out, like an elephant’s trunk (or perhaps an outrageously huge phallus). In China, Geoduck siphons are considered a culinary delight.

  Another tube protruding from the mantle is an extendable proboscis, tipped with sensory cells. Predators and scavengers use them to sniff out things to eat (while cone snails spit teeth out of t
heirs). Cooper’s Nutmeg Snails have an extra-long proboscis, several times their own body length, and for ages biologists wondered what it is they eat; clearly it’s something the snails don’t want to get too close to. The answer came after a chance encounter, when scientists from Scripps Institution of Oceanography were diving off the San Diego coast. They saw a nutmeg snail sneaking up to an electric ray, a flattened relative of sharks that can generate an electric shock to capture prey and deter predators; the jolt is equivalent to the shock from a car battery. But the nutmeg snails go unnoticed and un-zapped. They use a sharpened radular tooth at the end of their proboscis to make a small incision in a ray’s belly and suck its blood. These snails are the vampires – or perhaps the mosquitoes – of the mollusc world.

  Some molluscs have adapted their mantles for moving around. The marvellous Grimpoteuthis or dumbo octopuses (rarely are both scientific and common names so good) slowly glide around the deep sea, flapping extensions of their mantle that look like enormous ears. The mantles of cuttlefish extend into a fringe of long, narrow fins, like a frilly petticoat, that undulate in gentle waves as these cephalopods hover in the water and smoothly swim along.

  As for the hard calcium carbonate shells secreted by the mantle, these are first and foremost a means of protection and a safe place to hide: a portable home. Bivalves are the best protected of all the molluscs; with their two halves closed shut, they are extremely difficult to get into, as anyone who’s tried to open an oyster will know. Gastropod shells, on the other hand, tend to have a weak spot: the opening where their head sticks out. Limpets overcome this by fixing their shells tightly to rocks (they also use their shells in defence, when a predatory starfish shows up, by standing tall – so-called ‘mushrooming’ – then stamping down hard on the invading tube feet). Most snails can pull their heads inside their shells, and many have evolved a separate door, the operculum, which they swing shut behind them. This helps deter intruders, and prevents land-living snails from drying out.

  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

  On 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 rece
ntly 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.