Spirals in Time: The Secret Life and Curious Afterlife of Seashells (5 page)

This collection of mollusc body parts has proven to be incredibly malleable and adaptable. Rather than a Lego set, complete with all the specific parts to build a Star Wars
Millennium Falcon
, think of a box of modelling clay that can be made into anything your imagination allows. Similarly, each mollusc body part has been reconfigured, reshaped and repurposed over time by natural selection, allowing molluscs to wildly alter their appearance and way of life.

In effect the mollusc lineage has been riffing on a theme for half a billion years. They have been trying out experiments in how to eat and avoid being eaten, how to move about, and how to have sex and make more of themselves. This opened the way for molluscs to move into new habitats, to fill a huge range of ecological niches and ultimately to evolve into hundreds of thousands of species. Molluscs are supreme shape-shifters, and it’s this versatility that could explain their roaring success, as we can see by looking in turn at each of the main body parts.

All the better to rasp/chew/stab/harpoon you with

Peer into a mollusc’s mouth (preferably with the aid of a microscope) and be prepared for a terrifying show of fangs. They may be small, but they are some of the most complicated teeth on the planet.

The radula – a bristly tongue made of a protein called chitin – is covered in rows of tiny teeth, laid out across a conveyor belt that creeps ever forwards, with new teeth made at the back and old, worn-out ones falling out at the front. A single radula can have anywhere between a handful and many hundreds or even thousands of teeth, and each mollusc species has a unique arrangement of gnashers. Gastropods, in particular, have really gone to town with their teeth. They’re organised into groups with names that sound like Dr Who aliens; watch out for the rhipidoglossans, the hystrichoglossans and the toxoglossans. I’d like to imagine that molluscs grin at each other to identify themselves, but of course they don’t.

The precise shape and configuration of the teeth on the radula determines what molluscs can eat. Some radulas allow for quite simple but varied diets, sweeping up loose diatoms, slurping strings of seaweed like noodles or scraping at rocks and boulders covered in green slime. Limpets rasp microbes and seaweed sporelings from rocks, like a cat licking a bowl of frozen milk. The reason their teeth don’t instantly shatter when they do this is because they’re made from the strongest known biological material. A 2015 study found that limpet teeth, made of an iron-based mineral called goethite, are up there with the very strongest artificial materials. Limpets could chew holes in bulletproof jackets, if they wanted to. At low tide, you can see the zigzag marks they scrape across rocks and you can even hear them eating; quietly place a stethoscope on a rock near one of these little herbivores, and you should be able to make out the intermittent, sandpapery scratching as the limpet gathers its food.

Other vegetarian molluscs have evolved more specialised radulas, including the sacoglossans, a group of sea slugs that suck. They use their teeth to pierce the cell walls of plants and seaweeds, then suck out the sap inside. Many are incredibly picky eaters, feeding on a single species and, like fine gourmet diners, they have cutlery to match. Their teeth can be serrated triangles, sharp blades or shaped like wooden clogs; they’re adapted to pierce particular types of underwater growth, from leathery kelp to crusty seaweeds. With their specialised teeth and diets, sea slugs divide up habitats, allowing lots of species to evolve and coexist.

Snaggle-toothed radulas become frankly terrifying in molluscs that evolved to be hunters. Many have teeth like flick knives that stand on end, locking in place during attacks, then folding safely away when not in use. A few years ago, an eerie white slug was found in a garden in Cardiff, Wales. It was a species new to science and experts had a shock when they saw its teeth: it was the UK’s first predatory slug. Most land slugs, though, much to the annoyance of gardeners, are herbivores. And at a mere two centimetres (half an inch) long, the new slug is not exactly a sabre-toothed tiger, but it’s no less scary if you happen to be an earthworm.

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 theirs). 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.