Spirals in Time: The Secret Life and Curious Afterlife of Seashells Page 2
Following our trips to Cornwall, and clutching our dive certification cards, Helena and I both headed off to university. She studied languages and went into the wine trade, eventually moving to Australia and taking her dive kit with her. I studied ecology and marine biology and continued with what’s become a lifelong love affair with scuba-diving. Besides exploring the seas wherever and whenever I could, I decided to try and do my bit to help protect the oceans and ocean life from the onslaughts of the modern world. I had seen for myself the deterioration of marine habitats and I began to notice how every creature matters, no matter how small and apparently insignificant. For many years I have lived and worked in countries around the world, investigating the problems of overfishing and working on strategies to protect the species and ecosystems in the greatest jeopardy. And all the while, throughout my research and travels, seashells have followed me around.
I have seen living shell-makers going about their lives, ambling across coral reefs or sitting still where they are and gently sifting seawater. I’ve marvelled at the bright colours of nudibranchs – seashells without shells – and often wondered why it is that I can’t stand slugs on land, but give them a lick of colour and drop them in the sea and they become quite adorable. On one occasion, strolling along a tropical beach, I foolishly picked up what I thought was an empty seashell and got pinched by the hermit crab inside – it wouldn’t let go, no matter how much I yelled at it. Now I’m a lot more wary of the animals that borrow shells.
I have also seen how people use shells and how they depend on them in many ways. In the hot, dry forests of giant baobabs in Madagascar, I found shells of African land snails (relatives of seashells) filled with rum and honey and left as offerings to the spirits of the forest. Many times I’ve walked through the miasma of a tropical fish market – in the Philippines, Thailand and Fiji – and seen piles of cockles, clams and other shellfish that offer a cheap source of protein for everyone. I have also witnessed the darker, elite side of shellfish. In remote fishing villages in Borneo, I saw the shrivelled meat from hundreds of illegally caught giant clams drying in the sun, destined for Asian markets where people pay top prices for these chewy delicacies.
At a fancy restaurant in Malaysia, I was offered a bowl of giant mangrove snails and had to politely decline, not because they were rare and threatened but because I couldn’t quite bring myself to wrestle them out of their shells and then swallow them down. But on other occasions I’ve enjoyed myself much more eating shellfish, perhaps most of all on England’s Norfolk coast, a few hours north from where I now live. Bags of fresh mussels are left on a table by the roadside in the village of Morston, where marshes with slurpy blue mud stretch out to the flat grey waters of the North Sea. We help ourselves and I lean in the window of the cottage, passing in a five-pound note. ‘My husband gathered them this morning,’ says the voice inside.
In my years of study and diving, I’ve also learned that there are masses of shelled animals living in the oceans that we could quite legitimately call seashells. On Cornish dives, I kept trying to bring back one of the empty, hollow sea urchin shells the size of grapefruits that I would often find lying on the sand, but every time it got broken on its journey to land. Crabs, lobsters and shrimp (including little cleaner shrimp on coral reefs and in tide pools that I’ve occasionally persuaded to give me a manicure) also have hardened, external shells. There are myriads of intricate sea creatures that spend their lives drifting with the currents. Most can only be seen with the aid of a microscope and they are known, generally, as plankton: foraminifera and coccolithophores sculpt chalky shells, some looking like snowflakes, others like bits of popcorn stuck together; diatoms and radiolarians make their shells from silicon and look like miniature glass Christmas tree ornaments, triangles, diamonds and stars. All of these living things have their own stories to tell and important roles in life on Earth, but this book is about just one particular group that, to my mind, are the greatest shell-makers of them all. These are the animals that go by the name of Mollusca – the molluscs.
CHAPTER ONE
Meet the Shell-makers
No matter where you are in the world, you will never be far from a mollusc. These are some of the most abundant, most cosmopolitan animals on the planet, not to mention their being among the toughest, smartest and strangest creatures ever to evolve. They include familiar creatures like snails and mussels, clams and squid, as well as lesser-known varieties like chitons and nautiluses and argonauts. Molluscs (known in America as mollusks) are the animals that make seashells although, admittedly, not all of them do. Shell-free varieties exist, including octopuses and slugs as well as animals you’d be forgiven for thinking were shiny little worms. But the great majority of molluscs produce a shell of some kind. To tell tales of seashells we need to begin here, with the story of the molluscs.
No one knows exactly how many molluscs there are in total. Often-quoted numbers range from 50,000 to 100,000 known, named living species. The reason we don’t know for sure is because there hasn’t been a single mollusc catalogue. To name a new species, all you need do is publish a peer-reviewed paper describing it, showing why you believe it is new and hasn’t previously been named, then deposit a specimen – the type – somewhere that other people can go and look at it, usually a museum. You don’t have to inform some grand master of molluscs that you’ve found a new one, but simply add your piece of knowledge to the sprawling mountains of academic literature. And with many tens of thousands of species – including a number that have accidentally been named more than once – it’s no wonder things have got rather out of hand. That’s now changing, with the launch in 2014 of MolluscaBase, an online repository for mollusc species. It’s a gargantuan effort led by teams of malacologists – the people who study molluscs – and together they are painstakingly sifting through the literature to compile a definitive mollusc list. Every year more species will be added as other malacologists venture out and delve ever deeper into the world of molluscs. Because the truth is, if you want to find new molluscs, all you really have to do is go and look for them.
Back in 1993, a group of marine biologists arrived on the Pacific island of New Caledonia with one thing on their minds. They planned to find as many molluscs as possible in one month. Led by Philippe Bouchet, from the Muséum national d’histoire naturelle in Paris, the team of scuba-divers spent a total of 400 person-hours rummaging through the deepest recesses of coral reefs in a lagoon on the island’s north-west coast. They hand-picked specimens, brushed stones, cracked open solid chunks of dead coral, and even used a waterproof vacuum cleaner to carefully slurp up the tiniest hidden gems.
By the end of their expedition, Bouchet and his co-workers had gathered an astonishing 127,652 seashells. Then the really hard work began. It took years for experts to sift through the collection and divide it into specimens that looked like distinct species (known as morphospecies) but weren’t yet fully identified. That would have taken even longer.
In total they found 2,738 morphospecies. That’s more than all the marine molluscs that live throughout the entire Mediterranean Sea, and almost four times more than the number found around the British coasts. And it’s a higher diversity than in similar areas of habitat studied anywhere in the tropics, the most species-rich regions of the planet.
It’s hard to say exactly how many of the New Caledonian molluscs were previously unknown species, because they weren’t all identified. However, the team estimates that within the most diverse groups as much as 80 per cent of their shell haul was new to science. A lot of the shells they found were incredibly rare. One in five were singleton shells, that the divers found only once. If they had carried on looking, Bouchet and the team think they might eventually have taken their tally to well over 3,000 and perhaps closer to 4,000 species. That’s potentially 4,000 species of molluscs, in one coral lagoon, on one tropical island.
Based on findings like these, many experts – Bouchet included – estimate that, including all the
species that haven’t yet been found, there could be 200,000 types of molluscs. Bear in mind that, currently, there are roughly 1.2 million known and named species on Earth and around 250,000 of them live in the sea. The only animal group more species-rich than the molluscs are the arthropods, a gaggle of invertebrates that includes crustaceans, spiders, millipedes, centipedes and the stupendously diverse insects; they alone clock in at around a million described species. Nevertheless, the molluscs take a highly respectable second place (especially as insects don’t live in the sea, with a few minor exceptions that dip their toes in saltwater). It means that the insects are missing out on at least 90 per cent of the living space on Earth (including all the vast three-dimensional space that’s available in the open oceans, from the waves down to the deep seabed), which strikes me as a bit of an oversight.
There is undoubtedly a vast number of molluscs living in shallow tropical seas, but if we wanted to track down all the different varieties in the world we would need to visit a lot of other places, too. Molluscs first evolved in the ocean at least half a billion years ago, and since then they’ve moved into just about every available habitat beneath the waves and beyond. To find the very deepest marine molluscs we need to venture into the hadal zone, six kilometres (four miles) down, a place named after the underworld Hades of Greek mythology. This truly is a realm of fire and brimstone, and one of the most hostile parts of the planet. It’s there, along cracks in the Earth’s crust, that hydrothermal vents spew scorching, corrosive water from the deep beneath the sea floor, forming tall chimneys called black smokers. The only thing stopping the water from boiling is the crushing pressure. These weird, alien landscapes were first discovered in 1977 by researchers aboard the submarine Alvin, exploring the deep sea off the Galápagos Islands. They weren’t expecting to find anything living down there at all, but in fact they saw luxurious ecosystems, including masses of shell-making molluscs.
There are sea snails living on hydrothermal vents with spiralling shells the size of tennis balls, hundreds of them squeezed into every square metre of space. For food, down in the permanent dark cut off from sunlight, they rely on colonies of bacteria that grow inside their gills, and harness energy from sulphur compounds in the water. A recent study split these snails into five species, based on differences in their DNA. To look at, they’re impossible to tell apart. One of the new species is Alviniconcha strummeri, named as a joint tribute to the research submarine and to Joe Strummer, the lead vocalist and guitarist of the British punk band The Clash. It was a nod to these hard-as-nails snails that live in the most acidic, most sulphur-ridden hydrothermal vents in the Pacific Ocean, close to the islands of Fiji. And like many of the band’s 1970s punk followers, the snails have spiky hairdos in the form of a bristly layer of protein known as the periostracum, which covers their shells.
Above the hydrothermal vents, there are molluscs that browse the vast oozy plains of the abyssal zone, and others clinging to the sides of undersea mountains. Not all molluscs in the sea are confined to the seabed, and many abandoned a creeping, sedentary life and swam off into open waters, the biggest living space on the planet. Then there are the molluscs that marched into the shallows at land’s edge, burrowing down into shifting mud and sand and clamping themselves to rocks. Some molluscs didn’t stop at the high-tide mark but kept on going, migrating into briny estuaries, then freshwater, and on into rivers, lakes and ponds. And a few intrepid travellers hauled their shells out onto dry land. There are molluscs that live in trees, up mountains, in sizzling deserts and in other seemingly unlikely places. A 2010 expedition into one of the world’s deepest cave systems, the Lukina Jama–Trojama caves in Croatia, discovered the transparent shells of minute, blind snails that live more than a kilometre beneath the Earth’s surface. Even the snails and slugs nibbling plants in our gardens are essentially ocean migrants, whose ancestors were molluscs that came from the sea. Just about the only thing molluscs haven’t managed to do is take to the skies (although there are some that hitch-hike across the globe, stuck to the feet of migrating birds or lodged alive inside their gizzards).
With so many molluscs living in so many places, this raises a question. What is the key to their success? In order to figure that out, there is first a bigger, deeper question to ponder: what exactly is a mollusc?
‘There is nothing quite like a mollusc,’ wrote Colin Tudge in his book The Variety of Life. They are indeed a peculiar bunch, but the tricky part is figuring out exactly which features molluscs have that make them unmistakably different from everything else.
The word ‘mollusc’ has been around since Aristotle’s time. He used it to refer to cuttlefish and octopus, and other soft things. The current use of the word seems to stem from the eighteenth-century Latin term molluscus, from mollis, meaning soft. However, going around poking animals is not much help in deciding what is and what is not a mollusc.
Over the years, ‘molluscs’ have included an assortment of animals, slung together because they superficially look alike. Barnacles were once labelled as molluscs and, admittedly, if you squint at them from a distance, they do look rather like little limpets (which are molluscs), but they are in fact crustaceans that have the unusual habit of sticking themselves to rocks, head down, with their legs waggling in the water. In the past, microscopic creatures called bryozoans (also known as moss animals) were bunched in with molluscs, but they have since been separated out. Brachiopods, or lamp shells, look a lot like molluscs. Their twinned shells could easily pass as cockles or clams, and yet the way they build their shells and arrange their soft innards is different enough to mean that they too have been assigned a separate group. Having a soft body and a hard hat is not enough for an animal to be considered a mollusc.
Nowadays, various animals are confidently grouped together in the phylum Mollusca, one of about 35 phyla that divide up the animal kingdom. Other phyla include the arthropods (all those fluttering, scuttling insects, crustaceans and so on), chordates (all the vertebrates plus a few strange cousins like sea squirts and pyrosomes) and echinoderms (starfish, sea urchins and sea cucumbers). The mollusc phylum is divided into eight living classes, with several more that are now extinct. By far the biggest contains the gastropods. Take a random pick of all the molluscs, and four times out of five you’ll get a gastropod of some sort. These are the ‘stomach feet’ creatures (from the Greek words gaster meaning stomach and podos meaning foot) because they generally creep around on a single foot with a mouth on the underside. Most of them live inside a spiralling shell and are known, quite loosely, as snails. The ones that have reduced or lost their shells are known, equally loosely, as slugs (various different groups of gastropods have at one time or another lost their shells, so the things we call slugs are not actually that closely related to each other). Gastropods have evolved to live throughout the seas, in rivers, lakes and ponds, and they are the only molluscs that made it out onto land.
Clams, mussels, oysters, scallops and all the other molluscs with shells in two parts belong to the next biggest class. The bivalves have twin shells, connected by a hinge, that can open up and clamp tightly together, fully enclosing the animal inside. They live in seas and freshwaters and, along with the gastropods, they make up the bulk of species that we consider to be seashells.
The other mollusc classes are less diverse, and all of them are confined to the seas. Cephalopods are the cuttlefishes, octopuses, nautiluses and various types of squid. The name ‘cephalopod’ stems from the fact that in place of a single foot, these animals have a highly developed head (in Greek kephale means head). Many cephalopods have abandoned shell life entirely, but some have retained their hard parts and put them, as we will see, to various good uses.
Scaphopods or tusk shells are fairly self-explanatory: they look like little tusks. Often they live buried in the seabed, head down, with the tips of their shells poking out.
A little-known group of shell-making molluscs are the monoplacophorans; there’s only a handful of species an
d they all live in the deep sea. From their shells alone, they could be mistaken for untwisted gastropods, but with multiple pairs of internal organs they are undoubtedly something far stranger. Monoplacophorans were thought to be extinct until 1952 when one came up in a dredge bucket off the coast of Costa Rica.
Chitons are a rather different class of mollusc. In place of a single or a twinned shell, they have a fringe of scales and a suit of overlapping armoured plates across their backs. You can find chitons clamped to rocks in tide pools and along the shore. They can be the size of fingernails or larger than your hand (along the coasts of the North Pacific, from California through Kamchatka to Japan, lives the biggest of them all, the Gumboot Chiton). And if they get knocked off their rock by a wave or an inquisitive human, chitons will roll up into a ball like an armadillo.
That leaves two groups of the most enigmatic molluscs, the solenogastres and caudofoveates (they are so obscure that no one has given them an easier common name). These creatures look more like worms than molluscs and none of them make shells. Instead they are covered in bristles, known as sclerites, that make them look shiny and furry.
There’s no doubt that these various molluscs – the slugs, snails, squid, scaphopods and the rest – all belong together in the same phylum; their shared DNA sequences show this undeniably to be the case. Even so, the core concept of what it means to be a mollusc remains deeply contentious.
A major problem is that no one can point to a part of a modern mollusc and confidently proclaim, See that thing right there, that’s what makes this a mollusc. If we look back into the past, down towards the base of the mollusc family tree, we should be able to see which characters have been around the longest and are therefore the most fundamentally molluscan, the things that define the group. But unfortunately, when we do that, things there aren’t quite so clear-cut either.