Spirals in Time: The Secret Life and Curious Afterlife of Seashells (3 page)

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 and 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.

How it all began

The oldest known fossil shells date from the Cambrian period, around 540 million years ago, with the so-called ‘small shelly fossils’. This collection of minute marine fossils crops up in various places around the world. Among them are puzzling tube-like creatures that might be sponges or
corals as well as masses of titchy shells, one or two millimetres (about one-sixteenth of an inch) long, that look rather like molluscs as we know them today. In the mix are shells with tightly twisted coils; some are conical like a Christmas elf’s hat and some have twin shells like a clam. Most palaeontologists agree that these must have been molluscs, although a few remain cautious, pointing out that, although we have their shells, these fossils don’t tell us enough about the animals that made them for us to be sure what they really were.

Alongside these tiny shelled creatures, a troupe of enigmatic unshelled animals were creeping across the Cambrian seabed. Following their discovery more than a century ago in the world’s most famous fossil site, academic arguments have raged over the identity of these strange animals and whether any of them were in fact the very earliest molluscs.

On 30 August 1909, American geologist Charles Doolittle Walcott was riding his horse alone in the Yoho National Park in the Canadian Rockies when he made a ground-breaking discovery. He was looking for fossil trilobites – ancient arthropods that looked like giant, ornate woodlice – but on that day he came across some very unusual fossils. Several months later, in a letter to a geologist friend, Walcott referred to these new fossils as ‘very interesting things’, which was putting it mildly. In the coming years, he returned to the same spot in the Rockies many times, travelling by railway, horse and foot and in total collecting 65,000 extraordinary fossils, the likes of which no one had ever seen before. The site came to be known as the Burgess Shale.