Over 500 million years molluscs have become one of the most diverse and adaptable groups on Earth. Helen Scales reveals the secrets of their success.
A visit to the seaside wouldn’t be the same if we didn’t pause to admire the delicate spirals, curves and patterns of our most common shells.
But what we may not realise when we handle a limpet, mussel, winkle or cowrie is that they represent one of the oldest, most diverse and most successful group of animals on the planet: the molluscs.
Though molluscs are perhaps best known from our beaches and back gardens, they occur everywhere from ponds and treetops to mountains and deep-sea trenches.
In fact the only habitat these creatures have yet to conquer is the sky.
What are molluscs and why are they called molluscs?
The term mollusc comes from the Latin word mollis, meaning soft. Over the years scientists have wrangled over which soft animals are molluscs. Barnacles, bryozoans and brachiopods are among the former molluscs that have since been assigned to other groups.
Naming the true molluscs remains problematic and the group is rife with synonyms – single species named several times by different people. In fact a recent survey revealed that the most over-named species in the world is Littorina saxatilis, the rough periwinkle. This sea snail is common on rocky British shores and has accumulated more than 100 scientific names.
Scientists are now cutting through the confusion to compile a definitive list for a new database named MolluscaBase. Many new species are being added, but not fast enough. Countless specimens are stuck on museum shelves without full identification because the world just doesn’t have enough mollusc expert
How many species of molluscs are there?
Their diversity of habitat is matched by diversity of form – exact numbers are unclear, but there could be as many as 200,000 species alive today, from the humble common snail to the fantastic vampire squid (above), with its jet-black body, giant red eyes and lights on the tips of its arms.
When did molluscs evolve?
The mollusc story began in the seas more than half a billion years ago with the ‘small shelly fossils’ – a mysterious jumble of minute marine animals from
the early Cambrian period, some of which looked remarkably similar to the clams and snails we know today.
A little later, during the ‘Cambrian Explosion’, a flurry of peculiar animals evolved, including several that could have been early molluscs. Among them were strange creatures in the genus Wiwaxia that resembled a cross between a slug and a porcupine. Recent studies suggest that these were indeed molluscs, or at least forerunners to the mollusc lineage.
How many types of molluscs are there?
Despite an enigmatic start, the three major mollusc groups that we know today had all clearly evolved by the end of the Cambrian (485 million years ago): the gastropods (the snails), with single spiralling shells; the bivalves (clams, mussels and the like), with two-part shells; and the cephalopods (octopus and squid), with prominent heads and sets of arms or tentacles.
Other, lesser-known mollusc groups that emerged around the same time include the chitons, equipped with body armour comprising a series of overlapping shell plates, and the tusk shells, resembling miniature elephant tusks.
Of the cephalopod group, octopuses and squids are the most abundant today. Most have evolved a shell-free life or have their shells on the inside, such as the cuttlefishes (exceptions include the argonauts, the only octopuses that make shells – the females secrete a papery carapace with web-like membranes on the ends of two arms). However, during the Ordovician period (485–443 million years ago), when life was still confined to the oceans, shelled cephalopods ruled.
The largest was the straight-shelled Cameroceras which was the longest seashell ever to exist, exceeding the length of a double-decker bus. These enormous creatures probably led a sedentary life on the seabed, shovelling up their prey with grabby tentacles, while their smaller cousins darted through the open water like javelins. Then, in the Devonian period (419–358 million years ago), a new band of cephalopods came on the scene, a group that would dominate the oceans for the next 330 million years. Enter the ammonites.
In contrast to their mainly pencil-straight ancestors, ammonites did lots of different things with their shells. Many were rolled into elegant spirals. Some morphed into strange and elaborate shapes, including spiny corkscrews and giant trombones.
Others tied themselves in knots. Research suggests that their shells may have enabled ammonites to swim rather than sink, thanks to internal, air-filled chambers resembling those of modern nautiluses. Nautiluses slowly adjust the gas levels in their shells in order to rise and fall through the water, and swim by jet propulsion, sucking in and squirting out sea water through a tube known as the siphon.
This shell-bound life was clearly an enormous success. Thousands of ammonite species existed worldwide, ranging from diminutive buttons to 3m giants. But the reign was not to last. The ammonites were wiped out 65.5 million years ago, in the same mass-extinction event that killed off the non-avian dinosaurs – despite the similarities, nautiluses descend not from ammonites but from another ancient cephalopod group, the nautiloids, which lived alongside their famous cousins. It remains a great mystery why the less diverse, less numerous nautiloids clung on while the triumphant ammonites died out.
Cephalopods may have hogged the molluscan limelight for millions of years, but today it’s the gastropods and bivalves that are the most diverse and widespread. Plenty still live in the sea, co-opting their various body parts as swimming aids.
Scallops clap their twin shells together to travel in short bursts, like animated castanets. Scarlet seaslugs (see below) power themselves along with their mantles, which ripple like a flamenco dancer’s skirts.
Sea butterflies swim using feet that have split into two little wings (for which they are known as pteropods, the ‘wing-feet’ creatures), flitting around and snaring food with sticky webs like underwater spiders.
Some gastropods can even surf – namely the olive snails, which use their broad feet to ride waves up and down the beaches of Costa Rica. Interestingly these molluscs also use their feet to trap prey. Like a cat-burglar sneaking around with a sack, they can convert their feet into a pouch in order to engulf a victim – often another olive snail – before swiftly smuggling it down into the sand.
What do mollusc look like?
The immense success of molluscs comes down, in part, to the way they build their bodies. Their basic anatomical toolkit – comprising a calcium carbonate shell (secreted by a layer of soft tissue known as the mantle), a single foot and a radula (a set of teeth) – has proved extremely adaptable, enabling the animals to evolve into an eye-popping array of shapes and sizes and adopt a huge variety of lifestyles, including ponderous herbivore, speedy predator, oceanic swimmer and earthbound crawler. Indeed, throughout their long history on Earth molluscs have been phenomenal shape-shifters.
Can all molluscs swim?
Not all marine molluscs are swimmers – the adaptable body plan also allows for a more sedate life. Rather than chasing after prey, most bivalves stay put and let the food come to them.
They have lost their teeth but instead have enlarged gills, which they use to breathe and also to gather small particles of waterborne food, a process known as filter-feeding. Many clams live buried in sand and mud, using their tubular siphons to suck in water from above.
Some of the largest individuals are the geoducks (pronounced ‘gooey-ducks’), native to North America’s west coast. Their colossal siphons give them their other name, the elephant-trunk clam, and enable them to nestle a couple of metres beneath the seabed.
Heart cockles don’t dig but lie around on the sand in the sunlit shallows near coral reefs. Colonies of microscopic algae live inside them, photosynthesising to produce sugars on which the cockles feed. In return the algae bask in a safe haven with plenty of sunshine pouring through the shell’s transparent windows. Heart cockleshells essentially act as greenhouses.
How do molluscs evade predators?
Slow-moving molluscs have also evolved various tactics to avoid becoming someone else’s meal. Satsuma snails amputate their own feet to distract predators, replacing the lost limbs not long afterwards.
Cowries cover their shells with fleshy mantles that are textured and coloured to mimic their surroundings – a seagrass bed, perhaps, or a coral colony. In contrast Australian clusterwinks do their best to stand out.
These unremarkable yellow snails glow bright green when disturbed by a predator. The warning light, emitted via two spots on the mantle and diffused by the translucent shell, acts like a burglar alarm, hopefully scaring the hunter away
Avoiding predation may also have driven the evolution of the most convoluted shells of all. For land-dwelling snails (gastropods are the only molluscs to evolve a non-aquatic life), the shell is a vital feature that prevents drying out; terrestrial slugs have no shells, or very small ones, and only survive in damp conditions. Shells can also provide added protection.
The limestone hills across South-East Asia are home to a species of snail no bigger than 1–2mm across. What it lacks in size it makes up for with the design of its shell, boasting a highly ornate, twisted carapace covered in ribs and spines that may simply be too hard for a predator to handle.
The variety of molluscs is mind-blowing, from giant clams weighing as much as two newborn elephants to miniature snails that burrow into the skin of starfish. New species are constantly being identified, revealing just how versatile the molluscan anatomy can be.
These include snails that cling to the hydrothermal vents that form at cracks in the seabed at depths of several thousand metres.
Tough shells protected by a bristly protein layer protect them from the extreme temperatures and caustic waters, and these spiky ‘hairdos’ inspired scientists to name the species Alviniconcha strummeri after the late Joe Strummer, the lead singer of the punk band The Clash.
How many other new species of mollusc are out there just waiting to be discovered by science is truly anyone’s guess