In the summer of 2014, Philippe Bouchet led a team of mollusc-hunters to Nago, an island off the coast of Papua New Guinea, which lies in a coral-dotted lagoon stretching between the Bismarck Sea and the Pacific Ocean. This spot lies towards the eastern end of the Coral Triangle, the place where there are more marine species than anywhere else on the planet. Throughout years of field trips – in Vanuatu, Madagascar, the Philippines and elsewhere – Bouchet and colleagues from dozens of countries have been honing their collecting techniques. Divers venture out both day and night, so as not to miss the nocturnal species; they drop sampling devices down to varying depths where, like layers of a forest canopy, different assemblies of animals are found; they search between the tides; they even check among the spines of sea urchins and the tube feet of starfish for parasitic snails that suck the echinoderms’ bodily fluids. The teams have also begun taking snippets of tissue to preserve the animals’ DNA so species can be identified from their genetic fingerprint.
Using this suite of meticulous methods, the collectors are probing deeper than ever into the world of molluscs. In particular, they are uncovering an incalculable trove of micro-molluscs. These weeny animals are truly the secret gems of the sea. They come in a riot of exquisite colours, like a jar of jellybeans, with neon spots and stripes, yellow, purple, green and red. There are tiny clams with a sweep of pink tentacles sticking out, snails with glassy, transparent shells and kaleidoscopic mantles that show through from underneath, and bivalves that unfurl their colourful mantles over their shells and crawl about on their feet as if they were gastropods. Almost nothing is known about these animals. We don’t know what they eat, what eats them, what strange and useful molecules they might contain or where exactly they belong on the sprawling tree of molluscan life. There are so few experts who specialise in these minute species that many of the samples Bouchet and the team collect could remain for years on a museum shelf, found and logged but not fully identified. Taxonomists call these neglected species ‘orphans’. Even greater mysteries remain to be unravelled, hidden in oceanic nooks where molluscs reside but no one has yet worked out how to get hold of them.
Divers from Bouchet’s team went gathering molluscs along a vertical wall of coral that plunges into the Bismarck Sea off Nago, 1,000 metres beneath the waves. The wall is pockmarked with caves, inside which the divers found a tantalising array of previously unknown shells, all of them between one and five millimetres in size, and all of them empty and dead. No matter how hard they tried and how carefully they looked, they couldn’t find a single living mollusc responsible for making these enigmatic shells. The animals probably live deep within the cracks of this towering wall. The only reason we know they exist at all is because their shells drop down into the hands of the diving scientists. And we can only imagine what else lives in there, out of reach and out of sight.
There are undoubtedly many molluscs that will only be found by teams of experts with finely tuned searching skills and specialist equipment, but you don’t need scuba gear or microscopes or deep-diving submersible vehicles to have your own encounters with a host of curious shelled creatures. Next time you visit a beach or swim in the sea or even take a stroll somewhere a long way from the ocean, look out for the shells that are all around you. Then you can read the stories written into shells, the clues left here and there that tell you about the shell-maker’s life.
How big was your shell when it was a baby? Follow the spirals of a gastropod inwards towards its middle; the innermost whorls, the smoothest part often with an obvious line around the edge; this is the shell that the young snail wore when it first hatched. In some bivalves you can also spy a smooth inner part, right next to the hinge where the two halves of their shell fit together.
Which way does your shell coil? Hold it, tip pointing down, and see if you have a common right-coiler or perhaps it’s a rare sinistral specimen, one that may have found life difficult and sex an awkward, mismatched challenge.
The shape of your shell, its ornaments, crenulations, striations and spines, will tell you about its life; perhaps it is a flattened clam that lay on the seabed, or a screw-shaped gastropod that dug its way down. Or is it covered in prongs to lodge itself in the sand, to try to stop it from being swept away?
Take a close look at patterns drawn across shells, those notes-to-self written so they didn’t forget where they were in their shell-making efforts. Is there a point where the regular pattern goes awry? Did your shell get broken or attacked? Did it survive and keep on growing, eventually getting back in line and continuing with its elegant, decorated spiral?
Did your mollusc pick up any hitch-hikers, while it was alive or after it vacated its shell? You might spot barnacles or bryozoans (both formerly thought to be molluscs), or hydroids like tiny fir trees, or worms living inside white, spiralling tubes.
You might find gastropod shells with an elongated notch where a long siphon stuck out, probing and tasting the water in search of prey. These were the hunters, and you will also find their victims. Shells with neat holes drilled in them are testament to the evolution of so many molluscan ways of hunting and dining, and the fact that they’re not shy of eating each other. Some shells might have a ring etched in them but not quite a hole, the sign of an interrupted assault.
Once you’ve read these stories, you can either leave the shells behind or take a few home as a reminder of a day at the beach or a walk in the woods. And maybe you’ll be lucky enough to come face to face with shells that still have living occupants. Down at low tide you might spot dog whelks laying eggs like swollen grains of rice on the underside of boulders. In a rock pool you might catch a starfish attacking a limpet and getting its tube feet stamped on, or a pair of hermit crabs fighting over their shells. Perhaps, in shallow water, you’ll spy a scallop swimming past like a living castanet, a cockleshell hopping across the seabed or a razor clam swiftly and efficiently digging its way out of sight. And maybe you’ll find a sea snail, or a pond or land snail, and let it creep along your finger for a moment, watch it glide along its silvery trail with minute waves of its singular foot, before putting it carefully back where you found it.