Behold the beautiful sea slug

ANOTHER LIFE:Since the advent of digital cameras and e-mail, rather fewer small packages arrive for Eye on Nature, some of them a little past their smell-by date if their contents came from the sea. But beachcombers' puzzles are still welcome, and one the other day (quite odourless, as it happens) was new and intriguing: half-curls of thick sandy ribbon about the width of old 35mm film - "collars" in some descriptions. They appear every summer, it seems, on the beautiful, blue-flag Marble Hill beach in north-west Donegal.

I guessed at the eggs of something and was guided by Dr Dan Minchin, our guru in these matters, to one of the ocean's odder coastal molluscs, the moon or necklace snail. Its family, the Naticidae, is found around the world, where its egg capsules, embedded in a jelly-like strip full of sand grains, "always arouse curiosity", as my mollusc book says. Of three species in these islands, this one was Natica alderi (also called Polinices pulchellus - don't ask). It's a carnivore that spends its life buried in the sand offshore, below 10 to 50 metres of water. Spherical, polished, flecked with brown like an Art Deco glaze, the moon snail's globular shell may not seem best shaped for dragging through sand in a predatory hunt for baby bivalves.

But when the snail has emerged from its aperture, it is actually bigger than the shell could contain. It pumps itself up with sea water, then uses its foot like a ploughshare to thrust into the sand, pulling its shell along after it in "locomotory waves".

The muscular foot envelops its prey and holds it while its nail-file of a radula rasps away at the shell to reach the flesh (the little striped-venus bivalve is a favourite victim). But also, and remarkably, the foot is sometimes deployed as a defence against starfish, hooding the shell completely in a mucus-slippery skin.

In all my beachcombing I've never found an empty moon snail shell, though the slow tides of summer do deliver the most delicate things, amazingly intact. Nothing could be more fragile, for example, than the empty shell of Janthina, the violet sea snail that drifts at the ocean's surface beneath a raft of bubbles (in the drawing), feeding on little jellyfish. Our two prize specimens are for looking at, not squeezing.

Ireland's coastal snails can be tiny, even smaller than the pink three-spotted cowrie, that is like a baby's fingerprint rolled up. Of our eight or so species of periwinkle, only one, the dark Littorina littorea is really big enough to eat (the brown ones get harvested - almost 3,000 tonnes a year, worth €2 million or more - but winkles also come in red, green, orange and yellow).

The rest range through colourful, conical top shells, the spiralling tower shell and the pelican's foot shell, to the large whelks of offshore water. Their stout, whitish shells often get washed ashore, especially at Ireland's south-eastern corner, but on Atlantic coasts they are more heavily armoured, against the grip of the big brown edible crab.

Along with snails, Ireland has more than 100 bivalves, some as small as a millimetre, living between the middle shore and 200m down.

Among the debris of seashells that tumble in to our tidelines, ark shells, mussels, fan and file shells, scallops and oysters live on the surface of the seabed or attached to rock, and the cockles, razors, venus, carpet, trough, otter, wedge and tellin shells are burrowers into sand, gravel or mud.

The oval and fragile sand-gaper, consumed by the million in America as the soft-shell clam, has siphons reaching up perhaps 40cm to the sediment surface. Cockles, on the other hand, in ribbed and globular shells, live only in the top few centimetres.

The ocean's gastropods include the sea slugs (or Nudibranchs), beside whose frequent glamour the terrestrial, lettuce-eating variety must bow their drab tentacles. The biggest of the 80 or so species around Ireland is about 20cm and this is the size of the sea hare (named for its head flaps) that, caught in a fishing net, squirts purple ink in protest.

But translucent and voluminous appendages of various shapes, often finger-like and tipped or spotted with brilliant colours, can give sea slugs bizarre and beautiful forms that have captivated marine scientists (as UK marine scientist TE Thompson suggested: "They are to the molluscs what the butterflies are to the arthropods, or the orchids to other flowering plants").

Like most soft-bodied ocean creatures, sea slugs rarely arrive ashore. But the Nudibranch pages of the Ulster Museum's encyclopedic underwater website at www.habitas.org.uk offer a striking gallery: try Coryphella gracilis, Doto fragilis, Eubranchus farrani or Thecacera pennigera and see how lovely a slug can be.