As if in some well-used holiday cottage by the sea, our window sills carry glass vessels – a wine jug here, an outsized goblet there – packed with seashells from the shore. They speak for years of permanently sandy pockets and beachcombing delight at the variety of empty little mollusc homes left gently above the tide.
A good few of them are those of limpets, sea snails whose lives may seem among the least eventful of any at the edge of the ocean. Long before I learned about their deeper mysteries it was straight aesthetic pleasure that set me collecting. I loved the range of colours: oranges, greens, ivory whites, even tortoiseshell in the rims, the fluted textures and variable shapes of the cones, the lickable porcelain glaze of their interiors. Few, I have to say, have outlasted that delicious, sunlit instant of discovery, immured as they are behind glass.
The half-dozen limpet species around our shores share the same grand design: a tough tepee of a shell, tilted a bit for headroom, ribbed against storms and rounded for least resistance to the waves. But their exact conformation depends a lot on where they live, studded on boulders or starring the shallows of rock pools.
The common limpet, Patella vulgata, highest on the shore, has the tallest pointed cone and the tightest adhesion to the rock. In such exposed conditions it must hold water inside its shell between tides. Clamped down into its "home scar", worn gradually by abrasion from the heavily ribbed shell, its immovable stickability is triggered by the merest touch. As early as the 18th century a naturalist hung weights on limpets to break their hold: it took 30lb, or more than 13kg.
This limpet is among the most studied of all marine animals, not least for its mysterious powers of navigation. It goes foraging for food once the tide is in, and can find its way home even after hours away, grazing seaweeds in darkness. Then it turns its shell around in its scar, like a dog in its basket, to find the perfect fit.
Aristotle was among the first to mention such excursions, but the limpet’s means of homing still evade certainty. They have also defied scientific dirty tricks, such turning the home boulder around with crowbars, to make the snail’s return more problematic. The secret seems to lie in chemical cues laid down in the mucus of its outgoing trail, mysterious molecules that have survived even scrubbing the rock with detergent while the limpet was away.
The latest marvel, however, has been discovered by engineers at Portsmouth University, in the UK. Led by Prof Asa Barber, they have found that the teeth in the limpet's radula, while less than a millimetre long, are the strongest biological material ever tested – stronger even than the notable tensile powers of the toughest spider silk, or even the man-made Kevlar fibres of bulletproof vests.
The limpet’s radula is a strip of serried teeth, pressed down by powerful muscles to harvest seaweed sporelings on the rock. A high density of limpets can keep intertidal rock ledges virtually bare of the plants. Their teeth, it seems, are laced with very thin, tightly packed fibres of an iron-based mineral called goethite. The sound of their rasping can actually be heard on still nights; a research team at Lough Hyne, in Co Cork, recorded the noise by sticking gramophone transducers on the limpets’ shells and listening through headphones.
Almost at the other end of limpet size is the beautiful and fragile-seeming blue-rayed limpet, Helcion pellucidum, which specialises by feeding on offshore kelp and arrives ashore in Ireland on storm-swept fronds of the plant. It is the size of a fingernail, thin and shallowly domed, and would, indeed, fit right into current fashions in false nails, as its translucent shell is striped with dotted lines of a bright azure blue.
This limpet, too, has featured in recent research, by engineering scientists at Harvard and Massachusetts Institute of Technology, in the US, led by Mathias Kolle. They identified optical structures within the limpet's shell that shine brilliantly when catching the light. Like the Portsmouth engineers' thoughts of stronger new materials, the Americans have technology in mind. The blue-rayed limpet offers first evidence of an animal using mineral structures to produce optical displays – a possible means, say, of lighting up a map in one's windscreen, or information in one's glasses.
We've come far from the limpets as bairnigh – basic, if chewy, coastal snacks, often despised. Séamas Mac an Iomaire, in The Shores of Connemara, quoted some old advice: "Avoid the drinking house, or the limpet will be your food." However, he went on, "don't mind them, a batch of limpets is a sweet, healthy food beside the hearth on a cold spring afternoon when one is hungry."