The compelling beauty of sea anemones

Another Life: The smell of low tide, as Rachel Carson described it once, is "compounded of the faint, pervasive smell of worms and snails and jellyfish and crabs - the sulphur smell of sponge, the iodine smell of rockweed, and the salt smell of the rime that glitters on the sun-dried rocks". In her lyrical chronicle, The Edge of the Sea, she spoke for the continuum of North Atlantic shore life that links Maine with Mayo: an astonishingly similar parade of species stretching downwards from the tideline to the depths of the temperate ocean.

Among her favourites, as among mine, were the "handsome chrysanthemums" of sea anemones. I remember my wonder at a first sight of snakelocks in a shadowed, crystal-clear basin at Cruit Island in Co Donegal. The tentacles of this big anemone, green with purple tips, can have the exotic shock of a Medusan fright-wig, but its beauty is compelling. Nothing, indeed, is more graphic of the ocean's gift of life than the transformation of an intertidal sea anemone as the water returns and swirls above it. From a mere blob of coloured jelly on the rock, it expands into a "flower" of often exceptional elegance.

That so many seabed animals have the shapes of plants or flowers was a major confusion in the early sorting out of species. And there are huge differences in appearance between groups of species that actually share a basic body plan. The sea anemone, for example, belongs to the phylum, or supergroup, of the Cnidaria. So do jellyfish, corals and hydroids, sea-fans and sea-pens. The big Portuguese Man-of-war and tiny, feathery growths encrusting the snail shells lived in by hermit crabs turn out to come from the same sub-folder of marine designs.

That stumblebum word Cnidaria (kinnidaria will do) comes from the Greek word for nettle - knide - and the fact that nettles sting suggests the first shared characteristic: most Cnidaria are armed with stinging capsules. What they all have in common, however, is a basic cup-shaped, radial body plan which has a ring of tentacles and a mouth-cum-anus in the middle (in marine biology, where food flows in and out with the water, an economy of orifices often makes functional sense).

To fisherman Seamas Mac an Iomaire, chronicling the shores of Connemara a century ago, sea anemones were "useless things". "The crabs don't even like them, and very little else can be said about them." But science says quite a lot.

The big surprise about anemones is not that they sting for food, firing coiled poison darts to pierce and entangle but that they sometimes use the same tactics against each other in a struggle for territory. Crowded together in an aquarium, snakelocks anemones attack each other with their tentacles to space themselves out.

Some species have other weapons, such as stinging threads discharged through the mouth, and wart-like beads that are batteries of stinging cells..

There are solitary anemones that move around by sliding very slowly or by inflating and floating in currents and these may use their stings to claim a feeding space on an underwater cliff or overhang. But many live in colonies, budding off clones of themselves to increase their patches. It's at the perimeter of a patch, in encounters with strange clones, that the battle of stings is joined.

"The wars of the clones are unending," wrote Prof Steve Jones, the British geneticist, in Almost Like a Whale, his recent update of Darwinian theory. "Each (clone) has its own personality. Some are aggressive, while others are calmer but respond at once to attack. Some clones do not fight back, but instead throw more soldiers into the front line as their members are killed. Colonies are able to settle only next to those against whom they have some chance in a fight. In time, a resentful truce emerges and battle starts only when a newcomer arrives." He cites such behaviour in a "brainless" society to show that all animals have a life beyond the simple prescriptions of their DNA. "A little dose of judgement or reason comes into play even in animals very low in the scale of nature." There are more than 30 kinds of anemone known in Irish waters today. Commonest is the beadlet anemone, the familiar red blob on rocks at low tide, an old, tough species that has lived in aquaria for more than 60 years. But snorkelling over the kelp forest should find sheets of the brilliant jewel anemone beneath its stems or even the beautiful dahlia anemone, with 160 fleshy tentacles, almost 20cm across. Yes, but do they sting people? Poke your finger in a snakelocks anemone and you'll find it grasped, but its tiny harpoons make little impression on all but most sensitive skins.