Shelling out for the colour purple

Another Life: Even an April shower can sting when it swoops in, slate-grey, from the islands with a northerly wind behind it. A trudge along the strand is best described as bracing, the waves steep, green and jostling, the snowy gulls quite black against the dazzle of the foam.

At this point in spring a recharged sun takes stock of the steady erosion of the dunes. You'd have to mark out the seaward margin with surveyor's stakes to keep track of sand lost in a winter's hiss of sand-grains: from year to year one's memory offers only a tenuous topography. Once, we buried a dolphin's head at a high spot in a dune and have been waiting ever since for delivery of the skull, duly polished. But where there was sand there is now a void, a polished space, and of the skull there is still no sign.

Without such erosion we should know far less about those centuries in which the first people lived along the western shore. Successive storms have exposed whole layers of seashells - kitchen middens of coastal hunter-gatherers who relied on eating shellfish for at least part of the time. But along with oysters, cockles and periwinkles, a tough little animal called the dog whelk points up an aspect of Irish Stone Age life that has nothing to do with seafood.

The dog whelk, Nucillus lapillus, is a predatory sea snail that lives by drilling holes in other shells, notably those of mussels and barnacles, and sucking out the contents. But it has another striking characteristic. A vein near its head releases a clear liquid which, when exposed to sunlight, turns first pale green, then blue and finally a reddish-purple - the purple dye, indeed, that we know from Mediterranean and Roman history as "imperial" or Tyrian.

I first came across the notion of an early Irish purple dye industry in the writings of archaeologist Françoise Henry who, some 60 years ago, found piles of smashed dog whelk shells in the Early Christian landscape of the Inishkea Islands off north Co Mayo. Purple dye, as she wrote, was used in decorating manuscripts in both Ireland and England from the eighth century onwards.

But purple's symbolism as a "royal" colour was current in Ireland many centuries before that, travelling with Phoenicians along trade routes from Sidon and Tyre, where the dye was manufactured from even juicier whelks. Did it, indeed, go back even further, to a use of dog-whelk purple among the skin-clad Mesolithic Irish, for personal decoration - even, perhaps, as a badge of rank in religious or magical rituals? The idea of an ancient Irish dye industry was born as long ago as the 1890s, when middens of sorted dog whelks were found at Dog's Bay, near Roundstone in Co Galway. But the Mesolithic possibility is developed in a fascinating and persuasive article in the spring issue of Archaeology Ireland by the Connemara archaeologist Michael Gibbons.

At Ferriter's Cove on the Dingle Peninsula, at the Late Mesolithic site so closely studied by Prof Peter Woodman and his team, there are, as Gibbons describes, shellfish middens that are almost all dog whelks, many of them crushed or broken at the apex: they don't conform to anyone's idea of a snack. And he has now found a new midden of "processed" dog whelks at the cove of Culfin in north Connemara, in a dramatic setting at the mouth of Killary Harbour. We can even offer him another one, on the Mayo side of the bay, at a cove in the dunes at the foot of Mweelrea Mountain, where a scattered midden of the broken shells has intrigued us for years.

That the mysterious power of purple was abroad in the Stone Age is supported by the smears of "reddish-purple ochre" around the heads of Mesolithic burials in Denmark and elsewhere. And, as Michael Gibbons concludes, such conjectures about the middens do at least "have the virtue of considering the Mesolithic person as something other than a mobile stomach . . .".

The history of Ireland's shellfish and shellfisheries is also the theme of Alive Alive O, the new book by Prof Noel Wilkins published (most attractively, with paintings by Anne Korff) by Tir Eolas at €15.99. Already an authority on the science of aquatic creatures and a hands-on pioneer in the development of Irish aquaculture, Wilkins provides an enthusiastic, informative and entertaining account, studded with seafood recipes that would qualify for Babette's Feast. It also reminds us how rapacious has been the exploitation of native shellfish resources, from the rape of the western oyster beds in the 19th century to more recent, ungoverned plunder of razor clams and sea urchins for the French market. Such "pitiless harvesting", in Wilkins's words, is still maintained by a relentless pursuit of new stocks and species and the use of fluidised dredging that leaves a churned up sub-marine wasteland in its wake. This ought to turn us all purple - with rage.