The dense, drizzly coastal fog that is often our share of summer high pressure enveloped the Thallabawn hillside for three days, leaving every leaf glistening with water, every flower stem bowed with the weight of it. Such perfect conditions for slug and snail activity brought obvious thoughts to a gardener, but the naturalist in me had other concerns. As, for example, whether Cepaea nemoralis might still be "at it" down in the dunes, today mere shadows in the Labradorian gloom.
The mass mating of the sandy snails is more a phenomenon of soft days in late spring. Their celebratory orgy can then be so abundant that it's hard to know where to tread without trampling some entwined and frothy ecstasy. Much of it belongs to Helix aspersa, the so-called "garden" snail that here builds its shell from the powdered calcium of its cast-up ocean cousins – a truly economic recycling. But while its shell in our gardens is a glossy nut-brown, in the dunes it is often quite matt and blueish, this from its sand-blasted winter hibernation in the crevices of rocks and driftwood fences.
Cepaea nemoralis, on the other hand, its co-celebrant in these fertility rites, has spent the winter nestled in the moss or sand, so that only the top whorl or two of its shell has been bleached and worn to white by the constant sift of wind-blown grains. But the colour and pattern of the spiral-banded shell is notoriously variable.
As the fog dissipated, on the fourth day, I went on a foray across the channel and into the dunes, scattering the rabbits and nesting larks. There, indeed, were couples of Cepaea still in the tight lock of mating. A little delicate disturbance showed the undersides of the shells were yellow, when they might equally well have been pink or brown.
The sheer prettiness of these banded snails is enough to earn one's interest, But the distribution and variability of Cepaea has obsessed scientists for close on a century. Only last month, two biologists at Nottingham University, Dr Angus Davison and PhD student Adele Grindon, offered strong probability that the snails were brought to Ireland with Mesolithic travellers from the eastern Pyrenees, who used them as takeaway food.
There is a continuous fossil record of the snails in Ireland for at least 8,000 years, and this new study of the genetic history ("mitochondrial phylogenies") of living Cepaea populations across the whole of Europe showed most of those of the Irish west coast to be quite different from those, say, in Britain.
Their European match was found only in a restricted region of the eastern Pyrenees, with a few scattered examples from near Toulouse in southwest France and the Isle of Man, and a single specimen from Wales.
The resulting research is confidently titled: "Irish Cepaea nemoralis land snails have a cryptic Franco-Iberian origin that is most easily explained by the movements of Mesolithic humans".
(plosone.org/article/info:doi/10.1371/
journal.pone.0065792 ).
It offers yet more evidence from DNA detective work that many early arrivals of migrant life in Ireland, whether human, plant, molluscan or mammal, came from Iberia rather than across the Irish Sea.
Back in the 1950s, Cepaea's local permutations of shell banding and colour prompted a quite acrimonious debate among biologists studying evolution. Were the differences the result of random "genetic drift" over the centuries, or of natural selection? One powerful argument, backed up by experiment, for the influence of selection was that the snails' main predator – song thrushes – were less likely to seize upon snails whose shell colour most matched their background vegetation.
Anti-clockwise spiral
But Cepaea has been good for yet another twist – this quite literally, in sometimes spiralling anti-clockwise instead of the right-hand turn more usual in helical snails. In the 1920s, two British conchologists, Prof Arthur Boycott and Capt Cyril Diver, made an expedition to Bundoran, Co Donegal, to hunt for sinistral (left-handed) Cepaea. They had noted that many such specimens in British and Irish museum collections had come from the Finner dunes, where "old peasant women" collected empty shells by the thousand to make twopenny necklaces for the tourists.
The two men stalked the dunes, where the wind had sieved out shells from the sandhills and heaped them together in the slacks. In one small area "the size of a tennis-lawn" there were about 200,000. They spent a morning going through 8,000 of them and the 6,021st was sinistral – the only one. Then they found a live left-handed Cepaea on the roadside outside Ballyshannon -–but whether they crossed it with a dextral partner and what the result was, I cannot say. When last heard of, they were seeking collaborators in a further systematic scrutiny of shells, "aiming at numbers of the order of 100,000 or even half a million".