Another Life/Michael Viney:Among the marine bric-a-brac piled along our window sills, a couple of snail shells rest on a bed of cotton wool in a clear plastic box that once held posh chocolates.
They are much the same size as garden snails, but of a beautiful, deep violet, paling up to near-white, and of a translucent fragility: a pinch would crush them. We found them, empty but magically intact, on the strand below us some 16 autumns ago.
In the intervening years, Janthina janthina, this ocean drifter, or pelagic mesogastropod, has remained a comparatively rare trophy for Ireland's west-coast beachcombers. Shells, usually empty and part-broken, have mostly arrived in company with remnants of the snail's chief prey, the blue jellyfish, Velella velella, or by-the-wind sailor. The rainbow-tinted floats of Velella have also washed up on their own, sometimes by the million.
This autumn, however, in a startling first, it is the snails that have arrived in thousands on the strands of the north-west, sometimes alive and washed ashore still chewing on the jellyfish. If that were not enough of a novelty, the snails are of a species of Janthina scarcely known in Irish waters, and most of the jellyfish have their sails mounted the wrong way round. Something very strange has been going on in the mid-Atlantic.
The lives of these animals are already strange enough, starting with their means of travelling the ocean surface. Janthina floats upside down (as we would see it), suspended from a little raft of silver bubbles. It makes this by lifting its foot above the water to trap morsels of the wind in a film of mucus that hardens into something like cellophane.
The luscious colour of its shell has a definite function, helping to hide it from gulls looking down on a deep blue sea, where the bigger, violet whorls are good camouflage, and from squid looking up to the surface, where the shell's paler whorls merge with the liquid mirror of the sky.
The snail's carnivorous encounters with its prey are a matter of amazing chance: they have to bump into each other, like toy boats on a park pond.
Velella is not strictly a jellyfish, but a siphonophore - a colonial assembly like the great stinging Portuguese man-of-war, but scaled right down in size. Its jelly bits are deep blue, but what usually survives at the tideline is an oval disc a few centimetres across, with a little plastic-like flap of an iridescent, vertical sail set diagonally across it.
Caught by the wind, the sail keeps Velella moving to intercept fresh food. But on some it is set northwest-southeast across the float (left-sailing); on others, northeast-southwest (right-sailing). Both kinds occur in all the big oceans, where each drifts off on a different path, veering as much as 60 degrees from the wind's direction - an ingenious evolution of dispersal. In the Atlantic, it's mostly the left-sailing Velella that eventually collides with our shores. On America's Pacific coast and elsewhere, Velella are predominantly right-sailers.
Most of this autumn's arrivals, delivered by northwest winds to Donegal, Sligo and Mayo, are, surprisingly, right-sailers. The other big surprise is that their predators have switched species. All but a handful of the shells recovered so far are not the regular Janthina janthina but the paler, elongated Janthina globosa, a variation known worldwide and offered in shell-collectors' shops in the Bahamas or Hawaii but rarely recorded on an Irish shore.
The first snail flotillas, at the end of August, were discovered on Dunmoran Strand, Co Sligo by Dr John Mark Dick, the local GP and a regular beachcomber. Many animals were alive beneath their bubble floats and some floats even sheltered a layer of freshly-spawned eggs (see photograph).
Other snails were still firmly attached to the blue-jelly remnants of their prey. Dr Dick alerted Dr Don Cotton, the ecologist who teaches in the Science Department at the Sligo Institute of Technology.
A second wave of snails arrived earlier this month. The collection of shells from both Sligo and Donegal reached more than 1,000, and Dr Cotton travelled the shores of Mayo to bring the total number of strands sprinkled with violet shells to more than 40. Kerry marine biologist Dr Kevin Flannery added further armadas from encounters at sea some 200 kilometres west of Dingle.
Having identified Janthina globosa, Dr Cotton is now set on discovering their origins in the wide Atlantic. A chance encounter while gathering shells on a Donegal strand led him to Dr Stuart Baird, a Scottish expert on animal genetics and distribution of species, based in the University of Montpellier in the south of France. A match with DNA of specimens held in museums and marine centres in Europe or the US might begin to unravel the mysterious voyage of the new snails and jellyfish.
And that, of course, may have something to tell about climate-driven changes in the swirl and thrust of the North Atlantic Drift, if not of the Gulf Stream itself.
Eye On Nature
I was fishing on the rocks near Killary Harbour, when an otter
came along about three metres below me. On spotting me, it rolled
on its side to get a better look before slowly diving under the
water.
Bruce Vaughan, Kinadoohy, Louisburgh, Co Mayo
Early in September, close to the summit of Lughdubh near
Glendalough, I heard a loud noise coming from a small reed bed,
like someone ripping a tough cloth. It was a large blue dragonfly
attacking an equally large brown one which had yellow body stripes.
Or could it have been a mating ritual?
David Nolan, Santry, Dublin 9
It was the mating of one of the hawker dragonflies, most likely the migrant hawker, which flies in August and September. The male is blue and the female brown with yellow spots.
When fishing about 15 km out to sea off Skerries, my neighbour
saw a large basking shark, some nine metres long, and three others.
There was a large shoal of plankton in the sea.
Patricia Glynn, Glasnevin, Dublin 11
• Michael Viney welcomes observations at Thallabawn, Carrowniskey PO, Westport, Co Mayo; e-mail: viney@anu.ie. Include a postal address