Another LifeThe idea that those masses of jellyfish chose to "attack" the unfortunate salmon in their cages off the Glens of Antrim last month made striking headlines here and abroad, writes Michael Viney
This was factually a nonsense, of course, however it looked to awestruck workers trying to get to the nets through enormous, pulsing swarms of scyphozoa, or to the salmon company - Northern Ireland's only one - facing into a million-pound loss. What rounded up the jellies into such huge, dense masses - 10m deep and several kilometres across - was ultimately down to currents, winds and tides.
A natural phenomenon, therefore, but still urgently intriguing: even, perhaps, deeply ominous, given such a rare invasion of this species in such overwhelming numbers, at a time when so many of their global family are exploding in population. Pelagia noctiluca may not be the biggest stinger in the ocean, but the venom in the taser-like "darts" it fires from its eight thread-like tentacles (automatically, when brushed against) is quite powerful enough to disable the gills of salmon. What it normally hauls into its mouth is mostly the minutiae of zooplankton and crustacea, with a few small fish thrown in.
To call it the purple or mauve stinger - both are common names - can be puzzling, since only mature adults, all of 10cm across, have the rich colouration of the photograph (right, by Richard Lord of Guernsey). When younger animals - as small as a 20p coin - are washed ashore in millions, it can leave a strand carpeted with what looks very like marmalade. Off Glenarm, the sea was "red" with them.
Pelagia noctiluca translates as an ocean-going (pelagic) organism that glows in the dark. Slapped by a wave, it is said to flash spectacularly, and, kept in a jar of seawater overnight, can be shaken (not stirred) into performing. What makes it different from most jellyfish is that its young grow from eggs in the water without the need to spend time in a polyp moored on the seafloor.
It's also normally a warm-water species. Indeed, it first began to make news in the 1990s, when drifting swarms of Pelagia were stinging bathers in the Mediterranean. Their densities, too, could be spectacular - up to 600 animals per cubic metre when currents pressed them near the shore. Such massive blooms seemed to follow a definite cyclic pattern. Sifting through more than a century of records, marine scientists found "Pelagia years" in the western Mediterranean occurring about every 12 years and coinciding with dry, hot summers. This year seems to have been one of them.
The last big invasion of Irish waters was in the autumn of 1998, when millions of the jellyfish stranded along the west. The Sligo ecologist Don Cotton then estimated half a million on one short strand in Mullaghmore Bay alone. He found similar numbers on Streedagh Strand last month, and has collected records beginning in mid-October, when the jellyfish washed up in tens of thousands at Keel Strand on Achill Island. They also swirled around a fish farm at Killary Harbour, but while some salmon showed lesions, none died.
Pelagia has long been recognised as a characteric species of the Lusitanian current, flowing northwards past the west of Ireland and Scotland from vaguely Iberian origins and including water that has flowed out over the sill of the Mediterranean. Some of the Med's very salty water does, indeed, press north as a deep undercurrent as far as the Porcupine Bank, southwest of Ireland. But the northerly flow past the west of Ireland and Scotland is chiefly a current of warm near-surface water from the Bay of Biscay and north Spain. It follows the edge of the continental shelf, speeds up noticeably on its journey north, and is mixed to the surface in winter's rougher seas.
Pelagia can live six months, but the millions reaching Ireland and western Scotland in November were juveniles, leaving their exact source a mystery. Dr Cotton points to weeks of southerly winds across the west of Europe in late September and early October. It was almost certainly winds - first westerly, then northerly - that helped to carry the jellies round the corner of Northern Ireland, and the strong tidal currents at that end of the Irish Sea that herded them together in such numbers. While Pelagia can certainly swim - if a largely upward pulsing can be called as much - and while jellyfish may exercise more togetherness than science has so far allowed, they make unlikely strategists.
This autumn's event will be studied closely by the scientists of Eurogel, the nine-nation project investigating "key factors regulating the abundance and succession of jellyplankton species in European waters". A contribution from Irish consultants has been tests on salmon smolts with different jellyfish species. Pelagia noctiluca was already high on the list.