Jellyfish and climate change

After the mid-November storms, the strand offered disappointing beachcombing - a tangle of weed from the offshore kelp forest, a few, bright plastic fishing-floats for Christmas decorations, but no turtles or mermaids' eggs or glossy West Indian beans. A year from now, goodness knows what botanical exotica will arrive, washed down to the sea from the jungles of Central America in the wake of Hurricane Mitch.

At Thallabawn's tideline, however, there was still a scattering of Pelagia noctiluca, the luminescent jellyfish which arrived on the coast, from Clare to the north of Donegal, by the million this autumn. Most were very small - many no bigger than a 20p coin - but their deep amber colour was intense and the long drifts gathered up by converging currents were enough to tint the water, as they did, for example, in Sligo Bay in mid-October. Washed ashore, they sometimes formed a carpet of marmalade jelly on the sand, several metres across. "Pelagia" marks the species as a creature of the wide ocean. One of my heroines in Irish science, Maude Jane Delap, scooped up some Pelagia that arrived in the harbour near her home on Valentia Island in 1903 and bred them in bubbling bell-jars. She discovered that this species, unlike those of inshore waters, produces offspring that grow into jellyfish without ever attaching to the seabed.

"Noctiluca" - literally "nightlight" - refers to the creature's luminosity. Slapped by a wave, it is said to flash spectacularly, and kept in a jar of sea-water overnight can be shaken (not stirred) into performing.

Less attractive is its capacity for stinging. The warty goosebumps on the bell are part of the stinging apparatus, but the stings that count are in the eight thread-like tentacles trailing from the edge of the bell. These are used to capture the animal's food, but have a toxic impact on humans as well. Some bathers in the Adriatic have suffered painful weals on their legs, as if they had been whipped, but a diver who swam among the jellyfish in Connemara reported only a reddish rash.

Real casualties of such "tides" of stinging jellyfish are salmon and other farmed fish, trapped in cages. Not only the smaller jellies, but fragments of tentacles can sting sufficiently to blind the fish, suffocate them, or panic them into dashing against the nets.

Salmon farmers in Norway have suffered heavy losses of fish from mass invasions by a variety of stinging species, either small enough to pass through the cage meshes or fragile enough to be broken into fragments. In April this year, a sea-trout farm in Shetland lost 89,000 fish when billions of larvae of the common moon jelly, Aurelia aurita, only a few millimetres in diameter, stung the trout as they passed through their gills: the fish virtually bled to death.

Such events seem to be growing more frequent, and their economic implications, if no other, make them of urgent interest to marine biologists. The Marine Institute's Dr Dan Minchin (at the Fisheries Research Centre, Abbotstown, Dublin 15) is collecting all the observations he can on the strandings of Pelagia. For about 50 years, until the end of the 1960s, it seems to have gone unnoticed along Irish coasts, and nowhere in the records is there a mass "invasion" by the species lasting over such a long period, from August to November.

It may - or may not - have something to do with global warming, or with changes in the patterns of the Atlantic currents. To the west of Ireland, immediately off the continental shelf, are three thick layers of water. At the bottom, below about 1,000 metres, is the dense, cold water being pumped south at abyssal depths from the Arctic towards the Equator. Above that, but below the top 600 metres of the North Atlantic Drift (the Gulf Stream), is a northward-moving, "Lusitanian" current, including water which has flowed out over the sill of the Mediterranean at Gibraltar. Each of these three currents has its own characteristic mix of plankton. Pelagia noctiluca is common in the Lusitanian water and reaches our coast through intermixing with the North Atlantic Drift.

A reminder of how ignorant we are about Ireland's offshore ecology is offered in the current issue of Technology Ireland, where Dr Mark Costello writes about the coral reefs discovered last year in the Porcupine Basin, at depths of 600 to 700 metres. We tend to think of the seabed in such deeps as a bleak and virtually lifeless desert, but the structures revealed by underwater video have a biodiversity as rich as their tropical counterparts.

More than 860 species have been recorded on similar reefs elsewhere in the north-east Atlantic and as many as 300 on single reefs off Norway, Shetland and the Bay of Biscay. These are striking figures for just one kind of habitat, when the total for the whole of Ireland's coastal marine environment, with a wide variety of different habitats, is about 6,000 species. Rising up from a seabed of sand and gravel, the reefs are formed by two kinds of coral, interwoven with the calcareous tubes of a worm, Eunice norvegicus. These intricate structures teem with settled life of the sort that inhabits our lower, sub-tidal shores, and fish, crustaceans and molluscs find shelter and resting grounds among the coral branches.

The workings of the reef food webs and their importance in the deep-sea ecosystem are still, as Mark Costello says, a matter of speculation. But the thought of such centuries-old structures being blindly smashed to rubble by bottom trawling, as has been happening for 30 years, is truly appalling.

Dr Costello dedicates his article "to the government marine engineer who believed there was nothing much living in the deep sea off Ireland." As marine co-ordinator of the big, EU-funded BioMar project which has mapped the different underwater habitats around the coast of Ireland, no one knows better how wrong this was. The article, with links to associated research and other projects, can be read at the website maintained by his Dublin company, Ecological Consultancy Services, www.ecoserve.ie/projects/coral