Are jellyfish plotting a maritime coup d'etat?

Another LifeThere's still plenty of time for jellyfish. Even as human footmarks grow fewer on the strand, a few glistening moon jellies, Aurelia aurita, drift in to catch an autumn sun. But even as the ocean goes on warming (only slowly, in the well-stirred northern Atlantic), we seem to have skipped a summer for plagues, hordes, invasions and other untoward assemblies of menacing gelatinous zooplankton, the Scyphozoa.

Yet jellies as a whole have never had so much attention from marine science - this because their populations are more prone to abundance than they used to be, and because warnings that overfishing is tipping ecosystems in their favour are sounding less and less fanciful.

The term "regime shift" now describes such ominous effects.

Jellyfish ecology has been a slow-growing study, not least because jellies tend to fall to bits when being hauled up in plankton nets. Only as submersible video cameras probe the deeps, as echo-sounding becomes more selective, and as aerial surveys are brought to bear, is their full role in the ocean becoming clear. Among siphonophores, for example (the colonial variety of jellyfish) the deadly-tentacled Portuguese man-o'-war, drifting at the surface is matched in deep water by fluorescent armadas of many other species trailing toxic fishing nets.

They are among the hungriest, most effective carnivores in the ocean.

The most dramatic examples of true jellyfish plagues have come, so far, from distant points on the planet. Among them, great blooms of the giant Nemopilema nomurai, two metres across and 200kg in weight, have been choking the nets of fishermen in the Sea of Japan. Off Namibia, in southwest Africa, in waters heavily fished for sardines and anchovies, the biomass of jellyfish is now estimated to exceed the total weight of commercial fish stocks.

Nearer home, all projections warn of an increase in jellyfish, especially in years when high-pressure zones centred on the Azores bring warmer water northwards. In the North Sea, the abundance of jellyfish has been correlated with that of herring larvae. Both compete for zooplankton, and if jellyfish are relieved of competition by overfishing, their own abundance will increase, feeding both on plankton and on the herring larvae.

Around Ireland, the most immediate threat from jellyfish has been to fish-farming. Enough stings from small jellies, fragments of big ones or even the planktonic larvae of moon jellies can cause serious losses of captive salmon and sea trout. Ireland is one of nine partners in the Eurogel project, investigating "key factors regulating the abundance and succession of jellyplankton species in European waters". A contribution from Irish consultants has been to test the effects of various jellyfish stings on salmon smolts.

Despite being one of the most intensively studied waters of the world, the first major research on jellyfish populations off Ireland has come from the Irish Sea Leatherback Turtle Project (www.turtle.ie), a collaboration by University of Wales Swansea and University College Cork. Jellyfish are the turtles' principal prey, and they make regular northward migrations to these islands in summer to feed on them.

Using shoreline surveys, counts from ships and from low-flying aircraft, the team plotted the distribution of five regular jellyfish species over 8,000sq km of the Celtic and Irish seas. They found that the different kinds - moon jellies, compass, lion's mane, blue Cyanea and barrel jellyfish - tend to have separate distributions and a different north-south mix of species. The moon jellies - almost predictably - stuck together (as it were) in a special abundance along the Leinster coast and that of southwest Wales.

The most intriguing result of all came from aerial surveys, which discovered three consistent coastal hot spots of the big barrel jellyfish (also called the "dustbin lid") - Rhizostoma octopus - the leatherback turtle's favourite prey. Extending at times over tens of square kilometres, and reaching densities of one per square metre, huge numbers of the jellyfish crowded Rosslare Harbour, Co Wexford, and the bays of Tremadog and Carmarthen on the coast of Wales. The Welsh bays were already known for sightings of feeding leatherbacks, and the three locations accounted for about one-fifth of the records.

Such aggregations of jellyfish have yet to be fully explained. The swimming abilities of Scyphozoa, by pulsing water from their bells, are mostly vertical, leaving them passively drifting for the most part, so that the ocean's currents, eddies and fronts are often what round them up into crowds. These can also be found in plankton-rich bays, the same winds and waves having carried them both there. Or they may - prompted by temperature - have been "hatched" together as new, miniature medusae, lifting from the seabed polyps that account for one phase of their lives.