Attack of the killer walnuts

Italians are in a lather over the arrival of the fish-destroying Mnemiopsis leidyi, a tentacled comb jelly whose less threatening nickname, "sea walnut", belies its hearty appetite for fish eggs, larvae and crustaceans.

Sea walnuts can drift in vast blooms, and no larger animal (it can grow up to six inches in diameter) is thought to be more numerous. Swimmers, at least, are safe, as these beasties' sticky tentacles are harmless to humans. However, sea walnuts are known to have a terrible impact on other fish populations: a couple of years ago, leading Italian jellyfish expert Prof Ferdinando Boero described their impact in the Black Sea as "devastating". Knowing this, Italians are understandably nervous about the potential effects of sea walnuts on their own marine life.

The Italian "Look out for the jellyfish" (Occhio alla medusa) campaign has distributed leaflets (see left) detailing different species of jellyfish, urging keen-eyed beach-goers to e-mail Prof Boero with their sightings. This should enable scientists to track the movements of the walnuts. They were sighted in recent months on the coast of Israel, but lately the jellies have been drifting towards the Mediterrannean.

It's not clear what "Looking out for the jellyfish" can do to prevent damage to fish populations on the Italian coast. There is no safe way to reduce the spread and impact of the sea walnuts, and their spread suggests they have adapted to live in a range of sea temperatures. Says Prof Boero of the most recently spotted sea walnuts: "This is the first sighting in the eastern zone: and if it's here, it means that it is everywhere."