Will we feel the pinch of alien crabs?

Another Life: A few weeks ago I joined the growing legion of readers who have been engaged (and also disquieted) by a new novel called The Highest Tide.

Its author, Jim Lynch, lives near Puget Sound on the US's Pacific coast, and his hero, 13-year-old Miles O'Malley, is splendidly obsessed with life along that shore. Exploring the tideline, he comes across extraordinary things - sensationally, a stranded giant squid that gets him on TV, but also lesser aliens, not least "those new crabs with the hairy pinchers at Whiskey Point. I never saw them until about five weeks ago. Now they're everywhere out there. . . "

On a weekend in January a reader of this column, Jim Flinders, was fishing for codling on the Suir Estuary, Co Waterford, near the Belview container port, when he hauled in an unfamiliar crab, its sharp white pincers protruding from hairy muffs. He photographed it, went searching on the internet, and later received confirmation that he had caught the first Chinese mitten crab, Eriocheir sinensis, ever recorded in Ireland. What now remains to be seen is if "they're everywhere out there".

Of all the alien species to be spread around the world in ballast water, pumped out by merchant vessels in port, the crab is one of the most invasive, explosive and potentially disastrous ecologically, with a pattern of life that takes it far inland.

In Lynch's novel, the crab is burrowing holes into cliffs and collapsing holiday homes. Ireland could have other causes for regret. Along with riddling river banks with holes, a worst-case scenario would have the night-prowling crabs eating the eggs and food of native fish and attacking them as they sleep, evicting the native crayfish of the midlands, and extinguishing the EU-protected shad of the Suir and adjoining rivers.

An adult Eriocheir sinensis has a square, sandy-coloured carapace some five centimetres across or more, but it spends much of its busy life at a far smaller size. It hatches from an egg laid in the saltwater of an estuary - perhaps 900,000 from a single female - and spends some months there before using the tidal currents to begin travelling upstream, at thumbnail size, into fresh water.

It could be a long migration. In China, the crab's record is 1,400km (870 miles) up the Yangtze River. In Europe, it has reached Prague, some 750km (470 miles) from the sea. Given its habit of taking to the land if necessary, all the rivers and lakes of lowland Ireland are theoretically within reach. After a couple of years, with sexual maturity, it heads downstream again in the autumn, to breed in saltwater (a lifestyle called catadromous).

The mitten crab arrived in Europe at the German river Elbe in 1912, and is now at home in estuaries from Portugal to Sweden. Its initial invasions have been followed by cyclic population explosions, notably in Germany. In 1936, more than 21 million juveniles were caught on migration up five rivers. In 1998, in the Elbe, 850kg of migrants were caught in two hours. Taking to the land in these irruptions, they have pattered through streets and riverside houses.

Although the crab can tolerate pollution very well, cleaner rivers provide it with more living prey and thereby boost its numbers. In England, the first individuals were caught in the Thames in 1935, but it wasn't until the 1970s that numbers began to rise, showing up at the water intake screens of power stations. By the 1990s, in a sharp and continuing increase, the crabs had spread through half-a-dozen rivers joining the Thames Estuary, as well as to the Humber and possibly the Bristol Channel.

One Chinese mitten crab in the River Suir does not make an invasion or even a bridgehead. As pointed out by Dr Dan Minchin, the marine expert on aliens who confirmed the specimen's identity, single crabs have been caught in many ports throughout Europe where there are no established populations.

But there is now an urgent concern to monitor and manage the worst of Europe's alien invaders. A veteran of Ireland's efforts to deal with the inexorable spread of the freshwater zebra mussel, Dr Minchin is now helping the EU-funded Alarm programme with similar measures in north-east Spain. For another big European project, Daisie, he has mounted an inventory of alien marine species - some 70 in Ireland and 150 in Britain - for which the crab on the Suir is another ominous piece of data.

Does nothing eat the Chinese mitten crab? The Chinese do, for a start, and UK zoologists are among those urging efforts at a culinary control.

In Ireland, little egrets - snow-white alien waders now making themselves at home in the inlets of the south and east coasts - may have arrived from Europe just in time to help out native herons with a new estuarial prey.

[ elizabeth_sides@environ.ie ]