Another Life: Like Sunday morning in Temple Bar – ‘marine vomit’ and other visitors

The creatures of the seabed are not always attractive to the human eye, though many so-called worms can resemble the loveliest of flowers. Some organisms, indeed, might almost have been designed to offend the human aesthetic.

One of these has to be the tunicate Didemnum vexillum , a carpeting sea squirt quoted commonly in Wikipedia as "marine vomit" and elsewhere as "dripping candle wax". If I show, instead, the slightly more presentable Chinese mitten crab this does have passing relevance and avoids evoking a doorstep on Sunday morning in Temple Bar.

Such distaste is added to the remarkable alarm that this particular species of sea squirt has aroused from Japan and New Zealand to the US and Europe – and now including Ireland. It clings excessively to rocks, hangs massively from piers, pontoons and hulls, and smothers life on the seabed. Like the global numbers of jellyfish, it is seemingly out of control.

Within its glutinous colonies each squirt is a little barrel with a tunic of cellulose and an inside lined with mucus. The barrel has two siphons on top, one to suck in particles of plankton and the other, after filtering for food, to squirt out the waste.

Bizarrely (and excitingly to biologists) the short-lived larvae of the creatures are shaped like tadpoles, with gills, trunk and tail, and a backbone that adds them to the same evolutionary tree as people.

Ballast water
The spread of Didemnum vexillum , perhaps originally from Japanese waters, came from fragments dislodged from the hulls of ships and yachts or from larvae pumped out in ballast water. The species took time to pin down by name, but, apparently remote from its native predators, it has had dramatic impacts.

In the prime New England fishing ground of Georges Bank, for example, it has smothered more than 230sq km of seabed up to 60m deep, competing seriously for plankton food.

Its first recognised appearance in Ireland, in 2005, was as a distinctive and pendulous orangey beard on the hull of a yacht lifted for cleaning at Malahide marina, in north Co Dublin. Other enormous growths festooned the undersides of pontoons and mooring chains and smothered colonies of mussels.

A rapid survey was commissioned from Dr Dan Minchin, a veteran of the fight against the alien freshwater zebra mussel and otherwise expert on alien aquatic species. At other marinas, jetties and fish farms he found four new alien tunicates, among them Didemnum on hulls and pontoons in Carlingford Lough. It has also turned up on the west coast, trailing from oyster bags in Galway Bay and Clew Bay.

For the newest survey, just published (iti.ms/1op7Jmm), Minchin joined a Belfast marine biologist, Dr Julia Nunn. They examined 11 Northern Ireland marinas with a target list of more than 50 alien species, Didemnum high among them.

In Strangford Lough, where the pontoons of the Ballydorn marina are clustered at an old, red-painted lightship, beards of the sea squirt hung from its hull as profusely as at Malahide. On the floor of the lough it is beginning to grow over beds of native mussels.

Scraping or hosing it off, wrapping it in plastic sheeting, hoisting bags around it to douse it with a toxic chemical, stretching filter fabric over it: all have been tried internationally, but none yet with total success. Holyhead marina, in north Wales, spent €240,000 on a first round of treatment, and has sought a lot more than that for a second assault.

Meanwhile, the Northern Ireland survey has gathered a netful of new or expanding alien aquatic species, adding to well over 100 already on record.

Among them was Ireland's first fronds of wakame, Undaria pinnatifida , the sweet Japanese seaweed beloved of gourmet chefs. Like Didemnum , the report concludes, wakame can be expected to spread – if not, it must be hoped, on the scale of the Japanese wireweed Sargassum muticum , which now extends to every Irish coast.

Recent alien
Where does that leave another recent alien, the hairy-clawed Chinese mitten crab, Eriocheir sinensis , first reeled in by an angler fishing for codling on the Suir estuary of Co Waterford in January 2006?

It arrived, perhaps, with a vessel docking at Waterford container port, and others have been found since in lower reaches of the river.

Between rapid population explosions, the mitten crab can take time to spread – after almost a century in Britain it inhabits the River Thames and its tributaries and other rivers as far north as Yorkshire.

Even on its present showing in the Suir it is considered “established”, and Minchin has warned of its potential islandwide spread, burrowing destructively into riverbanks and attacking native species.