Tenacious Sea Squirt Invades North American Waters

Scientists warn that an aggressive sea squirt could wreak ecological havoc to marine environments and economically harm the aquaculture and fishing industries.

The sea squirt expands rapidly into large colonies, smothering everything it anchors to and killing shellfish, crustaceans and other sea life.

No-one knows where this ascidian originated from. Indeed, taxonomists haven't yet decided what species it is, assigning it only to the genus Didemnum. The little information available about its ecology indicates this highly adaptable and successfully competitive organism could soon cover massive expanses of docks, rocks and gravel along temperate North America's east and west coasts.

The primitive vertebrate thrives in many different habitats. It's been found growing everywhere from intertidal shores to 80 m underwater and in temperatures of -2 to 24 °C.

The sea squirt was first confirmed on the east coast in 1988 and now ranges from Maine to New York state including several offshore banks. On the west coast the sea squirt appeared at San Francisco Bay in 1993 and had since showed up elsewhere in California, in Washington's Puget Sound and by 2003 reached southern British Columbia.

Didemnum colonies brooding mature larvae were observed in Washington and BC during late 2004. But asexual reproduction where long, limp lobes break off of mats probably contributes most to its dispersal. When fouled oyster strings were pulled out of the sea at a British Columbian oyster farm, large chunks of the ascidian dropped off. These were thrown back into the water, likely enabling the sea squirt to colonize natural surfaces nearby.

Colonies grow into dense mats, covering 50 to 90% of the ocean bottom over large areas. By smothering the seafloor, Didemnum diminishes benthic life and may eliminate food for bottom-feeding fish. Scallops, anemones, sponges and anything else living on the substrate simply become overgrown and overpowered by the invading mat.

While sea stars have been photographed underwater feeding on Didemnum, the sea squirt could be well-equipped to deter other predators. Its acidic surface, at pH of 3 or less, probably protects it from many fish. The newcomer may also harbour potently toxic defence chemicals, as do other members of its genus.