WOODS HOLE, Mass. - A mysterious new breed of sea squirt found recently in waters in New England and the Pacific Northwest has spread unabated along those coasts and in a vital offshore fishing ground, scientists said on Friday at the first international conference on the issue.

The species, called Didemnum, has been found from Maine to New York and from northern California to Washington, according to scientists attending the conference at the Woods Hole Oceanographic Institution. The colonies are multiplying, and scientists, unsure how to stem their spread, are increasingly worried they could overwhelm valuable shellfish beds, including oysters, clams, mussels and scallops.

"It's ecological roulette. That's what we're trying to prevent here," said James Carlton, the conference's keynote speaker and a marine ecologist at Williams College in Williamstown, Mass., and at Mystic Seaport in Mystic, Conn.

Sea squirts are tunicates, invertebrates with a firm, rubbery outer covering called a "tunic." They come in all colors and attach themselves to hard surfaces, such as rocky sea bottoms or moorings in a harbor. Scientists have identified about 3,000 species of sea squirts worldwide. Most are not considered threats because they remain confined to a certain area and don't proliferate.

But certain species appear to be mobile, and can be transported to new areas, where they colonize and multiply. Scientists don't know how to stop those species from spreading, and as yet have been unable to figure out why certain species are invading other areas.

One of those species is Didemnum, which was first found in Cape Cod in 1993. Scientists began tracking it in 1998, and Didemnum is now found in half the bays and harbors on Cape Cod, said Mary Carman, a Woods Hole scientist who is seeking funding to study ways to kill sea squirts without killing shellfish.

Didemnum generally covers all in its path, "sort of like astroturfing the sea floor," according to Carlton. The mats choke off bottom-dwellers such as shellfish and may cover grounds needed by fish to lay eggs, scientists say. The spread has alarmed the shellfish industry, which has just recently contacted the scientific community for help.

Scientists are puzzled how Didemnum arrived in U.S. waters. Carman said they may have come from Japan, which, along with the Netherlands and New Zealand, have colonies. They also don't know what conditions allow it to thrive, though some believe waters rich with nitrogen from runoff and other pollutants may be a factor.

Scientists do know it is hardy, and adaptable. Studies have shown a colony can be torn, and pieces can plant itself in another area and regenerate. Also, it can live in waters with a wide fluctuation in temperature.

"Once they're here," Carlton said, "they're extremely difficult to manage and remove."