BOSTON – Geologist Page Valentine stared in disbelief at video screens aboard the research vessel Delaware. He expected to see a gravel-covered section of ocean floor. Instead, he saw an underwater invader, spreading in sheets over one of New England’s most fertile fishing grounds.

Colonies of an unknown species of sea squirt have infested about 6.5 square miles of ocean bottom in Georges Bank about 160 miles off Cape Cod, and they’re expanding.

Valentine saw the sea squirts in the same area a year before his trip last month – made to monitor the effects of fishing on ocean habitats. But it wasn’t like this.

The cream-colored vertebrate covered large areas like a mat, sometimes in layers, potentially choking off bottom dwellers such as scallops or killing the sea life on which fish feed.

“It covers everything on the bottom,” Valentine said. “The question is, ‘Does it make fish food unavailable?'”

Sea squirts are tunicates, which have a primitive spinal cord and a firm, flexible outer covering called a “tunic.” The animal resembles something out of a horror film when it hangs from hard surfaces, but looks more like “pancake batter” when it groups on the sea floor, said Larry Harris, a University of New Hampshire zoologist.

Sea squirts never have been found before in waters of this depth. Scientists still don’t know exactly what species it is, how harmful it is to marine life or how far it has spread.

If the answers to those questions indicate an urgent problem, it’s “potentially going to cause some problems managing the fisheries of Georges Bank,” Harris said.

Scientist speculate the sea squirt could be native to the West Coast, or be the same species that caused problems in coastal waters off New Zealand. Its move to the region – perhaps in a ship’s ballast water – introduced it to an environment with fewer of the diseases, predators or parasites that keep it in check. The absence of those checks is what makes nonnative species dangerous to new habitats.

Harris said this sea squirt was seen near Casco Bay in Maine as far back as 1988. More recently, it was discovered close to shore off Cape Cod in 2000, then later near Portsmouth, N.H., he said.

He added it’s not unusual for an invasive species to take time adjusting to a local environment, then suddenly explode.

“You know it’s going to spread,” Harris said. “It’s established now.”

In New Zealand, the sea squirt was attacked aggressively after it took hold in coastal areas. But since these sea squirt are offshore, they would be much tougher to eradicate, said Judy Pederson, a scientist with the MIT Sea Grant program.

This species reproduces with striking efficiency, either sexually or asexually after fragments break off and attach to hard surfaces. Scientists worry that scallop dredges and fishing nets in Georges Bank could spread the sea squirt by stirring up the surface and distributing the animal far and wide. Then again, Harris added, keeping the ocean floor active may stop the sea squirt from taking hold.

Jim Kendall, a longtime New Bedford scalloper who’s now an industry consultant, said scientists and fishermen should learn as much as they can about the sea squirt before making “rash” decisions. He worried their spread could be used as an excuse to needlessly close more fishing areas.

Sometimes, he said, the best way to deal with this kind of problem is leave it to nature to clean up.

“That [ocean] bottom has a way of being dynamic and changing itself in a matter of minutes and hours,” Kendall said. “[Mother Nature] has a way of making her own changes.”