GREEN crabs have long been known for their bad attitude but a new mad-as-hell variant of the species is on the move and destroying everything in its path, scientists warn.

Surprisingly, the furious crustaceans are from famously friendly Canada and are better suited to the ice hockey rink than the calm shores of Maine, on America’s east coast, where large clusters have been found.

Green crabs from Nova Scotia are the same species as their Maine cousins but infinitely more bad tempered, threatening to accelerate harm to the coastal ecosystem by gobbling up soft-shelled clams and destroying native eel grass.

The docile green crabs shrink from a threat, while the newcomers are more apt to wave their pincers and charge.

“What we’re seeing is this insane level of aggressiveness,” University of New England Professor Markus Frederich told AP.

The angry crab variant originated in northern Europe and is hardier and better suited to colder water than their gentle cousins, which originally came from southern Europe.

Green crabs, even the docile ones, are considered a scourge that can devour soft-shelled and juvenile clams. They can destroy eelgrass that provides a hiding place for juvenile sea creatures.

But the Canadian crabs take it to a new level.

Louis Logan, a University of New England graduate student, had the unpleasant task of labelling the crabs captured from Nova Scotia waters for the research.

The crabs were in no mood for games.

At a distance of 1.5m, the pint-sized brutes, which measure 10 to 12cm across, assumed a fighting posture. Those that grabbed him were in no hurry to let go.

“Any time I went down to grab one they went to grab me instead,” he wrote in an email.

One of them, in particular, would jump out of the water in its frenzy to attack.

In the lab, researchers unleashed both types of crabs on a bed of eel grass in a saltwater pool, and the difference was stark. The Canadian invaders shredded the eel grass like Edward Scissorhands in their efforts to scarf down marine organisms seeking refuge, Prof Frederich said.

The first round of study focused on 200 crabs from Canada, and will be published in coming months.

Prof Frederich said further studies will focus on whether a specific gene plays a role in the aggressiveness or if a factor called hybrid vigour is in play.

The hybrid vigour theory suggests that crabs could be more aggressive as they establish themselves, but will mellow out later.

The quarrelsome newcomers currently comprise only about 2 to 3 per cent of green crabs crawling on the ocean floor off Maine, but those numbers are certain to grow, Frederich said.

“It will be an entirely different ball game,” he predicted. “It’s just a question of when more of the crabs come and out-compete the Maine green crabs.”

The docile green crabs have been around for more than a century in New England waters, but they’ve emerged as a major problem as the Gulf of Maine has warmed.

The feistier crabs arrived off Nova Scotia in the 1980s, and currents brought their larvae southward into New England waters.

Eventually, the newcomers will move farther southward. “We can’t do anything about it,” he said. “The only thing that we can do is learn how to live with it.”

— with AP