In this Wednesday, Aug. 15, 2018 photo, University of New England marine sciences professor Markus Frederich holds a green crab at a campus research lab on in Biddeford, Maine.

In the "Crab Lab," the limits of feisty green crabs from Maine are put to the test.

Markus Frederich and his students at the University of New England in Maine placed about a dozen crabs in a bucket, pumped in nitrogen to deprive them of oxygen for 45 minutes and then put them on an underwater treadmill.

Some crabs ran for as long as five minutes and still survived.

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"That is pretty wild," Professor Frederich said. "Students were in awe."

In the mid-1800s, European green crabs hitched a ride on boats and came to the United States. But over the past few years, a genetically different European green crab from Nova Scotia, Canada — one that is more combative and more destructive of ecosystems — has appeared off the coast of Maine.

"To use nice words, I would simply describe them as highly aggressive," Professor Frederich said. In the lab, he and his students use more colorful language while working with the crabs, which come at them, pincers up.

The aggressive nature of the Canadian hybrid poses another problem for Maine, which already struggles to defend its soft-shell clam population from Maine green crabs.

The green crabs from Canada have been described as the "cockroach of the sea." Green crabs are voracious predators that can withstand changing temperatures, low salinity and low oxygen levels. They eat oysters, can prey on lobsters in groups and have been known to turn on each other, experts said.

Researchers worry that a changing climate and warming waters could make conditions more favorable for them.

Green crabs slice through eelgrass, an important habitat for other sea creatures, Professor Frederich said. Preliminary research shows that the aggressive crabs from Canada wreaked more havoc on eelgrass and soft-shell clam populations compared with their Maine counterparts.

A video posted by Professor Frederich shows a green crab from Maine scuttling away from a prodding finger while one from Canada menacingly accosted the camera.

"Simply walking up to the tank full of Canadian crabs made them go wild and jump at us," Professor Frederich said.

Attempts to reduce the impact of the crabs have included creating products out of them, such as commercial compost and food paste. Fences have also been placed in the water to protect soft-shell clam populations.

Brian Beal, a professor at the University of Maine at Machias, said the crabs from Canada have no native predator in North America, and only cold winters keep their population in check. Rising temperatures in the Gulf of Maine mean "conditions are becoming more and more favorable for green crabs to survive and populate areas," he said.

The aggressive green crabs have hurt the state's $15 million soft-shell clam industry, and they are ravenous in their pursuit, he said, adding, "If people could actually see that, Stephen King would love it."

Chad Coffin, president of the Maine Clammers Association, said as a shell fisherman, he already deals with aggressive crabs apart from the ones from Canada that are ready to fight when he pulls up a haul. It's hard to imagine things getting worse "because the crabs we have now are already eating everything," he said.

While the green crabs from Canada do not pose a threat to people, Professor Frederich predicted their population would continue to grow. He's studying their genetic makeup to determine why they're more aggressive.

"At this point, we just try to understand how those different populations are moving and why they are different," he said.