Correction: Previous versions of this article, including those appearing the March 21, 2011 print editions of The Post, incorrectly stated the name of a British Antarctic Survey marine biologist as David Clarke. The correct name is David Barnes. This version has been corrected.

Sven Thatje has been predicting an invasion of deep-water crabs into shallow Antarctic waters for the past several years.

But the biologist and his colleagues got their first look at the march of the seafloor predators while riding on an icebreaker across frozen Antarctic seas this winter.

The ship towed a robot sub carrying a small digital camera that filmed the seafloor below. It caught images of bright red king crabs up to 10 inches long, moving into an undersea habitat of creatures that haven’t seen sharp teeth or claws for the past 40 million years.

“There were hundreds,” Thatje said in an interview on board the Swedish icebreaker Oden, which docked at the main U.S. base in Antarctica, McMurdo Station, after a two-month research cruise. “Along the western Antarctica peninsula, we have found large populations over 30 miles. It was quite impressive.”

Thatje, an evolutionary biologist at the University of Southampton in England and chief scientist on the cruise, is part of a U.S.-Swedish team of marine researchers who are trying to figure out where, when and how fast this invasion is occurring. King crabs, of which there are 13 species, live in the deep waters off Alaska and Russia and across the Southern Ocean in the waters off New Zealand, Chile and Argentina. But here in Antarctica, crabs haven’t been able to survive because, until now, it’s been too cold. As a result, many bottom-dwelling creatures such as mussels, brittle stars and sea urchins have not developed any defenses against the crabs.

What’s happened is that the waters around the Antarctic peninsula have begun to get warmer. The air temperature has jumped almost 11 degrees Fahrenheit since the 1950s, and the average ocean temperature has increased by one degree over the same period. That slight change in water temperature has lowered a physiological barrier that had previously kept the crabs in check, Thatje said.

When the water is too cold — as it has been along the shallow waters of the Antarctic continental shelf — crabs can’t remove magnesium from their blood. Magnesium is a common mineral in seawater, and if they can’t get rid of it, it causes a narcotic effect that stops them from moving enough to survive.

Some scientists say the magnesium barrier may soon fall, as global climate change continues to affect wildlife at the polar regions.

The lack of clawed, beaked or toothed predators has led to a thick seafloor canopy of sorts, much like a submarine jungle comprising flowery feather stars, tube worms and squirming sea spiders along with clams and mussels.

As they sift through more than 120,000 digital images from their expedition, Thatje and other researchers are looking for evidence that crabs are preying on these creatures.

“The Antarctic shelf communities are quite unique,” Thatje said. “This is the result of tens of millions of years of evolution in isolation.”

The crab research team is analyzing the images of the seafloor, looking for clues into whether the crabs will invade and then leave or permanently colonize the shallow areas. Will their presence destroy the existing community or simply alter it? Previous cruises had spotted only one or two crabs, but now scientists are seeing entire populations, according to Richard Aronson, biology professor at the Florida Institute of Technology and co-investigator on the project, along with James McClintock of the University of Alabama at Birmingham.

The crabs are moving from the deep ocean, up the continental slope to the shallower shelf areas. Unlike most areas of the world, the shallower waters on the Antarctic continental shelf are actually slightly colder than the deeper waters of the Southern Ocean. That’s because of a clockwise current of water called the Antarctic circumpolar current.

That flow of cold water keeps Antarctic marine life — especially the bottom-dwelling creatures — isolated. There are no sharks, rays or fish with bony jaws, for example, in Antarctica.

“If you look at the warming trends on the peninsula, you would expect that the crabs would come back in 40 or 50 years,” Aronson said from his office in Melbourne, Fla. “But, boom, they’re already here.”

Not all experts agree that the crabs are destined to wreak havoc on the sea bottom. David Barnes, a marine ecologist at the British Antarctic Survey, said that the seawater temperature changes may be occurring on the surface of the ocean but that they’re too small to affect animals living on the bottom.

Barnes studies colonial animals in Antarctica, such as sponges and corals, and how they fit into the ecosystem. He said not enough is known about existing crab populations — where they live and how long they have been there — to declare that climate change is causing an invasion.

At the same time, he agrees with the Swedish and U.S. researchers that there are rapid changes underway in Antarctica, especially on the Western Antarctic peninsula, a thumb of land that juts northward to the bottom of South America. There’s less sea ice, for example, on the waters of the peninsula. That is causing problems for the penguins and seals that depend on sea ice for food and shelter.

“Yes, there is some cause for concern in that the rate of [environmental] change is greater than has been the case in recent millions of years,” Barnes said. “Obviously, for animals to tolerate or adapt to things in a very short period of time is going to be tricky.”

Niiler is a freelance writer.