Delicate marine ecosystems in the waters off Antarctica face a threat brought about by global warming: an invasion of shell-crushing crabs, which could become top predators in the chilly but warming waters, researchers say.

King crabs, notorious for their feeding habit involving cracking open the outer skeletons of creatures such as starfish and sea urchins, could wreak havoc on an Antarctic ecosystem that, for millions of years, has seen only soft-bodied filter feeders such as sea stars and marine worms.

The reappearance of king crabs in Antarctic waters, where they haven't been seen in tens of millions of years, is due to rising temperatures west of the Antarctic Peninsula, the portion of the icy continent that extends northward toward South America, the researchers say.

It is one of the most rapidly warming places on the globe, they note.

If the warming continues at its present rate, king crabs could move from their current deep-sea habitats and move onto the continental shelf in the coming decades, says Richard Aronson of the Florida Institute of Technology.

"Because other creatures on the continental shelf have evolved without shell-crushing predators, if the crabs moved in they could radically restructure the ecosystem," he says.

No barriers such as salinity levels, sediment types, or food resource limitations are present that would prevent the invasion of the crabs if the water becomes warm enough, the researchers report in the Proceedings of the National Academy of Sciences.

King crabs can't cope with temperatures much below 34 degrees Fahrenheit, which has tended to keep them out of chilly waters on the Antarctic continental shelf, but the waters off the Antarctic peninsula have been warmed up by around 3 degrees F over the past 50 years, almost double the global average.

The study does not prove that crabs have or will move into the shallower waters, the researchers acknowledge, but it does suggest there are no natural barriers to that movement if global warming continues.

"The only way to test the hypothesis that the crabs are expanding their depth-range is to track their movements through long-term monitoring," says study co-author James McClintock of the University of Alabama at Birmingham.

However, recent research with camera-equipped underwater vehicles has documented populations of the crabs near Antarctica in waters only a few hundred yards deeper than the continental shelf with its delicate ecosystem.

While the king crabs would face some predators if they moved onto the shelf, such as Antarctic toothfish and seals, many smaller creatures would be vulnerable and become easy prey for the crabs, the researchers say.