Biologists gathered here for the annual conference of the American Association for the Advancement of Science warned that the return of predators to Antarctica could prove devastating to its underwater ecosystem.

Antarctica's surrounding waters remain too cold for sharks and other fish capable of crushing shellfish similar to the mollusks living in the vast continent's seas, said University of Rhode Island biology professor Cheryl Wilga.

"As a result, the Antarctic seafloor has been dominated by relatively soft-bodied, slow-moving invertebrates, just as in ancient oceans prior to the evolution of shell-crushing predators," she told a news conference Friday on the sidelines of the conference.

But global warming has already pushed temperatures up by one to two degrees in the past 50 years, and the waters could become hospitable to sharks within the next 100 years, she said.

"The water only needs to remain above freezing year round for it to become habitable to some sharks, and at the rate we're going, that could happen this century," Wilga said.

"Once they get there, it will completely change the ecology of the Antarctic benthic community," she said.

While sharks may one day roam Antarctic waters, crabs are already crawling closer to the vast continent for the first time in ages, adding one more worry for a marine life left intact since the Paleozoic area of 250 million to 500 million years ago, biologists said.

"Predatory crabs are poised to return to warming Antarctic waters for the first time in millions of years, which will disrupt the composition of the archaic marine communities," Rick Aronson, of the Dauphin Island Sea Lab in Alabama, told the news conference.

"Shallow-water, benthic communities in Antarctica are unique," he said. "Nowhere else do giant pycnogonids, nemerteans and isopods occur in shallow marine environments, cohabiting with fish that have anti-freeze glycoproteins in their blood."

Sven Thatje of National Oceanography Center in Southampton, Britain, urged the international community to take action to curb greenhouse gases that cause global warming and prevent an ecological disaster.

"The crabs are on the doorsteps, they are sitting in deep water, and only a couple of hundred bathymetric meters now separate them from the slightly cooler shallow water in Antarctic shelf environment," he said.

The oceanographer made the crab discovery with other British journalists in January 2007.

He warned that the return of shell-cracking predators to Antarctica "would be a tragic loss for biodiversity in one of the last wild places on Earth."

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