[Photo] Surprise find: A king crab in the Antarctic deep.

Unique marine life in Antarctica will be at risk from an invasion of sharks, crabs and other predators if global warming continues, scientists warn.

Crabs are poised to return to the Antarctic shallows, threatening creatures such as giant sea spiders and floppy ribbon worms, says a UK-US team.

Some have evolved without predators for tens of millions of years.

Bony fish and sharks would move in if waters warm further, threatening species with extinction, they say.

In the last 50 years, sea surface temperatures around Antarctica have risen by 1 to 2C, which is more than twice the global average.

Loss of species

Speaking in Boston at the annual meeting of the American Association for the Advancement of Science (AAAS), the researchers said global warming could fundamentally change the ecosystem, leading to the loss of some species.

"Sharks are going to arrive in Antarctica as long as the warming trend continues, a bit more slowly than crabs - crabs are going to get there first," said Professor Cheryl Wilga of the University of Rhode Island (URI), US. "But once they do get there they are capable of eating the organisms that live there."

Professor Wilga said the arrival of sharks and shell-crushing bony fishes would lead to dramatic changes in the number and proportions of species found there.

Shrimp, ribbon worms and brittle stars are likely to be the most vulnerable to population declines.

Dr Sven Thatje of the National Oceanography Centre at the University of Southampton, UK, said animals living in shallow water in Antarctica were unique on Earth today because they evolved in a very cold environment over tens of millions of years.

Extreme conditions

"In the course of a process we call Antarctic cooling that started about 40 million years ago, all major seafloor predators such as sharks and crabs went extinct in Antarctica because they were not able to cope with these extreme conditions," he told BBC News.

"Today, global warming is removing barriers to invasions and we've seen recently that crabs, especially king crabs, are on the doorstep of Antarctica - they can potentially re-invade the shallow waters if warming continues."

The researchers say urgent local and global actions are needed to protect this last pristine environment.

"We have to act now in Antarctica as elsewhere to save the diversity of the planet," said Dr Richard Aronson of Dauphin Island Sea Laboratory in Alabama.

He said measures were needed to stop alien species being brought in through ships' ballast water.

"The local actions are to control ship traffic and control dumping of ballast waters," he told the BBC. "The global actions are what we've been saying for all other environments - we have to control emissions of greenhouse gases."

Strange creatures

Animals that live on the seafloor of Antarctica are some of the strangest creatures on Earth.

The extreme cold and lingering darkness has presented huge challenges to marine life over the passage of time, leading to the evolution of fish equipped with anti-freeze proteins in their blood, and a proliferation of filter feeders on the seabed.

Fast-moving shell-crushing animals such as crabs and ocean-going sharks that are normally key predators have long been kept at bay, as their bodies cannot cope with very cold conditions.

This has led to a dominance on the Antarctic seafloor of soft-bodied, slow-moving invertebrates, similar to those found in ancient oceans prior to the evolution of shell-crushing predators.