ROTHERA BASE, Antarctica, Jan 26 (Reuters) - Thriving only in near-freezing waters, creatures such as Antarctic sea spiders, limpets or sea urchins may be among the most vulnerable on the planet to global warming, as the Southern Ocean heats up.
Isolated for millions of years by the chill currents, exotic animals on the seabed around Antarctica -- including giant marine woodlice and sea lemons, a sort of bright yellow slug -- are among the least studied in the world.
Now scientists on the Antarctic Peninsula are finding worrying signs that they can only tolerate a very narrow temperature band -- and the waters have already warmed by about 1 Celsius (1.6 Fahrenheit) in the past 50 years.
"Because this is one of the most rapidly warming areas on the planet and because the animals are so temperature sensitive...this marine ecosystem is at higher risk than almost anywhere else on the planet," said Simon Morley, a marine biologist at the British Antarctic Survey at Rothera.
"A temperature rise of only 2-3 degrees (Celsius) above current temperatures could cause these animals to lose vital functions," he said.
In warmer waters, laboratory studies show that clams and limpets lose the ability to right themselves if they land upside down. Such a skill is vital in Antarctica's shallows, where icebergs regularly scrape across rocks on the seabed.
"Will they be here in 100 years' time?" Morley said, standing by blue tanks of sea cucumbers, worms and others. "I think that we will see changes in the ecosystems, more in some species and less in other species.
"It does look as if these mechanisms are truly applicable worldwide," he said.
Studies of clams in Singapore also show that they find it hard to burrow if temperatures rise, he said. Coral reefs can also suffer damage if temperatures rise even slightly.
The U.N. Climate Panel has a best estimate that air temperatures may rise by between 1.8 and 4.0 Celsius this century, due to a build-up of greenhouse gases.
Rothera's waters range from about minus 2 Celsius in winter, kept from freezing by saltiness, to 1 Celsius in summer.
On a recent trip into Rothera's bay, Ali Massey and Terri Souster, dressed in thick black dive-suits, disappeared into the water from a red inflatable speedboat and re-emerged 20 minutes later with a haul of the little-understood creatures.
"It is a fascinating place to dive," said Souster, a 24-year-old South African.
The inshore habitat is largely separate from the open ocean, where penguins and whales feed on krill that in turn consume algae. Big predators in the shallows are starfish and fish such as Antarctic cod.
In Antarctica, another linked threat is from icebergs that now scour each part of the shallow seabed on average once a year -- smashing many of the creatures.
Divers off Rothera are extending a 5-year study of iceberg scours by placing small white concrete blocks on the seabed. They are later retrieved to see how many are cracked by icebergs.
And iceberg poundings could become more frequent since warming could bring a decline in sea ice. Winter sea ice locks icebergs into position -- when it melts they can get moved around by winds and tides and swept into the shallows.
Another worry is that non-native species will arrive off Antarctica if the oceans warm, perhaps organisms floating on a piece of plastic or stuck on the hull of a ship. Invasive species, usually transported by humans, can oust local species.
"It's something we are really concerned about," Morley said, noting that at current rates of warming the danger was about 50 years away.
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(Editing by Louise Ireland)