Climate change could imperil the unique creatures which have made their home in the inhospitable waters of Antarctica, scientists believe.
By Alex Kirby
BBC News Online environment correspondent
The sealife which has developed there resembles some life forms around North America and Europe millions of years ago.
Ribbon worms' mucus is corrosive
But global warming could allow predators from warmer seas to colonise the Antarctic.
If that happens, a highly sensitive sea floor community could vanish, the scientists say.
The warning comes from Professor Rich Aronson, of Dauphin Island Sea Lab, Alabama, US, and colleagues, in a paper presented to a London conference.
The conference, The Role Of The Southern Ocean In Global Processes: An Earth System Science Approach, has been organised by the British Antarctic Survey (Bas).
The director of Bas, Professor Chris Rapley, said: "We are bringing together some of the top experts in the world to look at a key component of the Earth's system.
"The Southern Ocean influences the Earth's ocean currents and global climate. Its behaviour is as critical to the rest of the world as it is to the local environment.
"However, the impact of climate change and the commercial exploitation of its fish stocks could result in major and potentially damaging changes.
"The 5-10 year coordinated international research campaign we hope will happen would be a quantum step forward in our understanding of the region."
Professor Aronson told journalists the predators which still dominate temperate and tropical seas had vanished around 35 million years ago when Antarctica cooled, as the fossil record showed.
Antarctica had no lobsters or crabs, although there were crabs in the Arctic, where it was just as cold.
He told BBC News Online: "There are very few fish in the Antarctic, none of the fast-moving bone-crushing species you find in temperate seas, like wrasse and cod.
"Somehow - and we don't know how - the cooling of Antarctica cut out the fish, crabs, sharks and rays.
"That had enormous cascading effects, leading to the emergence of the suite of rather bizarre predators which dominates the ecosystem today.
A marine isopod, or woodlouse
"They're comparable with what you'd have found round North America and Europe several hundred million years ago.
"We don't know what will happen as the Antarctic warms, and whether modern predators may re-invade.
"The circumpolar current is a barrier to colonisation. If that were interrupted, it could have an enormous impact."
Professor Aronson said many Antarctic invertebrates used chemical defences against predation by other invertebrates.
He said: "The weirdest of all are the ribbon worms, great big fat things a metre long.
"They dive into the mud and go zipping in and obliterating clams. They secrete an acid mucus which can damage a diving suit.
"They and the seastars congregate round seal droppings and eat them. Then there are the sea spiders, which can grow up to 30 centimetres (12 inches) across, and may live for several decades."
Long-lived giant sea spider
Professor Aronson said changes in sea temperature of only a few degrees C could have significant impacts on predator-prey relationships, with profound ecological consequences.
El Nino events (the seasonal widespread climatic disturbance triggered by changes in the Pacific) appeared to be happening more often.
This meant the effects of upwelling cold water and consequent cooling should become more prominent in the next few decades.
Professor Aronson said: "More cold water could depress predation rates and potentially lead to the emergence of Antarctic-type communities."
The continued atmospheric build-up of greenhouse gases might also intensify these effects over several centuries.
The Antarctic Peninsula is one of the three regions of the world that has warmed most rapidly in the last half century - by 2.5 C.
Images courtesy of the British Antarctic Survey.