But this is about to change. 'The crabs are on the doorstep. They are sitting in deep water only a couple of hundred bathymetric metres away from the slightly cooler shallow water in the Antarctic shelf environment,' says Dr Sven Thatje of the National Oceanography Centre, Southampton (NOCS).

Predators like the shell-cracking crabs that dominate bottom communities in temperate and tropical waters have been shut out of Antarctica because it is too cold for them.

'Crabs have a problem in cold water,' says Dr Thatje, a biologist at the University of Southampton's School of Ocean and Earth Science, based at NOCS, 'They cannot flush magnesium out of their blood. So when they are already moving slowly because of the cold, the magnesium acts as a narcotic causing them to pass out and die.' Released from the dangers of predation, filter feeders such as brittlestars thrive in dense populations. Giant sea spiders and marine woodlice share the ocean bottom with fish that have antifreeze proteins in their blood.

Dr Thatje is discussing his findings at a science meeting of the American Association for the Advancement of Science in Boston February 15 with colleagues, Rich Aronson of the Dauphin Island Sea Lab in Alabama and Cheryl Wilga of the University of Rhode Island. Along with scientists from the British Antarctic Survey, BAS, the team studied the benthic communities living in different habitats around Antarctica.

'Antarctic marine communities are functionally Paleozoic - typical of around 250 million years ago,' says paleobiologist Rich Aronson. 'If the crabs' invasion succeeds, they will devastate Antarctica's spectacular Paleozoic-type fauna and fundamentally alter its ecological relationships.'

In January 2007 Dr Sven Thatje and a group of ocean biologists from NOCS and BAS discovered crabs massing in deeper slightly warmer waters, ready to move into the Antarctic shallows should they warm up sufficiently.

Understanding the changes of the past may help scientists to determine how a rise in temperatures may further transform this continent. In a report to be published in Ecology Life Hung By A Thread: Endurance of Antarctic Fauna In Glacial Periods, Sven Thatje reveals that harsh conditions during the Ice Ages pushed Antarctic life to the limit. With ice coverage ten times thicker than today's, the extreme climate conditions in Antarctica during past Ice Ages were so severe that animals were forced into migration to avoid extinction.

He and colleagues from BAS, and the Alfred Wegner Institute, Germany found that larger animals such as penguins, whales and seals were dependent upon areas of open water in the ice known as polynyas. These polynyas acted as vital oases providing access to food such as krill.

Dr Thatje said: 'The existence of open water was essential for the survival of marine plants - the base of the food web. Reduced to such refuges much of today's life in the high Antarctic would have hung by a thread. Limited resources restricted the abundance and productivity of both terrestrial and marine life.'

'Under Thin Ice: Global Warming and Predatory Invasion of the Antarctic Seas' is a presentation at the AAAS based on a paper published in the Annual Review of Ecology Evolution and Systematics: Climate Change and Invasibility of the Antarctic Benthos 2007. 38:129-54, Richard Aronson, Sven Thatje, Andrew Clarke (BAS), Lloyd Peck (BAS), Daniel Blake, (University of Illinois) Cheryl Wilga and Brad Seibel (University of Rhode Island).

Note: If no author is given, the source is cited instead.

• Fish
• Marine Biology
• Sea Life

• Global Warming
• Climate
• Oceanography

• Antarctic ice sheet
• Ice shelf
• Antifreeze protein
• Ice sheet
• Larsen Ice Shelf
• Horseshoe crab