Climate change is causing a major upheaval in the shallow marine ecosystems of Antarctica. 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.

"Antarctic marine communities are functionally Paleozoic," says paleobiologist Rich Aronson of the Dauphin Island Sea Lab in Alabama. "They look like primeval communities from hundreds of millions of years ago because modern predators-crabs and fish-are missing."

But this long stable situation is about to change. "The crabs are on the doorstep; they are sitting in deep water, and only a couple of hundred bathymetric meters now separate them from the slightly cooler shallow water in the Antarctic shelf environment," says Sven Thatje of the National Oceanography Centre, Southampton in the UK.

Seastars and ribbon worms under the waters in Antarctica. Photo by Rich Aronson.

Thatje, Aronson and Cheryl Wilga of the University of Rhode Island recently presented their findings at the American Association for the Advancement of Science (AAAS) meeting in Boston, MA.

Marine life on the Antarctic seafloor is unique. Nowhere else do giant sea spiders and marine pillbugs share the ocean bottom with fish that have antifreeze proteins in their blood. The shell-cracking predators that dominate bottom communities in temperate and tropical waters have been shut out of Antarctica because it is simply too cold for them. They have been physiologically barred from entry.

"Crabs have a problem in cold water," says Thatje, "They cannot flush magnesium out of their blood, so when they are already moving slowly because of the cold, the magnesium makes them pass out and die."

Magnesium is a narcotic for marine invertebrates. "My zoology students routinely use magnesium sulfate-Epsom salts-to anesthetize invertebrates so we can study them," Aronson adds.

Fast-moving, bone-crushing crabs, fish, sharks and rays are keystone predators in most places, but they cannot operate in the icy waters of Antarctica. The only fish there-the ones with the antifreeze proteins-eat small, shrimp-like crustaceans and other soft foods. The main bottom dwelling predators are slow-moving sea stars and giant, floppy ribbon worms.

Because crabs and predatory fish are not speeding around the seafloor smashing clams, snails and other animals with hard skeletons, marine food webs are different in Antarctica. Released from the dangers of predation, filter feeders such as brittlestars thrive in dense populations.

Antarctica began to cool off around 40 million years ago. Aronson and a team of paleontologists collected the abundant marine fossils found at Seymour Island off the Antarctic Peninsula. Linda Ivany of Syracuse University reconstructed changes in the Antarctic climate from chemical signals preserved in ancient clamshells. And Aronson discovered that, as temperatures dropped and crabs and fish were frozen out, the slow-moving predators that remained could not keep up with their prey. Filter feeders flourished in this new environment. Snails, once out of danger, gradually lost the spines and other shell armor they had evolved against crushing predators.

During the ice ages, conditions were even more harsh than usual for Antarctic sea life. According to Thatje's new study, life hung by a thread. Remnants of marine communities huddled beneath open, ice-free areas called polynyas in the vast, frozen wasteland that covered the Southern Ocean. Whales, seals, and penguins had to migrate northward to survive. When the ice retreated about 12,000 years ago, the Antarctic fauna once again spread out along the coast. These new insights are important to understanding how life is able to cope with climate oscillation and current global warming.

Now, coastal waters in Antarctica are warming rapidly. Temperatures at the sea surface off the western Antarctic Peninsula went up 1°C in the last 50 years, more than double the global average. As temperatures rise, magnesium poisoning becomes less of a barrier to crabs.

In January of 2007, Thatje and a group of oceanographers from the UK discovered that crabs are massing in deeper, slightly warmer waters, ready to conquer the Antarctic shallows. If the crabs' invasion succeeds, they will devastate Antarctica's spectacular Paleozoic-type fauna and fundamentally alter its ecological relationships.

"That would be a tragic loss for biodiversity in one of the last truly wild places on earth," says Aronson. "Unless we can get control of greenhouse-gas emissions, global warming will ruin the marine life in Antarctica and make the world a sadder, duller place."

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