by Larry O'Hanlon, Discovery News, 15 February 2008

Global warming is setting the stage for an invasion of predators on the sea floor around Antarctica, the likes of which have not been there for more than 40 million years.

Back in the late Eocene epoch, predatory animals such as sharks and crabs were driven away from Antarctic depths when the continent and its surrounding waters turned into an icebox, said researchers on Friday at a symposium at the meeting of the American Association for the Advancement of Science in Boston.

The result was a virtually predator-free zone on the seafloor and a paradise for worms, sea lilies, clams, brittle stars and other bottom-dwelling animals.

All that is about to end, however. Global warming is now raising water temperatures to the point where, very soon, those long-exiled predators could return and wreak havoc on the ocean floor, say biologists.

"It's going to be a smorgasbord," said researcher Cheryl Wilga of the University of Rhode Island. She studies the metabolic limitations of sharks that have kept them from Antarctic waters for millions of years, but may not do so much longer.

"The species in the Antarctic (seafloor) have no defense for shell-crushing predators," said extreme species researcher Brad Seibel, also of the University of Rhode Island. "I don't think that anyone was really aware of this issue."

Along the Antarctic Peninsula, the northernmost, warmest part of the continent, global warming is raising air temperatures quickly. Water temperatures have been warming as well, at a rate of about 0.04 degrees Celsius per year, Wilga says. That comes to about one degree per 25 years.

Compared to the relative stability seen for tens of millions of years, that's incredibly fast. Already, crabs are showing up, and some sharks are poised to pounce once the thermal dinner bell rings.

Antarctic King Crab?

The first exiled predator to return to Antarctica is the king crab. The leggy crustaceans have been found way down on the deep slopes off the Antarctic continental shelf -- where the water is slightly warmer than elsewhere.

There they are fighting the cold, explained marine scientist Richard Aronson of Dauphin Island Sea Lab and the University of South Alabama.

The frigid water makes it hard for the crabs to efficiently flush magnesium out of their bodies, said Aronson. Too much magnesium acts like a narcotic on a crab.

"When it's too cold, the magnesium makes them pass out and die," Aronson said. That's probably why the crabs have been absent for eons. Now, however, in those slightly warmer depths off the continental shelf, it's just warm enough for the crabs to survive there.

As the upper waters continue to warm, nothing will stop the king crabs from moving up onto the continental shelf and feasting. That will "hammer" the old seafloor communities, Aronson said.

"We expect the populations (of seafloor invertebrates) to take a dive," said Aronson.

Jaws II

Sharks are next in the supper line. Spiny dogfish, in particular, are already abundant off the coast of South America, poised for invasion as soon as the water gets a tad warmer, says Wilga.

The cold poses a different set of challenges for sharks, she explained. For one thing, sharks need to keep swimming to stay afloat, which requires a lot of energy. Very cold water slows them down to the point where it's hard to keep swim muscles moving.

Shark bodies are also infused with a chemical called triethylamine oxide (TMAO), which counters the toxic effects of urea that builds up in shark tissues. The colder or deeper the waters, the more TMAO sharks need to offset the urea, Wilga explained. Sharks appear to reach a TMAO limit before they can reach Antarctic waters.

But if there's any shark that can eke out a living first in the warming waters, Wilga is putting her money on the diminutive and virtually global spiny dogfish.

While this is more bad news for the Antarctic sea lilies and brittle stars, there will be a sliver of a silver lining to the sharks' return, said Wilga: "Sharks also eat crabs."

A Moving Feast

Some other marginally good news is that the seafloor invasion may have little effect on the better known animals of Antarctica -- penguins, fish and whales. These live on what is essentially a separate, krill-based, shallow-water food web, explains Antarctic marine biologist Robert Pitman of National Oceanic and Atmospheric Administration's Southwest Fisheries Science Center in La Jolla, Calif.

The top predator in the shallow water ecological realm is the orca, or killer whale, Pitman said. There are, in fact, more killer whales in Antarctica than anywhere on Earth. And if anything, dogfish will be welcome aperitifs for them, he explained.

Global warming might fill orcas' bellies in other ways as well, Pitman told Discovery News. Penguins, seals and Minke whales need lots of ice to hide from the fierce and intelligent marine mammal. The less ice, the easier the hunt.

"The best protection seals and penguins have from killer whales is on the ice," Pitman said. "Even offshore, the Minke whales migrate to pack ice for protection from killer whales."

So as the waters warm and the ice melts, Antarctica's existing predators may enjoy some feast days of their own.

Thwarting the Invasion

The predator invasion of Antarctica may be unavoidable, say the researchers, but it could be moderated if something is done immediately to reduce emissions of global warming greenhouse gases into the atmosphere.

"As global warming proceeds, the deeper water (around Antarctica) will only get warmer," said Aronson. The momentum of global warming is, indeed, huge and can not be stopped, he said, but it can be slowed.

"If we're going to do something about the planet we have to do it now." Aronson said. "The window is closing."

Link: http://dsc.discovery.com/news/2008/02/15/antarctica-predators.html