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Shell crushing crabs threaten ecosystem

Posted by: Saving Water SA (Cape Town, South Africa) – partnered with Water Rhapsody conservation systems – 09 February 2011

Warming waters along the Antarctic peninsula have opened the door to shell-crushing king crabs that threaten a unique ecosystem on the seafloor, according to new research by a U.S.-Sweden team of marine researchers.

Crabs haven't lived in coastal Antarctic waters for the past 40 million years.

On a two-month voyage of the Swedish icebreaker Oden and U.S. research vessel Nathaniel B. Palmer, marine biologists collected digital images of hundreds of crabs moving closer to the shallow coastal waters that have been protected from predators with pincers for more than 40 million years. They are the same kind of deep-water crabs with big red claws that you might find at the seafood counter.

“Along the western Antarctica peninsula we have found large populations over like 30 miles of transects. It was quite impressive,” said Sven Thatje, an evolutionary biologist at the University of Southampton in England and chief scientist on the cruise.

Finding crabs on the bottom of the ocean isn’t that big a deal. But here in Antarctica, crabs haven’t lived in coastal waters for the past 40 million years. Until now, it’s been too cold.

Bottom-dwelling creatures like mussels, brittle stars and sea urchins have not developed any defenses. They have thinner shells, for example. For the same reason, filter feeders, like clams and worms, burrow underground in most regions. The lack of predators has led to a thick canopy of sorts, much like a submarine jungle comprised of flowery feather stars, tube worms and squirming sea spiders.

During an interview on board the Oden just after it docked at the main U.S. base in Antarctica, Thatje described how the crabs are moving closer to an ecosystem with no defenses.

“The Antarctic shelf communities are quite unique,” Thatje said. “This is the result of tens of millions of years of evolution in isolation.”

To explore this underwater world, Thatje and his team of U.S. and Swedish scientists towed an autonomous underwater vehicle (AUV) that scanned the seafloor with a digital camera. Along the way, they encountered thick packs of ice, rough seas and lots of feeding whales.

Even though Thatje predicted the crab invasion several years ago in a research paper, he was surprised at seeing so many so quickly.

“The pace of changes that we are observing here in the Antarctic, which is the remotest continent on this planet, is quite frightening,” he said.

What’s happened is that the waters around the Antarctic peninsula have begun to get warmer. The air temperature has jumped 6 degrees Celsius (10.8 degrees Fahrenheit) since the 1950s, while the average ocean temperature has increased by 1 degree Celsius (1.8 degrees Fahrenheit) over the same time.

That change in water temperature has lowered a physiological barrier that has kept the crabs in check. Crabs are unable to process magnesium in their blood below a certain temperature, and the result is a narcotic effect on the crabs’ movement. Magnesium is a mineral that they absorb from the surrounding sea water. Scientists say that barrier may soon fall, as global climate change continues to impact wildlife at the polar regions.

The crab research team will spend the next few months analyzing 120,000 images taken of the seafloor by the AUV, which was designed and operated by the Woods Hole Oceanographic Institution. They want to know whether the crabs will invade and leave, or permanently colonize the shallow areas. Will their presence destroy the existing community or simply alter it?

Previous cruises had only spotted one or two crabs, but now scientists are seeing entire populations, according to Rich Aronson, lead investigator in the crab project and a professor of biology at the Florida Institute of Technology. The crabs are moving from the deep ocean, up the continental slope to the shallower shelf areas.

“As the surface waters warm up, that will make it possible to come running over the top and raise hell with the bottom communities,” Aronson said.

Unlike most areas of the world, the shallower waters on the Antarctic continental shelf are actually slightly colder than the deeper waters of the Southern Ocean. That’s because of a clockwise current of water called the Antarctic circumpolar current. That flow of cold water keeps Antarctic marine life — especially the bottom-dwelling creatures — isolated. There are no sharks, rays or fish with bony jaws, for example, Aronson explained, in Antarctica.

“If you look at the warming trends on the peninsula, you would expect that the crabs would come back in 40 or 50 years,” Aronson said from his office in Melbourne, Fla. “But boom, they’re already here. This is the last pristine marine system on Earth and it could get destroyed.”

By: Eric Niiler
Source: Discovery News

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