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Crushing blow

Scientists investigate invasion of shell-breaking crabs in West Antarctica


The Antarctic Peninsula is one of the fastest warming spots on the planet, as indicated by the shortened duration of sea ice and the decline of animals like Adélie penguins and krill that depend on it for their ecology.

Now scientists believe at least one species of king crab is slowly creeping from the deep ocean habitats onto the continental slope as the waters slowly warm, threatening marine communities that have evolved over millions of years without about the pressure of shell-crushing predators.

“The concern, of course, is that these crabs could have a very dramatic impact on this fairly defenseless suite of prey and alter the communities substantially,” said James McClintock, a professor and researcher at the University of Alabama at Birmingham.

Except for a few brief warm climatic spurts, king crabs and similar predators haven’t been in shallow-water Antarctic environments for about 40 million years, according to Richard Aronson, professor and head of Florida Institute of Technology’s Department of Biological Sciences.

“One of the main structuring factors in those modern bottom communities is a lack of predation by shell-cracking predators. … Those communities have a retrograde feel to them. They’re more like an anachronism — they’re more reminiscent of the Paleozoic,” said Aronson, referring to the geological era that ended about 250 million years ago.

Aronson and McClintock are teaming up with British and Swedish colleagues to assess the threat of Antarctica’s first invertebrate invasion. A nine-person team will travel aboard two research vessels for nearly two months beginning at the end of November to survey the seafloor and water column for adult crabs and their larvae around Antarctica.

At risk are a range of species, from ubiquitous brittle stars to bivalves to gastropods like clams and snails to starfish and sea urchins. “I don’t think there’s any question that there would be some really tasty things that they would be interested in,” McClintock said.

Since the 1950s, the average winter temperature in the region has spiked by 6 degrees centigrade, reducing the duration of sea ice in the area by three months. Over roughly the same time period, the average ocean temperature has ticked up about 1 degree centigrade.

It might not seem like much of an increase, but scientists believe the freezing cold temperatures of the peninsula’s continental shelf waters have served as a barrier to the cold-blooded crabs. Deeper seawater is slightly warmer because of its saline characteristics.

Aronson said scientists at the Alfred Wegener Institute, a German research center that focuses on the polar regions, found that crabs are unable to process magnesium in their blood below a certain temperature. McClintock said the effect on the crabs is like being drunk.

“It’s just warm enough for king crabs to not have this magnesium problem on the slope, but it’s still too cool on the shelf, but as the shelf warms, we’re expecting these king crabs to come over the top and invade the bottom communities in shallower-water environments,” he noted.

A Genghis Khan-esque marauding horde of crabs sweeping up the shelf may be the stuff of Hollywood, but Aronson said in geological terms, the invasion could be relatively swift.

Conservatively, he said, crabs could be on the continental shelf, at depths of 1,000 meters or less, in less than half a century. “My gut feeling is that it’s going to be faster than that,” Aronson said.

There is already evidence that king crabs are encroaching on the shelf, according to Sven Thatje, an evolutionary ecologist at the University of Southampton’s School of Ocean and Earth Science at the National Oceanography Center in Southampton, United Kingdom.

Thatje is leading the international team on the ship-based expedition, which will take scientists from Punta Arenas, Chile, to McMurdo Station on Ross Island in Antarctica.

In 2007, Thatje and British colleagues spied a species of crab, Paralomis birsteini, between 1,100 and 1,300 meters on the continental slope off Marguerite Bay, about in the middle of the Antarctic Peninsula. That’s where the team will begin its investigation before heading farther south to the Amundsen and Bellingshausen seas.

He and colleagues published the finding in 2008 in the journal Polar Biology. More recently, in November 2010, he co-authored another paper in Polar Biology that analyzed the distribution of 17 species of king crab, which lived at depths between 550 and 1,600 meters, in the Southern Ocean.

The results showed that king crabs prefer relatively warm waters, and the coldest waters in which they have been found were those of the Ross Sea, with 0.4 to 0.5 degrees Celsius. Water temperature at the surface can be as low as minus 1.8 degrees Celsius, the salinity keeping it from freezing.

“The paper provides a ‘physiological map’ to the known crab records in the Southern Ocean in relation to local temperature settings,” Thatje said via e-mail. “This is instrumental for any potential changes in their distribution. In theory we can now predict how the distribution of some of these species may change as waters continue to warm in key areas of the [Western Antarctic Peninsula].”

Earlier this year, scientists aboard the research vessel Nathaniel B. Palmer found crabs on the continental slope near Palmer Station, a small research base for the U.S. Antarctic Program.

The shipboard team will tackle the project from several angles.

Foremost among the tools the researchers will use is an autonomous underwater vehicle (AUV) from the Woods Hole Oceanographic Institution called SeaBED. The torpedo-shaped instrument is capable of getting high-resolution images of the seafloor that are good enough to identify individual animal species.

“It allows us to fly it just over the surface of the benthos and to see a level of detail that is just stunning,” McClintock said.

In this case, the AUV won’t work autonomously. The ships — first the USAP’s Nathaniel B. Palmer and later the Swedish icebreaker Oden – will tow the instrument on a sledge. The Oden will drag the SeaBED vehicle through ice as well — something rarely done before.

“That’s another exciting thing about this project: There’s a component of developing this technology to fly these AUVs,” Aronson said.

Another instrument called an epibenthic sledge will also cruise along near the bottom of the seafloor to sample the crab’s dermersal larvae, which develop just above the benthic communities. In addition, the ships will tow nets to capture plankton, floating organisms in the water, to see what species of crab larvae are being carried near Antarctica.

In previous work in Antarctica with other colleagues, McClintock has studied the function and evolution of chemical defenses used by some benthic organisms. Some of the compounds they have isolated in the past show promise as cancer therapies or even pesticides.

Now McClintock worries Antarctica’s natural laboratory may be irrevocably altered.

“These [compounds] are coming out of organisms that could be decimated by these crabs, and we might lose an opportunity to discover something that has benefits to mankind, because these communities are going to be so radically altered you lose 30 million years of evolution of these unique species.”

Warned Aronson: “We’re expecting some major changes as warming proceeds. … Crabs do have a big impact on benthic communities.”

NSF-funded research in this article: Richard Aronson, Florida Institute of Technology, Award No. 0838846; and James McClintock, University of Alabama at Birmingham, Award No. 0838844.

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Curator: Michael Lucibella, Antarctic Support Contract | NSF Official: Peter West, Division of Polar Programs