In the shallow ocean water of the Antarctic continental shelf, unique marine communities, dominated by starfish and giant ribbon worms, have been able to thrive. That's largely because the cold temperatures of the water on the shelf have kept skeleton-cracking predators like king crabs away for millions of years. But as climate change heats up the water surrounding Antarctica, a new study finds that in the near future there won't be any barriers left to keep king crabs from climbing up the shelf and decimating the delicate ecosystems that have thrived there until now.
"The absence of these predators gives us these odd communities," says Richard Aronson, a professor at the Florida Institute of Technology and lead author on the study, published today in Proceedings of the National Academy of Sciences. "The communities are unique, the way they operate is unique, and, on top of that, the species of animals that are there are almost all endemic," he says. "In other words, you only find them in Antarctica."
"If it gets warmer, they can come in and raise hell."
But in recent years, king crabs have been discovered on the continental slope, just below the outer edges of the shelf, which begins at depths of 400 to 550 meters. To find out just how far they've advanced, Aronson and his colleagues towed a camera behind a remotely operated underwater vehicle, snapping pictures of more than 180,000 square meters of the seafloor, from 385 to 2,285 meters deep, off the coast of Marguerite Bay on the western Antarctic Peninsula.
In the images, the crabs were still absent from the shelf, where the extremely low temperatures prevent the animals from flushing excess magnesium out of their blood—a fatal complication for the crustaceans, according to Aronson. "If it gets warmer, the magnesium problem goes away," he says. "They can come in and raise hell."
The images did reveal that the crab population in the continental slope's warmer waters was large enough to be reproducing—meaning the crabs were thriving just below the shelf.
"We are not saying that that the crabs are definitely on the march up," Aronson cautions. "This is a baseline to compare to whatever we find in the future."
It may be unclear if the crabs are inching up onto the shelf just yet, but the study did find that, should they find their way up, the conditions on the shelf are almost ideal for the crabs—there's plenty of prey, the salinity levels and seafloor sediments are just right, and the temperature is very nearly perfect. Temperatures on the shelf off the coast of the western Antarctic Peninsula have risen at nearly double the global water average. If the water temperature continues to climb at the current clip, there won't be anything to stop the crabs from colonizing the shelf in the next few decades.
"It's very disquieting that the one place on Earth that's the least disturbed by human activities is at risk as well," Aronson says.
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