Crabs are invading the shallow waters of the Southern Ocean

Mar 3rd 2011 | palmer, Antarctica | 

ANTARCTICA is a peculiar place. For instance, unlike those in most parts of the planet, the ocean depths around it are warmer, at 5ºC or so, than the shallows near the coast. Here, the temperature hovers around 0ºC and can drop to -2ºC. As a consequence, the animals of the Antarctic continental shelf have been free for millions of years from the attentions of predators, such as crabs and sharks, that cannot cope with the cold. The result is an unusual bunch of sea lilies, brittle stars, giant ribbon worms and mollusks that are armored only with thin, soft shells. 

But not, perhaps, for much longer—for crabs are on the march. In the past few years several groups of researchers have spotted king crabs on the continental slope of Antarctica. This slope, which connects the deep ocean with the continental shelf, and whose waters have an average temperature of between 1ºC and 5ºC, is a marginal habitat for king crabs. They die when the temperature drops below 1ºC because they are then unable to process magnesium ions. In small quantities, these ions are needed for energy metabolism. Too many, though, act as a narcotic that eventually kills the animal. Worryingly for sea-lily lovers, the latest research suggests the crabs may be creeping up the slope.

The expedition which made this discovery ended in January. On it, Sven Thatje of Britain’s National Oceanography Centre and his colleagues found hundreds of king crabs—beasts that grow to up to 20cm across, excluding their spindly legs—in waters that had been crab-free on the previous survey, in 2007. They did so by towing a submersible called SeaBED, which belongs to the Woods Hole Oceanographic Institution, in Massachusetts, along a 30-nautical-mile transect of Marguerite Bay, off the west coast of the Antarctic peninsula.

 

As SeaBED cruised over its eponymous target, its cameras took pictures every three seconds and its sensors measured the temperature, salinity and depth of the water. The result is 130,000 images showing hundreds of king crabs, together with details of their immediate environments. All of the crabs were still on the continental slope, but some of them were in shallower water than any seen in 2007. The question, yet to be answered, is whether that was a result of the vagaries of sampling, or because local waters are warming up.

Warming there has certainly been. Since the 1950s, when records began, the average temperature of the ocean to the west of the Antarctic peninsula has gone up by 1ºC. That has expanded the region in which king crabs might live. How much longer it will be before they invade the rich uplands of the continental shelf is hard to say. But, like the monsters in a bad sci-fi film, they are coming.