On a dim February evening, seven people crowded around a row of television monitors in a shack on the rear deck of the RV Nathaniel B. Palmer. The research icebreaker was idling 30 kilometers off the coast of Antarctica with a cable as thick as an adult's wrist dangling over the stern. At the end of that cable, on the continental shelf 1,400 meters down, a remote-operated vehicle (ROV) skimmed across the sea floor, surveying a barren, grey mudscape. The eerie picture of desolation, piped back to the television monitors, was the precursor to an unwelcome discovery.
The ROV had visited 11 different sea-floor locations during this 57-day research cruise along the Antarctic Peninsula in 2010. Each time, it had found plenty of life, mostly invertebrates: sea lilies waving in the currents; brittlestars with their skinny, sawtoothed arms; and sea pigs, a type of sea cucumber that lumbers along the sea floor on water-inflated legs. But at this spot, they were all absent. After 15 minutes, the reason became clear: a red-shelled crab, spidery and with a leg-span as wide as a chessboard, scuttled into view of the ROV's cameras. It probed the mud methodically — right claw, left claw, right claw — looking for worms or shellfish. Another crab soon appeared, followed by another and another. The crowded shack erupted into chatter. “They're natural invaders,” murmured Craig Smith, a marine ecologist from the University of Hawaii at Manoa. “They're coming in with the warmer water.”
Cold temperatures have kept crabs out of Antarctic seas for 30 million years. But warm water from the ocean depths is now intruding onto the continental shelf, and seems to be changing the delicate ecological balance. An analysis by Smith and his colleagues suggests that 1.5 million crabs already inhabit Palmer Deep, the sea-floor valley that the ROV was exploring that night (see 'A warming welcome'). And native organisms have few ways of defending themselves. “There are no hard-shell-crushing predators in Antarctica,” says Smith. “When these come in they're going to wipe out a whole bunch of endemic species.”
Researchers are worried that rising crab populations and other effects of the warming waters could irrevocably change a sea-floor ecosystem that resembles no other on Earth. Scientists are racing to document these effects, even as they continue to explore this little-understood region. “This could have a really major reorganizing impact on these unique and endemic marine communities,” says Richard Aronson, a marine biologist at the Florida Institute of Technology in Melbourne, who was part of a team that found crabs on another part of Antarctica's continental shelf in December 2010. “It's a fascinating thing,” he says. “A little scary, because it's a very obvious footprint of climate change.”
Cut off by cold
Aronson has worried about the fragility of life on the Antarctic shelf for more than a decade. He spent December 1994 collecting fossils from Seymour Island, on the northeast fringe of the Antarctic Peninsula. The island's bare, crumbling hills contain the remnants of an ancient sea floor. In 200 meters of layered rock and fossils exposed by wind erosion, Aronson saw evidence of the most pivotal event in Antarctica's history: the continent's final separation from South America, starting around 40 million years ago. This event allowed the emergence of the circumpolar ocean current, which isolated Antarctica from warmer air and water masses farther north, and plunged it into perpetual winter. Aronson and his students analyzed 10,000 fossils from before and after that sudden cooling, and a striking pattern emerged.