According to Domack's results, the incursion seems to have started following the end of the Little Ice Age — a period of relative cold that began in the Middle Ages — but it has intensified as anthropogenic warming and southern ozone depletion have taken hold. Average water temperatures west of the Antarctic Peninsula have risen by 1 °C in the past 50 years, and continue to rise by 0.01–0.02 °C per year. “The heat injection is going through the roof,” says Martinson. “It's going up exponentially.”
The first evidence that crabs were poised to invade along with the warm water came early in 2007. Sven Thatje, a marine ecologist at the University of Southampton, UK, launched an ROV to the outer slope of the Antarctic Peninsula to map glacial grooves on the sea floor. But its cameras also caught sight of 13 king crabs (Paralomis birsteini) between depths of 1,300 and 1,100 meters. Thatje had studied the cold tolerance of these crabs and concluded that they could probably survive farther north at 2,000–4,000 meters, where the water is a degree or two warmer — “but then we found them even on the continental slope” only 500 meters below the shelf itself, he says. “These crabs were thriving at 1 °C. They were basically at the physiological limit that I had anticipated.”
But it was Smith's discovery of Neolithodes yaldwyni king crabs in Palmer Deep, 120 kilometers in from the edge of the continental shelf, that demonstrated a true invasion. West of the Antarctic Peninsula, cold water sits on top of warmer water. To reach Palmer Deep from the outer ocean, crabs or larvae must have crossed what amounts to a cold, high mountain pass only 450 meters below sea level before settling into Palmer Deep, at depths of 800–1,400 meters.
In December 2010, Thatje, working with Aronson and McClintock, returned to Antarctica and towed a submersible up and down the continental slope near the mouth of Marguerite Trough. The ROV traced 100 kilometers of sea floor, capturing 150,000 photographs that revealed hundreds of P. birsteini crabs between 2,300 and 830 meters down. “If you extrapolate,” says McClintock, “we're talking about millions of crabs.”
The crab invasion could have started a decade or two ago. When Smith re-examined photographs taken at the bottom of Palmer Deep in 1998, he saw telltale claw marks in the mud — indicating that at least some crabs were already present, even if none had been caught on camera. Domack looked at 30 years of water-temperature data measured at Palmer Deep during earlier cruises, and found that the sea-floor valley had gradually warmed — becoming ever-more hospitable to the crabs. Smith is now comparing gene sequences from crabs sampled in Palmer Deep with ones collected from deeper, warmer waters farther north in the Southern Ocean. The data from these experiments should help him to zero in on the crabs' origins and the date of their arrival.
But even without knowing the exact history of the invasion, the implications seem clear. Animals living on the edge of their physiological limits often struggle to survive and reproduce, but 19 out of 27 crabs that Smith collected during a cruise in 2011 turned out to be females carrying larvae or eggs. “This population is reproducing like crazy,” he says. “It's probably here to stay and expand.” As the ceiling of cold water continues to lift over the next 10 or 20 years, crabs could spill out of Palmer Deep and Marguerite Trough — and colonize the broader continental shelf at depths of 400–600 meters, devastating the endemic sea life.
The warm waters will also bring other perils for Antarctica's sea-floor gardens. Many of the species here are exquisitely sensitive to increases in temperature. The brittlestars and other invertebrates have extremely slow metabolisms — an adaptation to the cold water — and only meager ability to absorb and transport oxygen. “So what do those guys do if it warms up and their metabolic rate speeds up?” asks Lloyd Peck, a biologist at the British Antarctic Survey in Cambridge, who has monitored these creatures in aquarium warming experiments. Their oxygen demand revs beyond what their gills can supply — and they slowly suffocate.