Prospects for the return of shell-crushing crabs to Antarctica

Rapidly rising sea temperatures off the western Antarctic Peninsula (WAP) are placing the Antarctic benthos at risk of increasing predation from durophagous (shell-breaking) brachyuran and anomuran crabs. Crabs are at present excluded from Antarctic-shelf environments by their limited capacity to down-regulate magnesium ions in their blood, a constraint that paralyses and kills them under cold conditions. Interspecific variation in the ability of Subantarctic crabs to flush out magnesium ions predicts which species should be capable of extending their ranges to shelf environments along the Antarctic Peninsula. In agreement with those predictions, the brachyuran Halicarcinus planatus was recently found in the shallow subtidal zone at Deception Island, an emergent volcanic cone located off the WAP. Geothermal activity at Deception Island provides a warm-water opportunity for persistence in a cold ocean, and heavy ship-traffic there could accelerate the delivery of non-native species. Large populations of anomuran king crabs (Lithodidae) have recently been discovered on the continental slope off the WAP, at ambient sea temperatures slightly warmer than the waters over the shelf. The lithodids are generalized, durophagous predators that could expand to shelf habitats as the overlying waters warm rapidly. Another brachyuran, the globally invasive, durophagous European green crab Carcinus maenas, also appears poised to expand to nearshore habitats in Antarctica. Successful expansion of predators and other benthic species to the Antarctic shelf would accelerate the global trend towards functional homogenization of the marine biota. Long-term monitoring programmes should be established immediately to assess the extent of the problem in Antarctica.