| Summary: | Cold-water conditions have excluded durophagous (skeleton-breaking) predators from the Antarctic seafloor for millions of years. Rapidly warming seas off the western Antarctic Peninsula (WAP) could now facilitate their return to the continental shelf, with profound consequences for the endemic fauna. Among the likely first arrivals are king crabs (Lithodidae), which were discovered recently on the adjacent continental slope. During the austral summer of 2010-2011, we used underwater imagery to survey a slope-dwelling population of the lithodid Paralomis birsteini off Marguerite Bay, WAP for environmental or trophic impediments to shoreward expansion. The average density was ~4.5 ind·1000m -2 within a depth-range of 1100-1500 m (overall observed depth-range 841–2266 m). Evidence of juveniles, molting, and precopulatory behavior suggested a reproductively viable population on the slope. At the time of the survey, there was no thermal barrier to prevent the lithodids from expanding upward and emerging on the outer shelf (400–500 m depth); however, near-surface temperatures remained too cold for them to survive in shallow, coastal environments (<200 m). Ambient salinity, composition of the substrate, and the depth-distribution of potential predators likewise indicated no barriers to expansion onto the outer shelf. Primary food resources for lithodids—echinoderms and mollusks—were abundant on the upper slope (500–800 m) and outer shelf. At present rates of warming, lithodids should emerge in outer-shelf environments within several decades. As sea temperatures continue to rise, they will likely play an increasingly important trophic role in subtidal communities closer to shore.
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