Biology of the king crab Paralomis birsteini on the continental slope off the western Antarctic Peninsula

Predatory king crabs (Lithodidae) structure benthic communities in their native habitats and cause shifts in the composition of benthic assemblages when introduced to new environments. Cold temperatures have apparently excluded skeleton-breaking predators from the continental shelf around Antarctica for millions of years, but recent increases in sea temperatures off the western Antarctic Peninsula (WAP) may be allowing lithodids to return. Imaging surveys have revealed dense populations of the lithodid Paralomis birsteini (Macpherson 1988) living on the continental slope off the WAP, but the biology of these populations remains poorly understood. We collected 51 adult P. birsteini in a trapping study on the slope off Marguerite Bay, WAP from depths of 1200–1400 m. Of the 51 crabs, 42 were males and 9 were females. Four females were ovigerous, carrying eggs at various stages of development. Rates of parasitism and limb regeneration were comparable to populations of lithodids elsewhere in the world, although the proportion of limb loss was relatively high. The parasite Briarosaccus callosus was obvious in both males and females, with one individual bearing hyperparasites. Gill necrosis was also observed in several dissected males. The success of the contemporary populations on the Antarctic slope suggests they have the potential to expand upward to the continental shelf

Climate change and the threat of novel marine predators in Antarctica

Historically low temperatures have severely limited skeleton-breaking predation on the Antarctic shelf, facilitating the evolution of a benthic fauna poorly defended against durophagy. Now, rapid warming of the Southern Ocean is restructuring Antarctic marine ecosystems as conditions become favorable for range expansions. Populations of the lithodid crab Paralomis birsteini currently inhabit some areas of the continental slope off Antarctica. They could potentially expand along the slope and upward to the outer continental shelf, where temperatures are no longer prohibitively low. We identified two sites inhabited by different densities of lithodids in the slope environment along the western Antarctic Peninsula. Analysis of the gut contents of P. birsteini trapped on the slope revealed them to be opportunistic invertivores. The abundances of three commonly eaten, eurybathic taxa—ophiuroids, echinoids, and gastropods—were negatively associated with P. birsteini off Marguerite Bay, where lithodid densities averaged 4280 ind/km2 at depths of 1100–1499 m (range 3440–5010 ind/km2), but not off Anvers Island, where lithodid densities were lower, averaging 2060 ind/km2 at these depths (range 660–3270 ind/km2). Higher abundances of lithodids appear to exert a negative effect on invertebrate distribution on the slope. Lateral or vertical range expansions of P. birsteini at sufficient densities could substantially reduce populations of their benthic prey off Antarctica, potentially exacerbating the direct impacts of rising temperatures on the distribution and diversity of the contemporary shelf benthos

No barrier to emergence of bathyal king crabs on the Antarctic shelf

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