A Role for Oxygen Delivery and Extracellular Magnesium in Limiting Cold Tolerance of the Sub-Antarctic Stone Crab Paralomis granulosa?

A low capacity for regulation of extracellular Mg2+ has been proposed to exclude reptant marine decapod crustaceans from temperatures below 0 degrees C and thus to exclude them from the high Antarctic. To test this hypothesis and to elaborate the underlying mechanisms in the most cold-tolerant reptant decapod family of the sub-Antarctic, the Lithodidae, thermal tolerance was determined in the crab Paralomis granulosa (Decapoda, Anomura, Lithodidae) using an acute stepwise temperature protocol (-1 degrees, 1 degrees, 4 degrees, 7 degrees, 10 degrees, and 13 degrees C). Arterial and venous oxygen partial pressures (Po-2) in hemolymph, heartbeat and ventilation beat frequencies, and hemolymph cation composition were measured at rest and after a forced activity (righting) trial. Scopes for heartbeat and ventilation beat frequencies and intermittent heartbeat and scaphognathite beat rates at rest were evaluated. Hemolymph [Mg2+] was experimentally reduced from 30 mmol L-1 to a level naturally observed in Antarctic caridean shrimps (12 mmol L-1) to investigate whether the animals remain more active and tolerant to cold (-1 degrees, 1 degrees, and 4 degrees C). In natural seawater, righting speed was significantly slower at -1 degrees and 13 degrees C, compared with acclimation temperature (4 degrees C). Arterial and venous hemolymph Po-2 increased in response to cooling even though heartbeat and ventilation beat frequencies as well as scopes decreased. At rest, ionic composition of the hemolymph was not affected by temperature. Activity induced a significant increase in hemolymph [K+] at -1 degrees and 1 degrees C. Reduction of hemolymph [Mg2+] did not result in an increase in activity, an increase in heartbeat and ventilation beat frequencies, or a shift in thermal tolerance to lower temperatures. In conclusion, oxygen delivery in this cold-water crustacean was not acutely limiting cold tolerance, and animals may have been constrained more by their functional capacity and motility. In contrast to earlier findings in temperate and subpolar brachyuran crabs, these constraints remained insensitive to changing Mg2+ levels.