A low capacity for regulation of extracellular Mg2+ has been proposed to exclude reptant marine decapod crustaceans from temperatures below 0°C and, thus, 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, 1, 4, 7, 10, 13°C). Arterial and venous oxygen partial pressures in hemolymph (PO2), heart and ventilation beat frequencies and hemolymph cation composition were measured at rest and after a forced activity (righting) trial. Scopes for heart and ventilation beat frequencies, and intermittent heart 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, 1, 4°C). In natural sea water, righting speed was significantly slower at -1 and 13°C compared to acclimation temperature (4°C). Arterial and venous HLPO2 increased in response to cooling even though heart 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 rise in hemolymph [K+] at -1 and 1°C. Reduction of hemolymph [Mg2+] did not result in an increase in activity, heart 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 sub-polar brachyuran crabs, these constraints remained insensitive to changing Mg2+ levels.