Haemolymph Mg2+ regulation in decapod crustaceans: Physiological correlates and ecological consequences in polar areas

Reptant decapod crustaceans are almost absent from the Southern Ocean south of the Antarctic Convergence. We tested the hypothesis that this may be du e to the reduced ability of this group to regulate Mg2+ levels in the haemo lymph ([Mg2+]HL). Mg2+ acts as an anaesthetic in marine invertebrates and i ts level is higher in Reptantia (crabs such as Cancer spp., Chionoecetes sp p,, Maja spp,, 30-50 mmol l(-1)) than in Natantia (prawns such as Pandalus spp,, Palaemon spp,, Crangon spp,, 5-12 mmol l(-1)). We varied [Mg2+]HL in three species of reptant decapod crustaceans, Carcinu s maenas, Hyas araneus and Eurypodius latriellei, and investigated heart ra te, the rate of oxygen consumption and levels of spontaneous and forced act ivity at different temperatures, The rate of oxygen consumption and heart r ate increased significantly with reduction in [Mg2+]HL over the entire temp erature range investigated in E, latreillei, In H, araneus, an increase in metabolic and heart rates compared with control values was found only at te mperatures below 2 degrees C, Forced and spontaneous activity levels increa sed significantly in the group of [Mg2+]HL-reduced animals below 0 degrees C, at which control animals were mostly inactive. At a reduced [Mg2+]HL Of 5-12 mmol l(-1), which is the [Mg2+]HL Of caridean shrimps in the Southern Ocean, Q(10) and activation energy were reduced for all these variables and extended the temperature range over which physiological functions were mai ntained. We suggest that the high [Mg2+]HL in Reptantia causes relaxation of the ani mals and reduces their scope for activity, especially at temperatnres below 0 degrees C. The hypothesis that the synergistic effects of high [Mg2+]HL and low temperature probably prevented the Reptantia from recolonizing the permanently cold water of polar areas is discussed.