Cold tolerance and the regulation of cardiac performance and hemolymph distribution in Maja squinado (Crustacea : Decapoda)

Elevated Mg2+ levels in the hemolymph ( [Mg2+](HL)) of brachyuran crabs hav e recently been demonstrated to limit cold tolerance by reducing motor and circulatory activity. Therefore, the limiting function of elevated [Mg2+](H L) on circulatory performance and arterial hemolymph flow was investigated by the pulsed-Doppler technique in the spider crab Maja squinado during pro gressive cooling from 12 degrees to 0 degreesC. [Mg2+](HL) were reduced fro m control levels of 39.9 mmol L-1 to levels of 6.1 mmol L-1 by incubation i n magnesium reduced seawater. At 12 degreesC cardiac output was 13.9 +/- 2. 4 mt kg(-1) min(-1) and stroke volume 0.2 +/- 0.04 mt kg(-1) min(-1) in con trol animals. In [Mg2+](HL)-reduced animals cardiac output increased to 43. 6 +/- 5.0 mL kg(-1) min(-1) and stroke volume rose to 0.6 +/- 0.1 mL kg(-1) min(-1). Temperature reduction in control animals revealed a break point a t 8 degreesC linked to a major redirection of hemolymph flow from lateral t o sternal and hepatic arteries. Cardiac output and heart rate dropped sharp ly during cooling until transiently constant values were reached. Further h eart rate reduction occurred below 4.5 degreesC. Such a plateau was not det ected in [Mg2+](HL)-reduced animals where the break point decreased to 6 de greesC, also indicated by a sharp drop in heart rate and cardiac output and the redirection of hemolymph flow. It is concluded that progressive coolin g brings the animals from a temperature range of optimum cardiac performanc e into a deleterious range when aerobic scope for activity falls before cri tical temperatures are reached. Reduction of [Mg2+](HL) shifts this transit ion to lower temperatures. These findings support a limiting role for [Mg2](HL) in thermal tolerance.