Bimodal respiration, water balance and acid-base regulation in a high-shore crab, Cyclograpsus lavauxi H. Milne Edwards

Abstract

Cyclograpsus lavauxi H. Milne Edwards, 1853 occurs under boulders near the littoral fringe, where it is wetted only briefly, if at all, in each tide. Mean water content of sea-water equilibrated crabs was 62.3 % of body weight. On warm, windy days crabs on the shore lose > 17.5% of body water. Water loss for crabs of 1.6 g mean body weight was 0.29 % water content · h −1 · mm Hg saturation deficit −1, which is similar to that of species occupying positions lower on the shore. C. lavauxi is, however, able to tolerate comparatively greater losses of body water (up to 36%) compared with species from lower shore levels. As the gills dry, the lamellae separate into regular clumps which may help to maintain gas exchange. Resting oxygen consumption ( V o2 ) after 3–5 h settlement was similar in water and air. During enforced activity, V o2 increased by factors of 5.3 in water and 2.6 in air at a standardized body weight of 1.5 g, indicating an appreciable aerobic scope in both media. Loss of up to 16% of body water did not depress resting aerial V o2 or aerobic scope. In resting crabs there was no change in haemolymph pH after 24 h of either immersion or emersion, but haemolymph [Ca 2+], [HCO − 3] + [CO 2− 3], and calculated P co2 all rose in air relative to aquatic values. These results suggest that cuticular or other endogenous CaCO 3 is mobilized to compensate the respiratory acidosis in air. The implications of such physiological properties for aquatic and aerial activity on the shore are discussed.