Haemolymph Acid-Base, Electrolyte and Gas Status During Sustained Voluntary Activity in the Land Hermit Crab (Coenobita Compressus)

After 3 h (50 m) of voluntary walking, the haemolymph pH of the land hermit crab Coenobita compressus (H. Milne Edwards) decreased by 0·4units. This was accompanied by increases in CO₂ tension bicarbonate (HCO₃^- + CO₃^2-) and lactate concentrations. The hypercapnic acidosis was partially compensated by metabolic bicarbonate accumulation and an H⁺ deficit developed. Unloaded crabs accumulated less of a proton load than crabs transporting mollusc shells. During activity, oxygenation of the haemocyanin (HCy) accounted for the release of 0·3 mmol CO₂1^-1, via the Haldane effect, which was seven times more than in settled crabs. Control acid-base balance was re-established within 1 h of recovery. At this time, acidic equivalents were excreted at a mean flux rate of 5 mequivkg^-1 h^-1 into a source of external water. [Na⁺] and the ratio of [Na⁺] : [Cl^-] increased during exercise.

Coenobita haemolymph had a high O₂-carrying capacity ⁠. HCy oxygen-binding characteristics were typical of other decapods (ϕ = -0·44), yet no lactate sensitivity was apparent. Settled in vivo values of O₂ tension and content were located around the half-saturation tension (P₅₀) of the dissociation curve. During exercise, increased and an unopposed Bohr shift decreased the O₂-binding affinity, thereby reducing postbranchial saturation. Quantitatively, however, compensations in cardiac output were more instrumental in increasing the O₂ delivery to respiring tissues. During recovery, haemolymph remained high and the venous reserve doubled.