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The Respiratory Gas Transport, Acid-Base State, Ion and Metabolite Status of the Christmas Island Blue Crab, Cardisoma hirtipes (Dana) Assessed in Situ with Respect to Immersion

Christmas Island blue crabs, Cardisoma hirtipes, were sampled in the field during two different seasons (November 1993 and June 1994) to assess the ability of this species to maintain gas exchange in air and water. Hemolymph samples from the pulmonary vessel, pericardial cavity, and infrabranchial sinuses were taken from crabs breathing air and after 30 min of immersion. The pulmonary partial pressure of O₂ (Ppo2) decreased from >90 Torr in air-breathing crabs to <15 Torr after 30 min of immersion but there was no increase in L-lactate in the hemolymph or the muscle tissue. Hemocyanin (Hc) in the pulmonary hemolymph was 100% saturated in air-breathing crabs but O₂ content and saturation were quite variable in the immersed crabs between seasons. The variability of hemolymph O₂ content in immersed animals was partially attributed to changes in He affinity, which in turn were brought about by different conditions of the spring water that the crabs were immersed in during the two sampling seasons. The spring water supplying the habitat was variably CaCO₃-enriched and hypercapnic. Blue crabs immersed in this water exhibited a marked metabolic alkalosis rather than a respiratory alkalosis that might result from hyperventilation in water. It is hypothesized that C. hirtipes trap air in their branchial chambers during natural submergence, which compromises gas exchange, but that net base loading from the water may occur across the gills. In the field, C. hirtipes is clearly not dependent on water for respiratory or acid-base regulation, exhibiting characteristics of an obligate air breather. The hemolymph osmotic pressure of C. hirtipes was low, ∼530 mOsmol · kg⁻¹, thus reducing the gradient for ion loss into the fresh water that these crabs inhabit. The requirement on water for osmotic and ionic regulation as well as nitrogen excretion requires investigation.