THE EFFECTS OF TEMPERATURE AND WATER AVAILABILITY ON ION AND ACID-BASE BALANCE IN HEMOLYMPH OF THE LAND HERMIT CRAB COENOBITA CLYPEATUS

¹ Department of Biology, University of Calgary, 2500 University Drive N.W., Calgary, Alberta, Canada T2N 1N4
² Zoology Department, University of Massachusetts, Amherst, Massachusetts 01003

Temperature acclimation (18-30°C) and dehydration (to 86% of initial mass) are two problems frequently encountered by the land hermit crab, Coenobita clypeatus. Their individual and combined effects on hemolymph ion and acid-base status were assessed. With free access to 10% SW, crabs maintained a constant degree of hydration, with hemolymph marginally hypo-osmotic to full strength SW. Increased acclimation temperature produced a reduction in pH, characteristic of ectotherms and consistent with maintenance of relative alkalinity which was accomplished by an elevation of CO₂ tension (P,CO₂). Hyperactivity resulted in some spillage of shell water and affected Cl^- balance. Under water deprivation, evaporative loss declined approximately exponentially and was negatively correlated with body mass. Hemolymph osmolality and electrolyte levels were significantly increased, ionic imbalance contributing largely to the hemolymph acidosis. Hemoconcentration was less marked when combined with temperature acclimation. Temperature-dependent pH regulation however in dehydrated crabs was accomplished as in hydrated crabs by ventilatory P,CO₂ control typical of air-breathers. The aquatic route of acid-base regulation (by ionic exchange) potentially afforded by the reservoir of water held in the molluscan shell was apparently not utilized.