SUMMARY

The compartmentation of calcium was studied in adult intermoult Gammarus pulex. The total body calcium concentration was 547 /imol g. wet wt.-1, which is weight for weight approximately one half of the calcium concentration found in decapod crustaceans. Over 96 % of whole body calcium in G. pulex was found in the exoskeleton. sCa exchange curves for haemolymph and hepatopancreas had similar time constants indicative of a comparatively large pool of freely exchangeable calcium. The parallel exchange of calcium between the haemolymph and other soft tissue compartments confounded the satisfactory use of an algebraic model for indirect calcium influx measurement using haemolymph specific activity. All calcium influx was subsequently measured on a whole-body basis. Passive absorption of calcium on to the cuticle was eliminated as a sig-nificant factor as the metabolic inhibitor 2:4-DNP almost completely inhibited any form of calcium uptake. The active nature of the calcium regulatory mechanism was further illustrated by potential difference measurements indicating that calcium is accumulated against an electro-chemical gradient from media having a calcium activity less than 1 mmol I"1. When placed in calcium-free media, intermoult Gammarus pulex are able to achieve at least a tenuous calcium balance between 12 and 50/miol Ca I"1 at a density of 1 animal per 10 ml medium. Some specimens show a slov; calcium loss. The calcium regulatory mechanism is saturated at an external calcium concentration of 2 mmol I"1, which is the normal field calcium concentration for this population, and has a half saturation value of 0-3 mmol I"1