AN OVERVIEW OF CALCIUM BALANCE IN CRUSTACEANS

Crustaceans have been extensively used as models to study Ca balance b ecause of their exoskeletons, which undergo Ca dynamics associated wit h the molting cycle. This article discusses major findings during the past 10 yr. Discussion of intermolt Ca regulation focuses on two areas : (1) mechanisms that enable extracellular (EC) Ca levels to be mainta ined irrespective of environmental levels and (2) evidence for acidosi s-induced exoskeletal CaCO2, mobilization. Marine crustaceans routinel y maintain EC Ca levels above ambient Ca levels by protein binding and in dilute seawater (SW) by active uptake; urine is isoionic with resp ect to Ca. Extracellular Ca rises when subtidal species are aerially e xposed. Freshwater (FW) crayfish tolerate passive branchial Ca efflux while influx mechanisms are inoperative. Filtered Ca is extensively re absorbed in the antennal gland so that renal loss is minimal. Circulat ing Ca can be altered by exposure to low-Ca water or hypersaline media . Acid-intolerant species may experience an elevation in EC Ca associa ted with a metabolic acidosis that is generally more pronounced in har d wafer. Calcium may also be elevated to maintain O-2 transport when c ertain crayfish are unable to effectively respire in air. Terrestrial crustaceans depend on external water and food for their Ca supply. Uri nary Ca can be post renally reabsorbed in the branchial chambers. Incr eased EC Ca has been associated with severe acidosis under certain exp erimental regimes. However, there is only indirect evidence that this Ca originates in the exoskeleton. The second part of the article compa res Ca dynamics and regulation throughout the molting cycle in crustac eans from different habitats During premolt, skeletal Ca is reabsorbed into the hemolymph and excreted or stored. Postmolt crustaceans remin eralize the new cuticle with stored Ca and external supplies. Calcium influx in marine crustaceans appears to be passive in full-strength SW although an active component has been demonstrated in dilute SW Simil arly, during postmolt in FW species enzyme-catalyzed mechanisms such a s Ca ATPase, which had been silent during intermolt, are activated (or increasingly expressed). The uptake mechanisms are dependent on exter nal HCO3, Na, and pH. Behavioral ecology determines the degree of Ca s torage in terrestrial crustaceans Calcium is stored in the hemolymph, and exoskeletal Ca is reingested. The article concludes with a section on hormonal control of Ca balance. There appears to be little correla tion between premolt Ca dynamics and the ecdysone peak immediately bef ore ecdysis. A calcitonin-like peptide, however peaks in the immediate postmolt period when Ca uptake is maximal.