An overview of calcium balance in crustaceans

Crustaceans have been extensively used as models to study Ca balance because of their exoskeletons, which undergo Ca dynamics associated with 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 maintained irrespective of environmental levels and (2) evidence for acidosis-induced exoskeletal CaCO₃ mobilization. Marine crustaceans routinely maintain EC Ca levels above ambient Ca levels by protein binding and in dilute seawater (SW) by active uptake; urine is isoionic with respect to Ca. Extracellular Ca rises when subtidal species are aerially exposed. Freshwater (FW) crayfish tolerate passive branchial Ca efflux while influx mechanisms are inoperative. Filtered Ca is extensively reabsorbed in the antennal gland so that renal loss is minimal. Circulating Ca can be altered by exposure to low-Ca water or hypersaline media. Acid-intolerant species may experience an elevation in EC Ca associated with a metabolic acidosis that is generally more pronounced in hard water. Calcium may also be elevated to maintain O₂ transport when certain crayfish are unable to effectively respire in air. Terrestrial crustaceans depend on external water and food for their Ca supply. Urinary Ca can be postrenally reabsorbed in the branchial chambers. Increased EC Ca has been associated with severe acidosis under certain experimental regimes. However, there is only indirect evidence that this Ca originates in the exoskeleton. The second part of the article compares Ca dynamics and regulation throughout the molting cycle in crustaceans from different habitats. During premolt, skeletal Ca is reabsorbed into the hemolymph and excreted or stored. Postmolt crustaceans remineralize 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. Similarly, during postmolt in FW species enzyme-catalyzed mechanisms such as Ca ATPase, which had been silent during intermolt, are activated (or increasingly expressed). The uptake mechanisms are dependent on external HCO₃, Na, and pH. Behavioral ecology determines the degree of Ca storage 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 correlation between premolt Ca dynamics and the ecdysone peak immediately before ecdysis, A calcitonin-likepeptide, however, peaks in the immediate postmolt period when Ca uptake is maximal.