Calcium utilisation during anaerobiosis induced by osmotic shock in a bivalve mollusc

Abstract

DURING aerial exposure, some littoral marine bivalve molluscs, for example Mytilus¹ and Mercenaria^2,3, have been shown to close their valves and respire anaerobically. The major by-product of anaerobiosis in the clam Mercenaria mercenaria is succinic acid which is neutralised by the dissolution of calcium carbonate previously deposited in the shell^2,3. The ability of bivalves to respire anaerobically has been well documented, an early review on this subject⁴ records a variety of species which are able to survive long periods in the absence of oxygen. Most evidence for anaerobic metabolism in bivalves is based on observations that on return to aerobic conditions, oxygen consumption rapidly rises to levels above normal^5,6. A similar ‘over shoot’ in heart rate has been reported^1,7,8. These observations have been explained as representing an increase in circulatory and respiratory activity associated with the repayment of an ‘oxygen debt’ incurred during the period of anaerobiosis. The changeover from aerobic to anaerobic respiration in bivalves has been reported to occur when the oxygen tension of the surrounding water falls to very low levels or to low tide when the bivalves may close the valves^7,9,10. We report here results which are part of an extensive study of osmotic and behavioural responses of Scrobicularia plana (Da Costa), a euryhaline estuarine species, to varying concentrations of seawater and demonstrate the use of calcium from the shell to buffer the products of anaerobiosis. The concentrations of sodium, potassium, magnesium and calcium were measured by atomic absorption spectrophotometry in the body fluids and medium under different conditions of osmotic stress at 10 °C