The effects of fluctuating salinity on the physiology of Modiolus demissus (Dillwyn)

Abstract

Specimens of the Atlantic ribbed mussel, Modiolus demissus (Dillwyn), were exposed to both gradual (sinusoidal) and abrupt (square wave) salinity fluctuations during which changes in haemolymph osmolality, Na + , K +, Ca 2+, Mg 2+, tissue water content, shell valve movements, oxygen uptake, and adductor muscle free amino acids (FAA) were measured. Shell valve closure was found to occur when the external sea-water concentration reached ≈60% sea water. The haemolymph osmolality, Na + , K + , Ca 2+ , Mg 2+ concentrations followed those of the external medium as long as the animals' shell valves remained open. There were no changes in haemolymph ionic or osmotic concentrations during periods of shell valve closure. Total tissue water varied inversely with salinity change reaching a maximum of ≈82% in wedged-open animals exposed to an abrupt salinity change. During exposure to decreasing salinities the adductor muscle FAA pool decreased; after shell valve closure the concentration increased, primarily due to an increase of alanine and glycine. Oxygen consumption by M. demissus in air saturated sea water was found to vary with body weight according to the equation : oxygen consumption = 0.295 dry weight 0.670. Oxygen consumption during declining oxygen tensions was found to be directly dependent on the environmental oxygen concentration. The rate of oxygen consumption during exposure to fluctuating salinities remained constant as long as the shell valves remained open.