VOLUME REGULATION IN 3 SPECIES OF MARINE MUSSELS

Three species of marine mussels were acclimated to the following range of osmotic concentrations: Modiolus modiolus L., 718-1000 mOsm; Mytilus edulis L., 375-980 mOsm; Geukensia demissa Dillwyn, 298-900 mOsm. Changes in the relative weight of whole animals, or changes in tissue water, extracellular water, and intracellular water of the posterior adductor muscle were measured in mussels acclimated to the different osmotic concentrations. These parameters were regressed against the reciprocal of relative osmotic concentration (1/ROC). The slopes of the regression lines were used to estimate the capacity for volume regulation: the larger the slope the lower the capacity. The relative weight of Modiolus acclimated to different osmotic concentrations changed significantly, indicating imperfect volume regulation, but no such change occurred in either Mytilus or Geukensia, indicating perfect volume regulation. For both tissue water and intracellular water, the slopes of the regression lines indicated that the capacity for volume regulation followed the sequence: Geukensia = Mytilus > Modiolus. Extracellular water decreased with an increase in osmotic concentration in Modiolus, but was constant in both Mytilus and Geukensia. The ratio of intracellular water to total tissue water followed the sequence Modiolus > Mytilus > Geukensia.