Patterns of Water and Solute Regulation in the Muscle Fibres of Osmo- Conforming Marine Decapod Crustaceans

1. The patterns of myoplasmic water and solute regulation were examined in several species of marine decapod crustaceans acclimated to various salinities. Three osmoconformers (Callianassa cahformemsis, Cancer antennarius, Emerita analoga) and one weak osmoregulator {Pachygrapsus crassipes) were studied. 2. Although haemolymph water and solute activities varied with adaptational salinity in these species, regulation of cell water content was apparent in all but 25 % s.w. acclimated Pachygrapsus. Cell water regulation was correlated with the regulation of ninhydrin-positive solutes. An estimate of the extent of the regulation was achieved by comparing the observed cell hydration changes to that predicted for an ideal osmometer and to that of a perfect volume regulator. It was concluded that the extent of cell volume regulation was variable and incomplete, particularly in dilute salinities. 3. Net losses of myoplasmic sodium, chloride, and ninhydrin-positive nitrogen were observed in animals acclimated to dilute sea water, while net gains of these solutes were noted in crabs acclimated to concentrated sea water. In contrast, the amounts of cell potassium and calcium plus magnesium were relatively constant fractions of the cell dry weight at any salinity. 4. A simple mechanism, br.sed upon Donnan considerations, is sufficient to account for the constancy of myoplasmic potassium and the variability of cell chloride with changing extracellular osmolarity.