1. Mytilus edulis could be adapted to salinities between 25% and 125% under laboratory conditions, with complete ionic and osmotic conformity of the body fluids.

2. Extracellular space, intracellular cation concentrations and water content were determined for the cerebro-visceral connectives of Mytilus adapted to either 100% or 25% salinity. These measurements suggested only a moderate degree of volume regulation (as indicated by relative cell hydration) and net losses of both potassium and sodium from the cells during acclimation to dilute sea water, although neither cation was reduced in proportion to the external concentrations.

3. Measurement of actual volume changes during acute hyposmotic stress indicated a greater capacity for volume control in the 25%-adapted connectives. However, these tissues also showed an increment of 11% in initial diameter, suggesting significant chronic swelling of the cells.

4. Fine-structural studies of Mytilus nerve indicated that the apparent swelling of the dilute-adapted tissues resulted from a roughly threefold thickening of the ensheathing neural lamella due to the deposition of extra collagen-like fibrils, with the axons in fact showing negligible volume increase. The connectives thus appear to exhibit almost perfect volume regulation.

5. To account for the conflicting estimates of volume regulating capacity, it is proposed that the cells are hyperosmotic to their environment at 25% salinity, the hydrostatic gradient thus created being countered by the restraint imposed by the thickened neural lamella. Physical stresses on the excitable membranes of the nerve would thus be minimized, and electrophysiological functioning in dilute media would be facilitated.

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