Mytilus edulis suffered reductions in trans-membrane sodium concentration gradient during dilute salinity acclimation. Nevertheless the nerves of this osmoconformer had a conventional basis for their excitability, irrespective of the salinity to which they were adapted, and could produce full-sized spikes after acclimation to 25% media.

2. The connectives showed rapid and predictable responses to sodium and to potassium ions, and the time-courses of these effects were unrelated to the presence or absence of the neural sheath at either of the acclimation salinities tested. Neural adaptation was therefore not a consequence of restricted access after acclimation to dilute media.

3. Classic pharmacological agents (tetrodotoxin, tetra-ethyl ammonium ions, 2,4-dinitrophenol and ouabain) elicited the expected responses, but invariably required rather high concentrations and long exposures for full effect.

4. Acute exposures of the connectives to hyposmotic or isosmotic dilutions resulted in changes in polarization accompanying the decline of the action potential. These were attributed to losses of potassium and of chloride from the axons, with a net loss of anion (and hence depolarization) during naturally occurring hyposmotic stress.

5. Mytilus connectives exhibited a critical salinity, with hyposmotic exposure below this level producing irreversible impairment of function. This salinity occurred at 20% s.w. for 100%-adapted tissues, and at 3.25% s.w. (i.e. 15% initial concentration) for 25%-adapted nerves. Gross isosmotic dilutions produced no permanent decrements in axonal function.

6. The observed patterns of response to chronic and acute osmotic stress in Mytilus are contrasted with those reported in annelid and crustacean conformers, and possible adaptations which might underlie these responses are discussed.

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