OSMOREGULATION IN THREE MOLLUSCS: ACANTHOCHITONA DISCREPANS(BROWN), GLYCYMERIS GLYCYMERIS (L.) AND MYTILUS EDULIS (L.)

¹ Laboratory of Marine Membrane Physiology, Duke University Marine Laboratory Beaufort, North Carolina 28516

The molluscs Glycymeris glycymeris, Mytilus edulis and Acanthochitona discrepans can be acclimated from sea water to salinities down to 25% sea water. During this acclimatization, these molluscs do not show any extracellular anisosmotic regulatory power except for potassium, the blood level of which is regulated at the concentration it has in the blood of sea water aclimated animals.

During rapid acclimatization to diluted media, a transistory hyperosmotic state can be recorded in both blood and perivisceral fluid of the two bivalves species (Glycymeris glycymeris, and Mytilus edulis) but not in the blood of the polyplacophora Acanthochitona discrepans. This hyperosmotic state, which can last for about 96 hours in both bivalves when placed suddenly in diluted media, is due to the ability of those molluscs to isolate themselves from the external medium by closing their valves tightly. This "shell-closing" mechanism may help the animals in withstanding a sudden osmotic stress but it cannot contribute to the osmotic regulation observed in these species.

In the studied species, it is the cell which has to cope with the osmotic stress and amino acids play a part in the cellular osmoregulation process. Taurine also appears as an important osmotic effector. Our results show that the concentration of the intracellular free amino acids and of taurine is higher in the muscle of sea water acclimated animals than in the same tissue taken from animals adapted to 50% sea water. The observed changes are greater than those which could be expected from the increase in the tissue water content recorded during the hypoosmotic stress.

The possible mechanisms implicated in the regulation of the amino acids and taurine concentration during the acclimatization of molluscs to diluted media are discussed.