Salt and water balance in the oligohaline clam, Rangia cuneata I. Anisosmotic extracellular regulation

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Abstract
The euryhaline mollusc Rangia cuneata maintains blood NaCl above that in the ambient medium in the range 0–2 ppt salinity and below ambient in the range 12–24 ppt. A hyperosmotic condition of the blood found at low salinity disappears at 12–16 ppt, and the blood medium‐difference becomes labile at high salinity. Hypersaline regulation of the blood at low salinity is accompanied by increased oxygen uptake, both in intact animals and in isolated gill and mantle tissue. However, only mantle andnot gill oxygen uptake is depressed at low salinity by ouabain, a specific inhibitor of the Na+ + K+ ATPases. These findings, together with other results on enzyme activity and the ionic effects on enzyme inhibition, suggest that hyperosmotic regulation of the blood at low salinity involves the absorption of NaCl from the medium, and that the process requires both oxidative metabolism and unimpaired functioning of the Na+ + K+ ATPases. The excretion of ammonia is also, depressed by ouabain, suggesting an exchange of Na+ and NH4+. The NH4+ may arise during deamination of intracellular free amino acids. The size of the free amino acid pool in gill, mantle and adductor muscle, varies directly with ambient salinity. Although the evidence is less complete, we also suggest that the hyposaline condition of the blood at high salinity results from an active process, and depends upon the uninhibited activity of the Na+ + K+ ATPases.