The serum and muscle ions, Na, K, Ca, Mg and Cl and serum osmotic concentration of two euryhaline and two stenohaline penaeids were studied. In the euryhaline forms, Penaeus setiferus and P. aztecus serum is hyposmotic to sea water (ca. 89 per cent) and the serum ions
account for 94-96 per cent of the osmotic concentration. In the stenohaline penaeids, Trachypeneus similis and Sicyonia dorsalis the serum is slightly hyposmotic to sea water (ca. 97-98 per cent). Serum ions account for 95 per cent of the osmotic concentration in T. similis,
but only 84 per cent in S. dorsalis. High Mg levels occur in S. dorsalis, but not the other penaeids and this relates to its more sluggish activity. Muscle K concentrations are highest in the euryhaline penaeids, intermediate in T. similis and lowest in S. dorsalis.
Thus, T. similis, in its serum ions, has affinities to the euryhaline species to which it is closely related and, with respect to muscle K and serum osmotic concentration, affinities to S. dorsalis, with which it shares some ecological similarities. Differences in serum regulation
of ions and osmotic concentration in P. setiferus and P. aztecus under hyposaline and hypersaline conditions, are shown to coincide with the different salinity distribution of the species in nature.
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