Adaptations with respect to salinity.

Amino acids contribute up to about 50% of the intracellular osmotic pressure of aquatic invertebrates. Since their concentration varies according to the salinity of the medium (high in sea water, low in fresh water) euryhaline invertebrates are good models for studying the mechanisms involved in the control of amino acid metabolism. During hyperosmotic stress CO2 production and O2 consumption decrease whereas the reverse is true when the animal is submitted to a hypo-osmotic stress. Nitrogen excretion (as NH3) increases in media of low salinity and the concentration of cyclic AMP increases during hyperosmotic stress. Moreover, blood proteins and haemocyanin are more concentrated in individuals adapted to media of low salinity. To explain the situation, three main mechanisms can be considered: (a) hydrolysis and synthesis of blood proteins; (b) transport of amino acids across the cell membrane; (c) control of the turnover rate of some amino acids. Results obtained on whole animals as well as on isolated tissues indicate that some amino acids are released from the cells and carried via the haemolymph to the posterior pairs of gills where they are oxidized (mechanisms b) or to an organ (hepatopancreas ?) Where they are used for blood protein synthesis (mechanism a). The use of labelled substrates demonstrates that the turnover rate of amino acids is controlled by the salinity of the environment (mechanism c). It is suggested that inorganic ions trigger the metabolic response by directing reducing equivalents toward oxygen or 2-oxo acids through control of the catalytic activity of dehydrogenases.