Effect of salinity adaptation on nitrogen metabolism in the freshwater fish Tilapia mossambica. I. Tissue protein and amino acid levels

Water, proteins and total free amino acids were estimated in different tissues of the euryhaline fish Tilapia mossambica after adaptation to various strenghts of sea water. The water content did not vary significantly in any tissue on salinity adaptation. The soluble and insoluble proteins displayed a general and considerable decrease in muscle, liver and heart; the decrease in the soluble fraction in the heart and the proteins of the muscle in 75% sea water (100% sea water=32.5 S) were significant. The gill proteins did not alter with salinity; the kidney proteins tended to increase slightly in 100% sea water (SW). The total free amino-acid content decreased insignificantly in all tissues on adaptation to 25% SW; in higher salinities, however, the content increased significantly. This increase was sudden and steep in 50% SW, and gradual and less steep in 75 and 100% SW. It is suggested that constancy in water content may contribute to the great adaptability of T. mossambica to heterosmotic media, and that the total free amino acids may be involved in isosmotic intracellular regulation. The possibility of amino acid increase as a result of protein breakdown is also indicated.