Effects of Ambient Salinities on Carbohydrate Metabolism in Two Species of Tilapia Oreochromis mossambicus and O. niloticus

Abstract

Metabolic aspects of osmoregulation were examined in two species of tilapia. Oreochromis niloticus, a moderate hypo-osmoregulator, was reared in fresh water (FW) or 50% seawater (SW) for 4 weeks with limited diet, while more euryhaline species, O. mossambicus, was kept in FW, 100% SW, or 160% SW. There was no significant difference in plasma concentrations of growth hormone, and Na+ and Cl- among the fish acclimated to different salinities in both species, indicating successful adaptation to each salinity. Plasma levels of prolactins (PRL177 and PRL188) were significantly higher in FW than in hypertonic environments in both species, confirming their important roles in FW osmoregulation. Activities of phosphofructokinase, glycogen phosphorylase, and glucose-6-phosphate dehydrogenase, key enzymes in carbohydrate metabolism in the liver, were all higher in 50% SW than in FW in O. niloticus, whereas no difference was seen in O. mossambicus among different salinities. Plasma glucose level also was higher in hyperosmotic environment than in FW only in O. niloticus. There was no difference in liver glycogen contents among the fish adapted to different salinities in both species. The increase in the liver metabolic activity in hyperosmotic environment observed only in O. niloticus, suggests a different efficiency in energy utilization for hypo-osmoregulation between the two species of tilapia.