Short-Term Salinity Acclimation Demands Thyroid Hormone Action in the Climbing Perch Anabas testudineus Bloch

Fishes have developed many complex physiological mechanisms to combat osmotic challenges. In this study triiodothyronine (T₃), thyroxine (T₄) and cortisol in the plasma were quantified and the indices of metabolic and hydromineral regulations analyzed in the climbing perch Anabas testudineus after exposing the fish to a selected salinity (20 ppt) for varied intervals (1, 7, 14 and 21 days) to study the physiological basis of short- and long-term salinity acclimation. It was found that transfer of fish to 20 ppt salinity for a day after transient salinity changes, plasma T₄ was elevated, and plasma T₃ decreased whereas plasma cortisol remained unchanged. The levels of these hormones, however, returned to basal levels when these fish were kept for a prolonged acclimation of three weeks. Plasma glucose and lactate showed no change in response to salinity acclimation, whereas plasma urea showed an increase. Substantial increase in the gill Na⁺, K⁺-ATPase activity was found in these fish during salinity acclimation, which remained high even after three weeks. Salinity transfer for a day produced significant increase in the intestinal Na⁺, K⁺-ATPase activity, though it remained unaffected during the long-term acclimation. Kidney Na⁺, K⁺-ATPase activity decreased on day 1 salinity challenge, but remained unaltered after prolonged acclimation. Liver Na⁺, K⁺-ATPase activity increased upon transient salinity challenge but the levels were maintained during prolonged salinity challenge. Our results indicate that salinity acclimation in climbing perch demands thyroid hormone secretion and its action and not cortisol as part of co-ordinating the acclimation processes in the early phase of salinity acclimation. The results also point to the ability of climbing perch to tolerate osmotic challenge, and the fish becomes fully adaptive to brackish water salinity of 20 ppt after three weeks.