Interactive effects of ambient acidity and salinity on thyroid function during acidic and post-acidic acclimation of air-breathing fish (Anabas testudineus Bloch)

The interactive effects of ambient acidity and salinity on thyroid function are less understood in fish particularly in air-breathing fish. We, therefore, examined the thyroid function particularly the osmotic and metabolic competences of freshwater (FW) and salinity-adapted (SA; 20 ppt) air-breathing fish (Anabas testudineus) during acidic and post-acidic acclimation, i.e., during the exposure of fish to either acidified water (pH 4.2 and 5.2) for 48 h or clean water for 96 h after pre-exposure. A substantial rise in plasma T4 occurred after acidic exposure of both FW and SA fish. Similarly, increased plasma T3 and T4 were found in FW fish kept for post-acidic acclimation and these suggest an involvement of THs in short-term acidic and post-acidic acclimation. Water acidification produced significant hyperglycaemia and hyperuremia in FW fish but not in SA fish. The SA fish when kept for post-acclimation, however, produced a significant hypouremia. In both FW and SA fish, gill Na+, K+-ATPase activity decreased but kidney Na+, K+-ATPase activity increased upon acidic acclimation. During post-acidic acclimation, gill Na+, K+-ATPase activity of the FW fish showed a rise while decreasing its activity in the SA fish. Similarly, post-acidic acclimation reduced the Na+, K+-ATPase activity of intestine but elevated its activity in the liver of SA fish. A higher tolerance of the SA fish to water acidification was evident in these fish as they showed tight plasma and tissue mineral status due to the ability of this fish to counteract the ion loss. In contrast, FW fish showed more sensitivity to water acidification as they loose more ions in that medium. The positive correlations of plasma THs with many tested metabolic and hydromineral indices of both FW and SA fish and also with water pH further confirm the involvement of THs in acidic and post-acidic acclimation in these fish. We conclude that thyroid function of this fish is more sensitive to environmental acidity than ambient salinity and salinity interference nullifies the toxic effect of water acidification.

► Role of thyroid in acidic acclimation was studied in air-breathing fish.
► Activated thyroid suggest that they are involved in acidic acclimation.
► Fish thyroid shows more sensitivity to acidity than salinity.
► Ambient salinity attenuated the toxic impact of water acidification.