Aquaculture of Etroplus suratensis is being practised at various water salinities. In this investigation, morphological responses of gill tissue of E. suratensis to various osmotic changes induced by freshwater (0%) and sea water (36%) were studied. Ultra-structural studies revealed significant decrease in the density of type I Mitochondrial rich cells in the apical surface of gills at 18% and complete disappearance of these cells at 36%. Type III subtype density increased consistently from 0% to 36%. Variations in type II cells density were comparatively less in any of the experimental groups. Light microscopic changes included reduction in mucous cell number with increasing salinity. Haemorrhages, lamellar fusion, lamellar curling, lifting of epithelium & hyperplasia are major histological responses to salinity shock. Observable clinical signs in fish exposed to increased salinity changes included agitated behaviour and respiratory distress.


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