Effects of different salinity on number and area of chloride cells in gill of juvenile grouper (Epinephelus coioides)

The purpose of this study was to evaluate the ability of osmoregulation by mitochondria-rich cells in grouper fish with high tolerance of salinity variation levels. For this purpose, groupers were transferred from salt water with an average salinity of 40 ppt to the waters with 10 ppt, 20 ppt and 60 ppt rates of salinity. Changes in the number and area of chloride cells in the different treatments were observed during two months adjustment period. Experiment was carried out by sampling three fish from each tank within 8 phases at the moment of the transition, 12 hours, day 1, day 3, day 7, day 14, day 30 and day 60. Histomorphological and immunohistochemical observations were done after fixing in Bouin's solution for 24 hours. Samples were dehydrated with increasing series of ethanol, followed by paraffin, and cleared by xylene. Paraffin blocks were cut at 5 microns and stained by hematoxylin – eosin. Changes in the number and area of chloride cells in 5% level were very different at high and brackish salinity. So in the early hours of the transfer, number and area of chloride cells were low in the brackish water than to that in high salinity. From second week towards the end of period, number of chloride cells in 60 ppt was more than that in control treatment from seventh days towards the end of period, the area of chloride cells was more than that in control group. The high ability and adaptability of this species in response to different environmental salinities might be associated with changes in gill histological evaluation and model osmoregulation mechanisms at different salinities.