Fine structure of the skin cells of a stenohaline freshwater fish Cyprinus carpio exposed to diluted seawater.

Seawater diluted to half (1.750% salinity) is lethal for adult carps after 3 h and 15 min. At lower salinities (0.350%-0.875%), the fish survived for longer periods, but only 0.175% salinity was innocuous. In carps, adapted to 0.175% salinity, the secretory activity of pavement cells was very high and their external ridges flattened or even disappeared. Mucus secretion was conspicuous, characterized by holocriny of old cells and apparition of young ones in large numbers. The intracellular mucus droplets often coalesced. Pavement cells and mucus cells were disconnected from their neighboring tissue fabric and were sloughing off. Mitotic figures of filament cells were frequent, suggesting high turnover. Club cells appeared near the epidermal surface. The number of pinocytotic vesicles of the basal cell layer markedly decreased, indicating a possible decrease in dermis-epidermis molecular transfers. Leucocytes, mainly lymphocytes penetrated into the epidermis, where also rodlet cells appeared. The low salinity tolerance of the carp might be related among other possible factors to the absence of chloride cells in the skin.