Fine structure of the skin cells of a stenohaline freshwater fish Cyprinus carpio exposed to diluted seawater M. Abraham, Y. Iger, L. Zhang Abstract. 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. ß 2001 Harcourt Publishers Ltd Keywords: carp, skin, seawater, ultrastructure Introduction The usual classi®cation of ®sh into stenohaline and eu- ryhaline species does not indicate the degree of adaptabi- lity of a euryhaline ®sh to a biotope different from its own. The rainbow trout and the brook trout are both considered euryhaline, although the former can adapt to seawater while the later shows signs of a pronounced osmotic disequilibrium when transferred into seawater, and dies a few weeks after the transfer (Boeuf and Harache, 1982). Aphanius mento was considered a steno- haline species due to its normal occurrence in freshwater. When tested experimentally it was shown that A. mento is euryhaline, although much less than A. dispar (Lavie, 1980). Carassius auratus is considered a stenohaline fresh- water species although it survives in 400 mOsm-diluted seawater (Kikuchi, 1977). The carp is considered a species that is very resistant to oscillations in temperature, oxygen content of water and pH (Wohlfarth and Hulata, 1995). At the same time, it is known for its high sensitivity to the rise of salt content of the water in which it dwells. It is one of the most dissem- inated aquaculture species and were it not for its strict freshwater stenohalinity, it would probably occupy the ®rst place in the world of ®sh breeders. Aside the theore- tical interest to understand the physological challenges presented to a freshwater animal to survive in a saline biotope, there is a practical interest of inestimable im- portance for mankind to enable aquaculturists to grow rigorous stenohaline freshwater ®sh species in saline water. The effect of changes in environmental osmolality on the ultrastructure of gill and skin of ®sh has been recorded in several publications (King et al., 1989; Abraham et al., 1991). Particular attention was given to Tissue & Cell, 2001 33 (1) 46±54 ß 2001 Harcourt Publishers Ltd DOI: 10.1054/tice.2000.0149, available online at http://www.idealibrary.com Tissue &Cell 46 Department of Cell and Animal Biology, The Hebrew University of Jerusalem, Jerusalem, Israel Received 18 April 2000 Accepted 12 September 2000 Correspondence to: Prof. M. Abraham, Department of Cell Biology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel; Tel.: 972 2 6585941; Fax: 972 2 5617918; E-mail: morab@cc.huji.ac.il