ULTRASTRUCTURE OF THE PSEUDOBRANCH IN THE EURYHALINE CYPRINODONTID FISH, RIVULUS-MARMORATUS

The ultrastructure of the pseudobranch of the euryhaline, self-fertili zing fish, Rivulus marmoratus (Cyprinodontidae) was studied with thin section and freeze-fracture transmission electron microscopy. In speci mens raised from birth in 1% or 200% seawater, the pseudobranch contai ns two mitochondria-rich cell types, pseudobranchial cells and chlorid e cells, each of which has an extensive tubular system. Chloride cells only occur on the lateral aspects of the pseudobranch, with their api cal crypts open to the environment. Apical crypt invagination to a ''p it'' structure and multicellular complexes occur in both salinity extr emes. The ultrastructure of chloride cells in 1% and 200% SW is consis tent with that described previously in the opercular epithelium, operc ular skin, and gill of this species; elaboration of mitochondria and b asolateral membrane accompanies increased environmental salinity (King et al.: Cell and Tissue Research 257:367-377, 1989). Pseudobranchial cells constitute the majority of the cells; they do not extend to the surface but have a more organized tubular system that is continuous wi th the basal membrane. These cells do not exhibit ultrastructural chan ges in response to increased salinity. (C) 1993 Wiley-Liss, Inc.