Salinity-induced changes in the fine structure of the gills of the semiterrestrial estuarian crab, Uca uruguayensis (Nobili, 1901) (Decapoda, Ocypodidae).

The posterior gills of Uca uruguayensis are mostly lined with a thick tissue which presents the characteristics of a typical salt-transporting epithelium. Electron microscope analysis of gill tissue from crabs acclimated to both low (2.5 per thousand) and high (44 per thousand) salinity showed significant development of the basolateral membrane interdigitations with numerous mitochondria and conspicuous apical membrane infoldings. In high-salinity acclimated crabs, the basolateral interdigitations extended to the apical membrane. Under these conditions, apical infoldings were expanded laterally (forming wide subcuticular spaces), while the apical infoldings of low-salinity adapted animals appeared as regular leaflets. Septate desmosomes were also much more developed in low-salinity exposed animals than in those kept under high-salinity conditions. These morphological observations were analyzed for correlation with the currently-accepted ion hyporegulation model for crustaceans, which is mainly based on transcellular sodium flow. In this study, we propose an ion hyporegulation model involving apical paracellular sodium flux.