Ontogeny of Osmoregulatory Structures in the Shrimp Penaeus japonicus (Crustacea, Decapoda).

The ontogeny of differentiated osmoregulatory epithelia in the branchial chamber (gills, branchiostegite, pleura, epipodite) was studied by transmission electron microscopy throughout the postembryonic development of Penaeus japonicus. These epithelia are characterized by typical cytological features, including apical microvilli and numerous basal infoldings associated with mitochondria. Differentiated osmoregulatory structures are not observed in the early larval stages: nauplii and zoea 1. In the next larval stages, zoeas and mysis, gills and epipodites are developed as buds only, but osmoregulatory epithelia are observed in the branchiostegites and pleurae. The differentiated structures of the branchiostegites and pleurae are still present in postlarvae but disappear in juveniles and adults. Gills and epipodites develop progressively in the postlarval stages, with early differentiation of osmoregulatory epithelia in the epipodites. In juveniles and adults, the gill epithelium is poorly differentiated; in contrast, abundant differentiated osmoregulatory structures are observed in the epipodites. Ontogenetical comparisons of these observations with previous studies in the same species reveal strong correlations between the development of osmoregulatory epithelia, the ability to osmoregulate, the activity of Na+-K+ ATPase, and salinity tolerance during the postembryonic development of Penaeus japonicus.