Branchial chambers constitute the main osmoregulatory site in almost all decapod crustaceans. However, few studies have been devoted to elucidate the cellular function of specific cells in every osmoregulatory structure of the branchial chambers. In decapod crustaceans, it is well-known that the osmoregulatory function is localized in specific structures that progressively specialize from early developmental stages while specific molecular mechanisms occur. In this study, we found that although the structures developed progressively during the larval and postlarval stages, before reaching juvenile or adult morphology, the osmoregulatory capabilities of Litopenaeus vannamei were gradually established only during the development of branchiostegites and epipodites, but not gills. The cellular structures of the branchial chambers observed during the larval phase do not present the typical ultrastructure of ionocytes, neither Na+/K+-ATPase expression, likely indicating that pleura, branchiostegites, or bud gills do not participate in osmoregulation. During early postlarval stages, the lack of Na+/K+-ATPase immunoreactivity of the ionocytes from the branchiostegites and epipodites suggests that they are immature ionocytes (ionocytes type I). It could be inferred from IIF and TEM results that epipodites and branchiostegites are involved in iono-osmoregulation from PL15, while gills and pleura do not participate in this function.