Osmoregulation and immunolocalization of Na+/K+-ATPase during the ontogeny of the mitten crab Eriocheir sinensis (Decapoda, Grapsoidea)

The ontogeny of osmoregulation was studied in laboratory-reared early developmental stages of the Chinese mitten crab Eriocheir sinensis from the Elbe estuary (North Sea, Germany). At salinities ranging from 0.16 to 44.3‰, survival rate was quantified, hemolymph osmolality was measured, and osmoregulatory capacity was calculated as the difference between osmolality of the hemolymph and that of the external medium. Zoea larvae hyper-regulated in dilute media, but osmoconformed in seawater and at higher salinities (≥32.2‰). Megalopae and Stage I–II juveniles hyperregulated at low salinities and hypo-regulated at ≥32.2‰, with an ontogenetic increase in osmoregulatory capacity. Survival at ca. 10 to 32‰ was generally high (90 to 100%), while complete mortality occurred in all zoeal stages (except for zoea I) at 0.16 to 5.3 ‰. By contrast, nearly 50% of the megalopae and all juvenile crabs survived at such low salinities. The expression of Na+/K+-ATPase and the development of transporting epithelia were studied by means of immunofluorescence light microscopy (ILM) and transmission electron microscopy (TEM). In early (stage I & II) zoeae, fluorescence staining was observed along the inner epithelium of the branchiostegites, and epithelial cells showed typical features of ionocytes. In the megalopa and juvenile crab stage I, ionocytes and immunolabeled Na+/K+-ATPase were located in the filaments of the most posterior gills, while no immunolocalization occurred in the anterior gills. Comparison of histological and physiological results shows a close relationship between the ontogeny of osmoregulation and the expression of Na+/K+-ATPase within the transporting epithelia of the branchial chamber. In conclusion, the adult pattern of osmoregulation develops in E. sinensis through 2 molts, (1) from a moderately hyper-isoregulating zoeal phase to the moderately hyper-/hypo-regulating megalopa, (2) from the megalopa to a strongly euryhaline, hyper-/hypo-regulating first juvenile crab stage. The results of this study are consistent with an export strategy in this holo-euryhaline crab species.