Ontogeny of osmoregulatory structures and functions in the green crab Carcinus maenas (Crustacea, Decapoda)

The ontogeny of osmoregulation, the development of branchial transporting epithelia and the expression of the enzyme Na+/K+-ATPase were studied in Carcinus maenas (L.) obtained from the North Sea, Germany. Laboratory-reared zoea larvae, megalopae and young crabs were exposed to a wide range of salinities, and hemolymph osmolality was measured after 24 h exposure time (72 h in juveniles). Zoea I larvae slightly hyper-regulated in dilute media (10.2parts per thousand and 17.0parts per thousand) and osmoconformed at >17parts per thousand. All later zoea I stages (II-IV) osmoconformed in salinities from 10.2parts per thousand to 44.3parts per thousand. The megalopa hyper-regulated at salinities from 10.2 to 25.5parts per thousand. Young crabs hyper-regulated at salinities from 5.3parts per thousand to 25.5parts per thousand, showing an increase in their osmoregulatory capacity. The development of transporting epithelia and the expression of Na+/K+-ATPase were investigated by means of transmission electron microscopy and immunofluorescence microscopy. In the zoea IV, only a very light fluorescence staining was observed in gill buds. Epithelial cells were rather undifferentiated, without showing any features of ionocytes. Gills were present in the megalopa, where Na+/K+-ATPase was located in basal filaments of the posterior gills. In crab I juveniles and adults, Na+/K+-ATPase was noted in the three most posterior pairs of gills, but lacking in anterior gills. Ionocytes could first be recognized in filaments of megalopal posterior gills, persisting through subsequent stages at the same location. Thus, the development of the gills and the expression of Na+/K+-ATPase are closely correlated with the ontogeny of osmoregulatory abilities. The morphological two-step metamorphosis of C maenas can also be regarded as an osmo-physiological metamorphosis, (i) from the osmoconforming zoeaI stages to the weakly regulating megalopa, and (ii) to the effectively hyper-regulating juvenile and adult crabs.