Effects of long-term exposure to different salinities on the location and activity of Na+–K+-ATPase in the gills of juvenile mitten crab, Eriocheir sinensis

The euryhalinity of mitten crab, Eriocheir sinensis, is based on osmoregulation, and thus on the activity of Na+–K+-ATPase. We studied location and activity of this enzyme in gills of juvenile crabs exposed to 5‰, 25‰, and 40‰ salinity. The posterior gills showed always a high number of immunopositive cells (IPC), staining with fluorescent antibody against Na+–K+-ATPase, covering at 5‰ the entire lamellae. At 25‰, they showed fewer IPC which occurred only at the bases of the lamellae. Enzyme activity was consistently higher in posterior than in anterior gills. Low salinity stimulated the activity only in posterior gills. Both histochemical and enzymatic results are consistent with previous ultrastructural observations showing that the epithelial cells of the posterior, but not the anterior gills exhibit typical traits of ionocytes. While an increase in Na+–K+-ATPase activity at a reduced salinity is consistent with a strong hyper-osmoregulatory capacity in juvenile crabs, a low activity at an enhanced salinity suggests a physiological response, directed towards a reduction of Na+ uptake. The activity increase of ion-transporting enzymes is directly related to spatial changes in their distribution along the osmoregulatory tissue, i.e. an enhanced number of IPC scattered along the entire lamellae. In juveniles, this allows for successful development and growth at reduced salinities.