Changes to chloride and rodlet cells in gills, kidney and intestine of Dicentrarchus labrax (L.) exposed to reduced salinities.

The effect of osmotic shock was investigated mainly in the chloride cells (CCs) and rodlet cells (RCs) of gills and RCs of intestine and kidney of the European sea bass (Dicentrarchus labrax L.) obtained from a farm in northern Adriatic Sea. During the experiment, fish were abruptly transferred from seawater to 15‰ seawater (15‰ SW) or to freshwater (FW). Numeric variation and ultrastructural changes of both cell types were evaluated at 24 h, 48 h and 96 h after the transfer to lower salinity levels, using light and transmission electron microscopy (TEM). Exposure to freshwater produced a significant increase (Anova, P< 0.05) in the number of branchial CCs and RCs within 96 h and 24 h, respectively. Following osmotic challenge (either transfer to 15‰ SW and/or transfer to FW), kidney and intestine showed an evident increase in RC numbers. The cellular damage detected by TEM was the same for each sampling time (24, 48, 96 h) but appeared more severe in fish exposed to FW (higher osmotic shock) than in those exposed to 15‰ SW. In RCs cytoplasmic vacuolisations, autophagosomes and autophagolysosomes with myelinoid bodies, dissolution and shrinkage of the typical inclusions were documented. Nevertheless, CCs showed vacuolisation of endoplasmic reticulum and cytoplasmic dissolution and maintained the apical crypt typical of seawater acclimated fish. Renal tubular cells and intestinal epithelial cells showed similar changes to those reported for CCs and RCs.