Morphofunctional features of ionocytes in Japanese eel Anguilla japonica leptocephali acclimated to half-diluted and full-strength seawater

Abstract

Wild anguillid eel larvae inhabit the ocean during their early life stages and never experience low-salinity water until the glass eel stage. The larvae show less mortality in half-diluted seawater than in full-strength seawater in captivity; however, physiological influences of environmental salinity on eel larvae have not been clarified. In this study, we compared the distributional and functional features of ionocytes between Japanese eel larvae acclimated to half-diluted and full-strength seawater. The mean tissue fluid osmolality in larvae acclimated to half-diluted seawater (300 mOsm/kg H2O) was slightly lower than in those (344 mOsm/kg H2O) acclimated to full-strength seawater. The density and opening size of ionocytes in the skin were not significantly different between the two salinities. Na+/K+-ATPase-immunoreactive ionocytes showed Na+/H+ exchanger-3 (NHE3) and cystic fibrosis transmembrane conductance regulator (CFTR) immunoreactions in their apical region and Na+/K+/2Cl- cotransporter-1 (NKCC1) immunoreaction in their basolateral region, suggesting that the skin ionocytes are involved in salt secretion in both salinities. In transmission electron microscopic observation, the ionocytes of larvae in full-strength seawater were characterized by the electron-dense cytoplasm, expanded tubular system and well-developed mitochondria, compared with those in half-diluted seawater, suggesting that the salt-secreting function was more activated in full-strength seawater than in half-diluted seawater. These results suggest that the energy metabolism cost of ion regulation could be lower in the intermediate salinity environment, which is closer to their osmolality than full-strength seawater. Hence, it is hypothesized that the saving of energy required for osmoregulation in half-diluted seawater could be favorable to better survival and growth of artificial eel larvae.