Ontogenic change of gill chloride cells in leptocephalus and glass eel stages of the Japanese eel, <Emphasis Type="Italic">Anguilla japonica</Emphasis>

The development of gill chloride cells was examined in premetamorphic larvae (leptocephali) and juveniles (glass eels) of the Japanese eel, Anguilla japonica. Branchial chloride cells were detected by immunocytochemistry using an antiserum specific for Na⁺,K⁺-ATPase. The specificity and availability of the antiserum for the detection of Japanese eel chloride cells were confirmed by Western blot analysis. The chloride cells first appeared on the developing gill filaments in a mid larval stage of leptocephalus (32.2 mm). Both immunoreactivity and the number of chloride cells gradually increased as the fish grew to a late stage of leptocephalus over 54 mm. In glass eels just after metamorphosis, gill lamellae developed from the gill filaments, and a rich population of chloride cells was observed in the gill filaments. In glass eels collected at a coastal area, chloride cells were extensively distributed in the gill filaments. The chloride cell size decreased progressively in glass eels transferred from seawater (SW) to freshwater (FW), whereas there was no difference in cell number. In contrast, some Na⁺,K⁺-ATPase immunoreaction distinct from typical chloride cells was observed in the gill lamellae throughout FW-transferred fish, but disappeared in control fish maintained in SW for 14 days. These findings indicate that the gill and gill chloride cells developed slowly during the extremely long larval stage, followed by rapid differentiation during a short period of metamorphosis. The excellent euryhalinity of glass eels may be due to the presence of the filament chloride cells and lamellar Na⁺,K⁺-ATPase-immunoreaction, presumably being responsible for SW and FW adaptation, respectively.