Developmental features of Japanese eels, Anguilla japonica, from the late leptocephalus to the yellow eel stages: an early metamorphosis to the eel-like form and a prolonged transition to the juvenile.

Organogenesis of Japanese eels (Anguilla japonica) was investigated histologically from the late leptocephalus to the yellow eel stages. Early organogenesis, such as the formation of inner ears and the appearance of round blood cells that might be larval erythrocytes, had already begun at the late leptocephalus stage. During the first developmental phase (M1-M3 stages) of metamorphosing into early glass eels (G1 stage), the formation of gills and lateral muscles progressed conspicuously with a drastic body shape change from leaf-like to eel-like. In contrast, obvious regression in oesophageal muscle and pancreas occurred during metamorphosis. Formation of lateral line canals advanced continuously until the yellow eel stage. When the second developmental phase was initiated at the G1 stage, cone photoreceptor cells appeared, and the formation of oesophageal, stomach and intestinal muscles was initiated. Differentiation of gastric glands began at 1 week after metamorphosis. Erythrocytes increased continuously in density in glass eels and elvers (G1-E2 stages), and the morphological features of cone cells and olfactory epidermal cells became clearer with stage progression. In early elvers (E1 stage), the swimbladder initiated inflation, the stomach fully expanded and the rectal longitudinal fold changed to a circle. Swimbladder gas glands appeared in late elvers (E2 stage). In the yellow eels (juvenile stage), almost all organ structures were formed. These observations indicate that the organogenesis of A. japonica is ongoing after metamorphosis into glass eels, and the M1-E2 stages are considered to be a homologous phase to first metamorphosis, which is a transformation from the larval to the juvenile stages in other teleosts. In comparison to conger eels, the completion of the body shape change to eel-like occurs at the G1 stage, when organogenesis is still in progress, being followed by a prolonged duration of the G1-E2 stages before reaching the yellow eel juvenile stage, which may be a unique characteristic that is related to the early migratory life history of A. japonica.