Drastic changes in otolith microstructure and microchemistry accompanying the onset of metamorphosis in the Japanese eel Anguilla japonica

Otolith microstructure and microchemistry were examined in Anguilla japonica final stage leptocephali and glass eels. Otolith increment width, relatively constant and narrow in the leptocephalus, increased sharply from age 80 to 160 d. Sr:Ca ratios in the otolith, which increased during the leptocephalus stage, thereafter showed a rapid drop, coinciding with the increase in increment width. A comparison of the ontogenic change patterns in otolith increment width and Sr:Ca ratios between leptocephali and glass eels strongly suggested that these coincidental changes between 80 and 160 d indicate the onset of metamorphosis in the leptocephali. Metamorphosis appeared to have been completed before the increment width had reached its maximum, suggesting a duration of the former of some 20 to 40 d.