Correspondence between otolith microstructual changes and early life history events in Anguilla marmorata leptocephali and glass eels

To determine the exact correspondence between otolith characteristics and early life history events such as metamorphosis, coastal migration and recruitment to estuaries, both otolith microstructure and microchemistry analyses were applied to a sequential developmental series of samples, e. g. leptocephali, a metamorphosing larva, oceanic glass eels, and coastal glass eels. Total length and age were 10.1-50.7 mm and 22-137 d in leptocephali, 46.3 mm and 147 d in a metamorphosing larva, 47.8, 48.6 mm and 159, 160 d in oceanic glass eels, and 47.9-54.8 mm and 119-168 d in coastal glass eels. Checks at hatching and first feeding were observed in all specimens, but metamorphosis and freshwater checks were observed only in some specimens. It was confirmed that the abrupt drop in otolith Sr : Ca ratios and drastic increases of otolith increment widths in the metamorphosing larval stage correspond to the onset of metamorphosis, and the decrease after the peaks suggested the completion of metamorphosis, because the metamorphosing larva had no decrease in incremental widths. The relatively conserved Sr : Ca ratios decreased sharply in synchrony with the increasing increment widths. This study provides the first direct evidence that these drastic changes in otolith microstructure and microchemistry actually occur during metamorphosis, which has been only hypothesized