Daily growth increment formation in otoliths of Pacific tarpon Megalops cyprinoides during metamorphosis

The otolith microstructure of Pacific tarpon Megalops cyprinoides was examined to determine whether there were influences on the deposition of otolith daily growth increments (DGIs) during metamorphosis from leptocephalus to juvenile. Leptocephali collected in a Taiwanese estuary in August 2003 were fully grown and completed metamorphosis in approximately 10 d during rearing in environmental conditions similar to those in the wild. The metamorphic process was divided into Stages II an IIID based on temporal changes in external morphology, otolith growth rate, and Sr/Ca ratio. The otolith increment width (OIW) was comparatively small for the leptocephalus (Stage I) before the experiment, increased rapidly at Stage II, and reached a maximum at the beginning of Stage III. In contrast, the otolith Sr/Ca ratio decreased sharply after Stage II. Metamorphosis was accompanied by a breakdown of Sr-rich sulfated glycosaminoglycans (GAGs) in the body, a change of body organization, body length shrinkage, and otolith rapid growth; growth increments in otoliths were deposited daily throughout. The metamorphic process of the Pacific tarpon leptocephalus is analogous to that of the anguillid eel, and the mechanism of otolith DGI deposition during metamorphosis may be similar.