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Sr/Ca ratios in the otoliths of 2 marine eels, the moray eel Gymnothorax reticularis and the pike eel Muraenesox cinereus, decreased greatly when the crystal structure of otolith daily growth increments changed from concentric to radiate form. This decrease is proposed to correspond to the timing of metamorphosis from the leptocephalus to glass eel stage, similar to freshwater eels (Anguilla spp.). In freshwater eels, ontogenetic and habitat shifts might influence the decrease. However, marine eels do not migrate into freshwater after completing metamorphosis, so the decrease in these species must be the result of metamorphosis-related physiological, rather than environmental, effects. The mean age at metamorphosis of the moray eels was significantly lower than that of pike eels, and both were lower than that of freshwater eels (p < 0.05). Consequently, the spawning ground of marine eels is presumed to be closer than that of freshwater eels to the continental shelf.