International eel symposium, 2003 - Quebec August 11, 2003
Poster Session

Interpretation of the Dramatic Decrease in Otolith Sr/Ca Ratios and the Difference in Daily Age at Metamorphosis of Two Marine Eels

Ling, Y.-J.* Department of Zoology, College of Science, National Taiwan University, Taipei, Taiwan 106, ROC.
Iizuka, Y. Institute of Earth Science, Academia Sinca, Nankang, Taipei, Taiwan 106 ROC
Tzeng, W.-N Department of Zoology, College of Science, National Taiwan University, Taipei, Taiwan 106, ROC. wnt@ccms.ntu.edu.tw

Presenter email address: wnt AT ccms.ntu.edu.tw

Abstract Text: The Japanese eels (Anguilla japonica) display a dramatic decrease in otolith Sr/Ca ratios during metamorphosis from the oceanic leaf-like larva (Leptocephalus) to the glass eel entering the freshwater. Two possible causes exist, the environmental and physiological. To validate hypotheses about the causes, we selected two marine eels: the pike eel (Muraenesox cinereus) and the moray eel (Gymnothorax reticularis), which metamorphose without entering the freshwater. We then examined the daily growth increments in otoliths for the differences between species and inferred the evolutionary meaning of their age at metamorphosis. The Sr/Ca ratios in the otoliths of both eels decreased sharply at metamorphosis, as it does for the Japanese eel. The decrease in otolith Sr/Ca ratios may result from physiological factors such as ontogenic change, rather than the environmental factors such as salinity. The mechanism of the decrease in otolith Sr/Ca ratios during metamorphosis was reviewed. The mean (±SD) age at metamorphosis is 36.7 ± 7.1 days (n= 22) for the rock moray eel, which is significantly lower than 66.4 ± 15.9 days for the pike eel (n= 18) (p<0.01). Both marine eels metamorphose at a younger age than does in the Japanese eel, 115.8 ± 8.1 days (Cheng and Tzeng 1996) (p<0.01). The differences in age at metamorphosis may relate to the distance of the spawning area from the coast, larval dispersal pathway, or reproductive and developmental strategies.