Relationships between water chemistry and otolith elemental concentrations of Pacific tarpon (Megalops cyprinoides) during metamorphosis

The Pacific tarpon Megalops cyprinoides are similar with Anguilliformes which has a willow-leaf-shaped larva called leptocephalus. Previous research indicated a drastic ontogenetic change in behavior, habitat, body shape, and recombination of tissue during metamorphosing from leptocephalus to juvenile stages. A maximum otolith increment width (OIW) and sharp decreasing in otolith Sr/Ca ratios resulting from glycosaminoglycan (GAGs) broken–down were observed during metamorphosis. In addition to such physiological factor, otolith microchemistry was affected mostly by ambient concentration rather than salinity and temperature. In order to compare with the affected degree of physiological factors and water chemistry during metamorphosis, we designed a controlled experiment with adding strontium (Sr) and barium (Ba) in rearing water to discuss the relationships between otolith chemistry and life stages in Pacific tarpon.
Leptocephali (entering StageⅡ) were collected in the estuary of Gong-shy-tyan Creek, a tributary of the Tanshui river in August to September 2005. After acclimating 1 day, the leptocephali were immersed in a tetracycline solution to mark the otoliths with a fluorescent band as the onset of the experiment (Day 1) and then reared in the treatments spiked with different concentrations (2×、4×、8×) of Sr and Ba. According to the changes of otolith daily growth increments (DGIs), the leptocephalus completed metamorphosis is approximately 14d during rearing conditions. The results of Sr:Ca and Ba:Ca ratios in the rearing water were measured to be 1×、5.3×、13.4×、29.0× and 1.00×、0.99×、1.15×、1.53× respectively after spiking of Sr and Ba. The Sr:Ca ratios of the otoliths were analysed with EPMA and the period of metamorphosis (M-zone) increased times in:1.0×、2.5×、7.4×、16.0× and juvenile (J-zone) in:1.0×、6.5×、20.0×、47.1×, and the phenomenon of Sr:Ca ratios rapid decreasing only presented in control treatment (Sr1×). Other Ba:Ca ratios in the otoliths were analysed with LA-ICPMS and the concentration ratios at M-zone were: 1.00×、1.19×、1.15×、1.16× and J-zone: 1.00×、1.37×、1.60×、1.81×. Although Ba:Ca ratios has less magnification effects than Sr:Ca in the otoliths, both of them reflected the relationships of non-equal times of the element spiked in rearing water. Whether in metamorphasis or juvenile stages, the Sr:Ca ratios in the otoliths were highly related to water chemistry (r=0.99) which indicated that otolith Sr:Ca ratios were mostly influenced by ambient elemental concentrations. Besides, the relationships only appeared within juvenile (r=0.70) in Ba treatments. Sr and Ba belong to alkali metals, but mean partition coefficients DMe in Sr:Ca ratios are around 20 times more than Ba:Ca ratios. In this study the great discovery is the effects of water chemistry are more important than physical factor in metamorphosis, and selective absorption rate from ambient element to otoliths is also existed especially in different life stages.