大眼海鰱 Megalops cyprinoides 變態過程中耳石的微細構造與微化學改變之研究

Abstract

The development of Elopomorpha in early stage is characterized by a metamorphosis process from leptocephalus to juvenile. The previous study indicated that the deposition rhythm of the daily growth increments（DGIs）in otoliths of the fishes will be diffusive during metamorphosis. The present study is to use Pacific tarpon (Megalops cyprinoides) as a model animal to validate if DGIs is deposited normally during metamorphosis and their otolith microstructure and microchemistry are also examined. Leptocephali of Pacific tarpon at their Stage I (TL range 25.43 mm~31.26 mm) were collected from Gongshytyan Creek, a tributary to the Tanshui estuary between June and August 2003. The leptocephalus were immersed in a tetracycline (TC) solution with a concentration of 350μg ml-1 to produce a fluorescent TC mark in the otoliths as the onset of the experiment. They were then reared under a constant condition of 25oC, 15psu and light cycle of 10D/14L for 16 days to investigate the ontogenetic somatic and otolith changes during metamorphosis. The results showed that total length of the leptocephali dramatically decreased during the 1st to the 4th day of experiment in the Stage II. In the Stage III（6st ~10th days） the total length of the leptocephali stayed unchanged, the fish body is not transparent any more, organs and systems grew continuously. The juvenile restarted to increase (TL range average 25.66 mm~27.91 mm) after the 10th day. Otoliths remained steadily growing during metamorphoses, the number of otolith DGIs deposited after TC marks as consistant with the rearing days(p＜0.01) which validate the daily periodicity of the otolith increment deposition of Pacific tarpon leptocephali during metamorphoses. As for otolith microchemistry, Sr/Ca ratios in otolith of the fish decreased in accompany with metamorphosis, while Na/Ca ratios in otolith changed oppositely as Sr/Ca ratio. Additionally, the effect of salinity and feeding condition on the deposition of DGIs during metamorphosis were also examined, in which the leptocephali were reared in two different salinities (10psu、35psu) and two different feeding condition of feed (＋) and starvation (－) for 18 days using a Two-Way ANOVA experimental design. As a result, the ontogenetic changes and the formation of daily growth increments in otolith of Pacific tarpon leptocephali were neither affected by salinity nor starvation during metamorphosis. The Sr/Ca ratios in otoliths were also decreased rapidly at this process. The otolith growth rates were not affected by salinity but significantly affected by the feeding conditions. It also showed that the daily deposition rate of otolith was not changed by either salinity or starvation; here findings are discrepant with previous study.