The studies of the changes on the morphology, physiology and endocrinology during silvering process of the Japanese eel Anguilla japonica

• Main Authors: Han, Yu-San, 韓玉山
• Other Authors: 廖一久
• Format: Others
• Language: zh-TW
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/34262531484713336538

博士 === 國立臺灣大學 === 動物學研究所 === 91 === This study aims to understand the changing morphological and physiological indices associated with the changes in endocrine system of the Japanese eel Anguilla japonica during silvering. Eels were collected from the Kaoping River estuary in southwestern Taiwan from November 1998 through August 2002. Based on external skin coloration and gonadal development, the females were classified into yellow, pre-silver and silver eel stages. The males were classified into yellow and silver eel stages. The maturity of eel gonads and oocyte diameters were judged by histology. The related genes of the pituitary and their expression levels were determined by reverse transcriptase-polymerase chain reaction method. The serum levels of estradiol-17β、testosterone and thyroxine were measured by radioimmunoassay. The results showed that the sex ratio of the wild eels in Taiwan skewed to the females. Most pre-silver eels were found in the summer, and silver eels were found in the winter. The eels reached maximum size at silver stage, and the mean silver size was larger in females than males. The mean age at maturity was 6.8 and 6.0 years for female and male eels, respectively. The mean muscle fat contents were higher in the silver eels than yellow ones, but no significant difference was found between male and female silver eels. The gonadosomatic index of both sexes significantly increased during silvering. The oocytes developed from the chromatin nucleolus stage in the yellow eels, to the peri-nucleolus stage in the pre-silver eels, and the oil-drop stage in the silver eels. Spermatogonia, mainly in late type B stage, were active in mitosis in silver males. The changes in skin color, eye size and digestive tract were synchronous with the development of gonad, irrespective of natural silvering or induced maturation, indicating that the gonadal development was closely correlated to silvering process. The expression of the pituitary gonadotropin and thyroid-stimulating hormone mRNA significantly increased during eel silvering, which paralleled with the increase in serum sex steroids and thyroxine levels. On the other hand, the expression of the pituitary prolactin hormone mRNA significantly decreased during eel silvering, together with the increase of Sr/Ca ratios in the otolith edge, inferring the shifting of the silvering eels from freshwater to high saline water. These results indicated that the onset of silvering in the Japanese eels was dependent on both environmental cue (season) and internal physiological cues (size, age and muscle fat content). The changes in morphology and physiology of the eels during silvering process were effected by pituitary-gonad axis for pre-adaption to long-distance spawning migration.