Summary of Research Project Results under JSPS FY2001
"Research for the Future Program"

1.Research Institution The University of Tokyo
2.Research Area Life Sciences
3.Research Field Molecular Bioengineering of Food Animal Protein Resources
4.Term of Project FY 1997 〜 FY 2001
5.Project Number 97L00901
6.Title of Project Elucidation of Life Cycle of Eel and Artificial Control of its Reproduction

7.Project Leader
Name Institution,Department Title of Position
Katsumi Aida The University of Tokyo, Graduate School of Agricultural and Life Sciences professor

8.Core Members

Names Institution,Department Title of Position
Katsumi Tsukamoto The University of Tokyo, Ocean Research Institute professor
Toyoji Kaneko The University of Tokyo, Ocean Research Institute associate professor
Yuzuru Suzuki The University of Tokyo, Graduate School of Agricultural and Life Sciences professor

9.Summary of Research Results

1. Elucidation of Life Cycle
It was clarified that the Japanese eel spawns periodically once a month during new moon near three seamounts located Northwest of the Mariana Island. Leptocephali of Anguilla celebesensis and A. borneensis were collected in the Celebes Sea and Tomini Bay. This discovery indicated that tropical eels make shorter spawning migration compared to temperate species. Trajectories of satellite tracked Argos buoys suggested that leptocephali have been transported from the seamounts. Otolith studies revealed that Japanese eel leptocephali drift passively about 100 days and after metamorphosing from leptocephali into glass eels, they recruit to their freshwater habitats about five months after hatching. A long spawning season lasting about 8 months from May to December was also indicated. Analysis of eels collected from various areas in Japan indicated that 25-50% of eels inhabiting coast or estuarine areas had never entered freshwater. This indicates a need for studies on the reproductive contribution of this "sea resident population" from a fisheries perspective. Stable 18O/16O in the otolis of leptocephali and glass eels indicated that there was no remarkable change in water temperature between spawning and larval periods, suggesting that they spawn at 16-28C and 50-250m depth. The same swimming depth of migrating silver eels in the Pacific Ocean was also suggested by tracking artificially matured female Japanese ells using pressure-sensitive ultrasonic transmitters.
2. Artificial control of reproduction and larval rearing
Techniques for induction of ovarian and testicular maturation were developed. Weekly injection of salmon pituitary homogenate followed by a final injection of DHP was most effective for induction of ovulation. For males weekly hCG injection was effective. Hatched larvae were constantly obtained by artificial insemination. Artificial food was developed using freeze dried powder of shark eggs, oligopeptides, vitamin mixture, mineral mixture and krill extract. Hatched larvae survived more than 1 year, and developed to leptocepali of 5cm body length. It was the first success in the world. High pressure conditions of speculated spawning depth (about 200m) delayed hatching. This result suggests that hatching occurs at more developed stage of larvae under natural spawning conditions. Chloride cells were found to appear on the yolk suck membrane and body surface before the gill appears. New lectins were isolated from the skin, and primary structures were identified. In order to improve techniques for artificial maturation and egg quality, reproductive mechanisms following artificial maturation were elucidated. cDNAs encoding enzymes for the production of steroid hormones in the ovary, estrogen receptors and vitellogenin were cloned, and their expressions were analyzed. cDNAs encoding factors involved in the spermatogenesis were also cloned, and their functions were determined. Functional recombinant GTH was produced using yeast to induce gonadal maturation by homologous GTH.

10.Key Words

(4)lifecycle、(5)population structure、(6)spawning
(7)reproductive mechanisms、(8)artificial maturation、(9)larval rearing