1Department of Life Science, International Christian University, Mitaka, Tokyo 181-8585, 2Department of Biosphere Sciences, School of Human Sciences, Kobe College, Nishinomiya, Hyogo 662-8505, Japan
The population of Japanese medaka Oryzias latipes has decreased in size because of recent changes in the aquatic environment, and medaka are now recognized as an endangered species. For the efficient conservation of medaka, information on the reproductive ecology of medaka is essential. However, there have been no reports on how wild medaka spawn in nature. In the present study, in order to clarify the spawning behavior and the environment appropriate for such behavior, we observed the spawning behavior of wild Japanese medaka which have been reproducing naturally for several generations in an outdoor pond. During the spawning period, the courtship behavior of males to females started early in the morning and spawning (egg release and sperm release) was observed in the morning. No special substrate was necessary for spawning behavior. After spawning, females held the fertilized eggs on their abdomen and deposited the eggs on aquatic plants growing near the surface of the water. This egg deposition was observed from the morning until the afternoon. For the successful conservation of wild medaka, it is important to collect such information on environmental conditions where medaka reproduce naturally.