野生メダ力の繁殖生態と保全-メダ力はどこで卵を産むか?-

摘要

Japanese medaka Oryzias latipes has been recognized as an endangered species because its population size has decreased by recent changes in the aquatic environment in Japan. For the effectual conservation of medaka in the wild, information on the ecology of medaka, especially their reproductive ecology, is essential. However, while well known in the laboratory there have been few reports on how wild Japanese medaka reproduce in nature. We have been studying the reproductive ecology of wild medaka in order to provide basic biological information for the conservation of medaka. First, we observed reproductive behavior of wild medaka in outdoor ponds. According to the field observations, reproductive behavior consists of three patterns: aggressive behavior among males, spawning behavior (egg release and sperm release) by male and female, and egg deposition behavior by female holding fertilized eggs. No special substrate is necessary to elicit spawning behavior but a substrate, such as aquatic plants and mosses, is necessary for egg deposition by the female. We also conducted laboratory experiments to examine the suitability of various materials for egg deposition. Laboratory experiments revealed that aquatic plants and mosses were suitable as egg deposition substrate but sand and a concrete wall were not suitable as a substrate for egg deposition. Results of our field and laboratory studies indicate that a suitable substrate is essential for successful egg deposition of medaka and suggest that maintaining suitable vegetation in natural environments is important for the conservation of wild medaka.