The Japanese medaka is designated as an endangered species because of its decreasing population, and conservation of the medaka is an urgent concern. In the present study, in order to obtain basic information for conservation of wild medaka, we observed the reproductive behavior of female medaka (Oryzias latipes, orange-red variety and wild fish) in experimental aquaria under various environmental conditions. Female medaka normally deposited eggs on aquatic plants or aquatic mosses which were solid with a soft surface in experimental aquaria. However, the fish did not deposit eggs but discarded eggs in aquaria provided with sand, floating aquatic plants, and no substrate, and in an aquarium lined with concrete. These results 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 conservation of wild medaka.
