Aquarium medaka collected from a park biotope located in Urayasu, Chiba Prefecture, Japan

Abstract

“Aquarium medaka” finding their way into natural ecosystems is a concern for causing negative effects on wild medaka populations. One such negative effect could be genetic pollution because they easily intercross with each other. Recently, aquarium medaka was collected from some regions in Japan. We collected medaka from a park’s biotope in Chiba Prefecture and they were identified as aquarium medaka by their morphological characteristics and body color. Most of these medaka had incomplete traits. These would be released with such traits to keep them from breeding with wild populations. The release and expansion of species with dominant traits causes many problems such as genetic pollution and the mediation of infections and parasites. Therefore, public awareness is needed to prevent intentional and unintentional releases of aquarium medaka.