Bitterlings are a freshwater fish that use freshwater mussels as spawning host. Recently, many bitterling species in Japan are threatened so that conservation of these species is very important. Decreasing bitterlings(bitterling　populations)was caused by a habitat degradation, decreasing freshwater mussels as spawning site and the impact of invasive species. So, this study showed the interaction between invasive and domestic species of bitterlings about a reproductive behavior.

We tested using Rhodeus ocellatus ocellatus, invasive species in Japan, and Rhodeus atremius atremius, threatened species, and two species of mussel were used, Unio douglasiae nipponensis and Anodonta　Japonica. At first we investigated a host species preference in aquarium experiment. Which mussel species was selected of spawning site. Next, to assess the effects on the reproductive success, we observed competitive behavior. In this test, two species bitterlings were contained in a same aquarium and differences of behavior between individuals were observed. Two experiments are conducted in the 600×300×360mm aquarium and behaviors of fishes were recorded by video. Records were made of the rate the mussels into which females spawned and males' inspection. We also recorded frequency of attacks on rivals by territorial males and the frequency that sneakers released sperm into mussels during spawning.