The introducedT. limbatawas more abundant, had a longer breeding period, and produced more juveniles than the nativeT. lanceolata. Hybrids between the two species occurred frequently, and in total 101 of the 837 juveniles genotyped were hybrids. The density ofP. japanensiswas low, at most 0.42 individuals/m².Nodularia douglasiaeandS. lautahave nearly or totally disappeared from these sites. Hybrid clutches of theTanakiaspecies occurred more frequently where the local density ofP. japanensiswas low. The mussels were apparently overused and used simultaneously by three species of bitterlings.

The decline of freshwater unionid populations has heightened hybridisation of native and invasive bitterling fishes by increasing the competition for a breeding substrate. We showed that a rapid decline of host mussel species and an introduction of an invasive congener have interacted to cause a rapid decline of native bitterling fish. The degradation of habitat and the introduction of invasive species interact to cause a cascade of extinctions in the native species. In our study, obligate parasite species are threatened because the host species are disappearing, resulting in a serious threat of coextinction.