1. Bitterling fishes (Subfamily: Acheilognathinae) spawn in the gills of living freshwater mussels and obligately depend on the mussels for reproduction. On the Matsuyama Plain, Japan, populations of unionid mussels—Pronodularia japanensis, Nodularia douglasiae, and Sinanodonta lauta—have decreased rapidly over the past 30 years. Simultaneously, the population of a native bitterling fish, Tanakia lanceolata, which depends on the three unionids as a breeding substrate, has decreased. Furthermore, a congeneric bitterling, Tanakia limbata, has been artificially introduced, and hybridisation and genetic introgression occur between them. Here, we hypothesised that decline of the unionids has enhanced this invasive hybridisation through competition for the breeding substrate.
2. Three study sites were set in three streams on the Matsuyama Plain. We collected adult bitterling fishes (native T. lanceolata, introduced T. limbata, and foreign Rhodeus ocellatus ocellatus) once a week from April to October 2013 to measure their densities in streams and to examine seasonal differences in female ovipositor length, which elongates in the breeding season. Simultaneously, we set quadrats and captured unionids and measured environmental conditions. Each unionid individual was kept separately in its own aquarium to collect ejected bitterling eggs/larvae. Tanakia eggs and larvae were genotyped using six microsatellite markers and the mitochondrial cytochrome b gene.
3. Introduced T. limbata was more abundant, had a longer breeding period, and produced more juveniles than native T. lanceolata. Hybrids between the two species occurred at all sites, and in total 101 of the 837 juveniles genotyped were hybrids. The density of P. japanensis was low, at most 0.42 individuals/m². Nodularia douglasiae and S. lauta have nearly or totally disappeared from these sites. Hybrid clutches of Tanakia species occurred more frequently where the local density of P. japanensis was low. Mussels were apparently overused and used simultaneously by three species of bitterlings.
4. Decline of freshwater unionid populations has enhanced hybridisation of native and invasive bitterling fishes through increasing competition for breeding substrate. We showed that rapid decline of host mussel species and introduction of an invasive congener have interacted to cause a rapid decline of native bitterling fish.
5. Degradation of habitat and the introduction of invasive species interact to cause a cascade of extinctions in native species. In our study, obligate parasite species are threatened because the host species are disappearing, which means there is a serious threat of coextinction.

中文翻译：

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贻贝贻贝的下降通过竞争繁殖底物增强了本地和引进苦味鱼的杂交

1. 苦涩的鱼（亚科：Acheilognathinae）在活的淡水贻贝的g中产卵，并只能依靠贻贝繁殖。在松山平原，日本，蚌的人口mussels- Pronodularia japanensis，节球douglasiae和Sinanodonta lauta-已经在过去30年迅速下降。同时，依靠三种union虫作为繁殖底物的本地苦涩鱼Tanakia lanceolata的种群减少了。此外，一种同类苦味酒，Tanakia limbata被人为引入，并且它们之间发生杂交和基因渗入。在这里，我们假设，通过竞争育种底物，融合子的减少增强了这种侵入性杂交。
2. 在松山平原的三个溪流中设置了三个研究地点。我们从2013年4月至2013年10月每周一次收集成年苦味鱼类（天然T. lanceolata，引入的T. limbata和外来Rhodeus ocellatus ocellatus），以测量它们在溪流中的密度并检查雌性产卵器长度的季节性差异，该长度在鱼卵中延长繁殖季节。同时，我们设置了四边形并捕获了联合体并测量了环境条件。每个工会会员单独存放在自己的水族馆中，以收集弹出的苦鸡蛋/幼虫。使用六个微卫星标记和线粒体细胞色素b基因对Tanakia卵和幼虫进行基因分型。
3. 引入的T. limbata比本地的T. lanceolata更为丰富，繁殖期更长，并且幼鱼更多。这两个物种之间的杂种都出现在所有位点上，在837个基因型少年中，共有101个是杂种。日本对虾的密度很低，最多为0.42个/ m ²。结节虫和沙门氏菌已从这些部位几乎或完全消失。Tanakia物种的杂种离合更频繁地发生在日本对虾的局部密度较低的地方。贻贝显然被三种苦味药过度使用并同时使用。
4. 通过增加对繁殖底物的竞争，淡水工会动物种群的减少已经增强了本地和侵入性苦味鱼的杂交。我们表明，寄主贻贝物种的迅速减少和侵入性同类动物的引入相互作用，导致本地苦涩鱼的迅速减少。
5. 栖息地的退化和入侵物种的引入相互作用，导致了自然物种的一系列灭绝。在我们的研究中，专性寄生虫物种受到威胁，因为宿主物种正在消失，这意味着存在严重的共灭威胁。