The bitterling–mussel symbiosis: a model for host‐parasite adaptation

Bitterlings (Acheilognatinae) are a monophyletic group of cyprinid fishes that lay their eggs in the gill chamber of freshwater mussels. They have evolved many behavioural, morphological and physiological adaptations to the symbiosis. Female bitterling develop a long ovipositor that insert into the exhalant siphon of a mussel and males fertilize the eggs by releasing sperm over the inhalant siphon of the mussel. Embryos hatch within 2 days but develop inside the mussel for further 3 to 6 weeks. Embryos are adapted to the low oxygen environment in the mussel's gill chamber. Both males and females discriminate among mussels in relation to their quality as host for developing embryos. On the other hand, mussels used for oviposition have larvae that obligate ectoparasites on fish. Here I review current knowledge on the status of the symbiosis, developmental and behavioural adaptations by bitterling and mussel and summarize costs and benefits to both symbionts. Further, I use a recent well‐resolved bitterling phylogeny to emphasize the potential of this model system to study the evolution of this symbiosis, which is a part of the ongoing study.