Adaptive host choice and avoidance of superparasitism in the spawning decisions of bitterling (Rhodeus sericeus)

Choice of a site for oviposition can have fitness consequences. We investig ated the consequences of female oviposition decisions for offspring surviva l using the bitterling, Rhodeus sericeus, a freshwater fish that spawns ins ide living unionid mussels. A field survey of nine bitterling populations i n the Czech Republic revealed a significantly lower rate of release of juve nile bitterling from Anodonta cygnea compared to three other mussel species . A field experiment demonstrated that female bitterling show highly signif icant preferences for spawning in A. anatina, Unio pictorum, and U. tumidus . Within a species, female bitterling avoided mussels containing high numbe rs of bitterling embryos. Mortality rates of bitterling embryos in mussels were strongly density dependent and the strength of density dependence vari ed significantly among mussel species, Female preferences for mussels match ed survival rates of embryos within mussels and females distributed their e ggs among mussels such that embryo mortalities conformed to the predictions of an ideal free distribution model. Thus, female oviposition choice is ad aptive and minimizes individual embryo mortality.