| Summary: | <i>Clinostomum</i> metacercariae are common endoparasites of fish and frogs. In this study, we examine taxonomic identity and developmental strategy of <i>Clinostomum</i> metacercariae infesting reed frogs <i>Hyperolius kivuensis</i> and <i>H. viridiflavus</i> in Rwanda. Moreover, we evaluate the impact of infestation on demographic and morphological life-history traits of the hosts. Morphological and molecular features, particularly genital morphology and COX1 sequences, provided evidence that the metacercariae belong to <i>C. chabaudi</i> Vercammen-Grandjean, 1960. Depending on the host’s defensive behavior and the availability of resources, metacercariae develop either as sedentary “yellow grubs” encysted in the lymphatic sacs or mouth of the host or as initially encysted, but later free-ranging individuals invading the host´s body cavity. Nutrition on lymphatic fluid within the cyst leads to yellow-colored gut content, feeding on blood or host tissue, to brownish green gut content in free-ranging individuals. Almost all metacercariae opted for the first developmental strategy in <i>H. kivuensis</i>, whereas the second strategy dominated in metacercariae infesting <i>H. viridiflavus</i>. <i>Hyperolius kivuensis</i> suffered significant morphological modifications, when infested with encysted metacercariae. Both developmental modes permitted a coexistence with the host of less than one year. We hypothesize that the presence of alternative development modes is an adaptation of <i>C. chabaudi</i> to cope with resource limitation within host-produced cysts.
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