In the Afrotropic region, the genus Clinostomum is represented by four accepted and four unnamed species distinguished using molecular data. Here, we describe one of the four unnamed species as Clinostomum ukolii n. sp. based on metacercariae from siluriform fishes (Synodontis batensoda, Schilbe intermedius) collected in Nigeria and South Africa. The new species is distinguished by molecular data (39 new sequences of partial cytochrome c oxidase I ≥ 6.7% divergent from those of other species) and morphological differences from named and unnamed species in the same region. Metacercariae of C. ukolii n. sp. can be distinguished based on size, tegumental spines, and various aspects of the genital complex, including its position, lobation of the anterior testis, and the disposition and shape of the cirrus pouch. Although descriptions of new species of digeneans are typically based on the morphology of adults, we argue that in cases where data are available from metacercariae from regionally known species, new species can be described based on metacercariae, particularly when supported by molecular data, as here. Moreover, sub-adult reproductive structures can be clearly visualized in metacercaria of Clinostomum. Considering metacercariae as potential types for new species could advance clinostome systematics more rapidly, because metacercariae are encountered much more often than adults in avian definitive hosts.
Keywords: Catfish; Halzoun; Helminth; Yellow grub.