The description and delineation of trematode species is a major ongoing task. Across the field there has been, and currently still is, great variation in the standard of this work and in the sophistication of the proposal of taxonomic hypotheses. Although most species are relatively unambiguously distinct from their congeners, many are either morphologically very similar, including the major and rapidly growing component of cryptic species, or are highly variable morphologically despite little to no molecular variation for standard DNA markers. Here we review challenges in species delineation in the context provided to us by the historical literature, and the use of morphological, geographical, host, and molecular data. We observe that there are potential challenges associated with all these information sources. As a result, we encourage careful proposal of taxonomic hypotheses with consideration for underlying species concepts and frank acknowledgement of weaknesses or conflict in the data. It seems clear that there is no single source of data that provides a wholly reliable answer to our taxonomic challenges but that nuanced consideration of information from multiple sources (the ‘integrated approach’) provides the best possibility of developing hypotheses that will stand the test of time.

The best way to study digenean diversity combines molecular genetic methods, life-cycle studies and elaborate morphological descriptions. This approach has been barely used for one of the most widespread digenean taxa parasitizing fish – the superfamily Hemiuroidea. Here, we applied the integrative approach to the hemiuroideans from the family Derogenidae parasitizing fish at the White and Barents Seas. Analysis of 28S, 18S, 5.8S rDNA, ITS2 and cox1 gene sequences from sexually adult worms (maritae) showed genetic heterogeneity for 2 derogenid species known from this area: Derogenes varicus and Progonus muelleri. Thus, 2 pairs of genetic lineages were found: DV1 and DV2, PM1 and PM2, respectively. Data from other regions indicate that 2 more lineages of D. varicus probably exist. Based on previous records from the White and Barents Seas, we hypothesized that the cercariae found in the moonsnails (family Naticidae) belong to the Derogenidae and may help to differentiate these lineages as species. According to our results, Cercaria appendiculata from Cryptonatica affinis matched DV1, similar nameless cercariae from Euspira pallida and Amauropsis islandica matched DV2, and Cercaria octocauda from C. affinis matched PM1. We provide new data on the structure of these cercariae and discuss the life-cycle pattern of the studied digeneans.