First rDNA sequence data for Haplosplanchnus pachysomus (Digenea: Haplosplanchnidae) ex Mugil cephalus from the Black Sea, and molecular evidence for cryptic species within Haplosplanchnus pachysomus (Digenea: Haplosplanchnidae) in Palaearctic and Indo-West Pacific regions.

Adult trematodes, morphologically similar to Haplosplanchnus pachysomus (Eysenhardt, 1829), were extracted from the intestine of Mugil cephalus Linnaeus, 1758, collected in the Black Sea basin. Morphological, morphometric and 28S ribosomal DNA (rDNA) partial sequence data were obtained for these trematodes following comparative analysis with previous data on this species. Worms from this study were morphologically identical to all previously reported H. pachysomus specimens from different locations. The results of the morphometric analysis indicated general similarity between H. pachysomus from the Black Sea and trematodes from Vietnam and Australia. Trematodes from the Black Sea and specimens from Spain were identical based on 28S rDNA partial sequences; however, these sequences differed from that of H. pachysomus from Vietnam and Australia by eight fixed substitutions. Overall, our results indicate that H. pachysomus from Spain and the Black Sea and from Vietnam and Australia can presently be considered as two cryptic species, one in Palaearctic and one in Indo-West Pacific regions. Our results provide a molecular base for including Haplosplanchnus purii in the genus Provitellotrema or to consider H. purii, P. crenimugilis and H. pachysomus within the same genus.