Resurrection of Pomphorhynchus tereticollis (Rudolphi, 1809) (Acanthocephala: Pomphorhynchidae) based on new morphological and molecular data

Pomphorhynchus tereticollis (Rudolphi, 1809) is here redescribed on the basis of Rudolphi's material, deposited in the Museum fur Naturkunde Berlin, and on acanthocephalans recently collected from the type host Platichthys flessus (L.) and the region embodying the type locality. Out of the paratypes of P. tereticollis, the lectotype and paralectotypes have been designated. Their morphology fits well with that of newly collected material of P. tereticollis dissected from the type fish host from the Baltic coast near Stralsund. The resurrection of P. tereticollis, previously considered a synonym of Pomphorhynchus laevis (Zoega in Muller, 1779), is supported by several morphological features distinguishing the two Pomphorhynchus species: 1. The basal parts of the proboscis hooks located on the posterior proboscis half possess proximal projections in P. tereticollis but not in P. laevis. This shape of the hook bases is clearly visible only in unfixed fresh worms; 2. The last hooks are situated at the anterior part of the bulbus or rarely at the posterior-most end of the proboscis in P. tereticollis, while they lie anterior to the end of the proboscis in P. laevis; 3. The proboscis hooks No. 5 or 6 are markedly stout (robust) and clearly distinct in comparison with the surrounding hooks in P. tereticollis, while less robust and more similar to the hooks in P. laevis. In addition, genetic divergence between P. tereticollis and P. laevis based on ITS1, ITS2 and COI sequencing supports the existence of two distinct species and reveals that some isolates previously identified as P. laevis were actually P. tereticollis. Previous and present morphological and genetic data show that both Pomphorhynchus species occur in freshwaters throughout Europe and may infect the same fish hosts, such as chub and barbel, and also several species of isopods (Gammaridae). This study also provides morphological evidence that Pomphorhynchus intermedius Engelbrecht, 1957 is a synonym of P. tereticollis, because the only discrimination character of the former species, the "existing but small proximal projections of basal parts of the proboscis hooks located on the posterior proboscis half" are present also in P. tereticollis.