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Capelin (Mallotus villosus) is among the most abundant fish species in the Barents Sea, and represents a critical food source for many predators in the area including Atlantic cod and harp seal. In Norway, the fish is of economic importance since whole capelin and roe are valuable export products. Despite its economic and ecological importance, the parasites of Barents Sea capelin are poorly known. However, the presence of parasites in the edible parts may adversely affect product quality and consumer safety. During the main annual catching seasons of 2009-2012, we investigated the diversity and infection dynamics of ascaridoid nematodes in capelin (n = 620) from the southern Barents Sea. Three anisakid species were identified by genetic or molecular methods; Anisakis simplex (s.s.), Contracaecum osculatum sp. B, and Hysterothylacium aduncum, with C. osculatum sp. B as the most prevalent and abundant species. The present findings suggest that the ascaridoid species composition in capelin reflects its trophic position in the Barents Sea ecosystem. There appears to be a link between infection level of the nematode species and the preferred prey organisms of the different developmental phases of capelin. Thus, the higher abundance of C. osculatum sp. B compared to A. simplex (s.s.) and H. aduncum may be related to more extensive feeding on calanoid copepods over a wider ontogenetic size range including adolescence, while the main intermediate hosts of the latter nematode species, i.e. euphausiids and amphipods, appear to be the preferred prey of larger capelin.

A new taxon, Rhabdias esculentarum n. sp., is described based on DNA sequence analysis at multiple loci (i.e. mtDNA cox-1, 12S rRNA, ITS-1 and partial ITS-2 regions of the nuclear rDNA) and morphometric analysis carried out on specimens collected from the green frogs of the Rana esculenta species complex in Italy (i.e. R. lessonae Camerano and R. esculenta Linnaeus, identified genetically by diagnostic allozyme loci). Rhabdias esculentarum n. sp. was differentiated genetically, at both mitochondrial and nuclear levels, from Rh. bufonis (Schrank, 1788) (sensu Hartwich, 1972) and Rh. sphaerocephala Goodey, 1924 recovered from the toad Bufo bufo Linnaeus collected sympatrically with the specimens of Rana lessonae and R. esculenta examined in the present study. Moreover, the new taxon proved to be different from the other species of Rhabdias from anurans, which had previously been sequenced using the same genes and deposited in GeneBank. Phylogenetic analyses (MP and ML) inferred from mitochondrial (mtDNA cox-1 and 12S ribosomal RNA) and nuclear (ITS-1 and ITS-2 of the rDNA regions) sequences datasets were congruent in depicting Rh. esculentarum n. sp. as forming a highly supported clade distinct from the sympatric species Rh. bufonis, as well as from Rh. sphaerocephala, characterised on the basis of the same loci. Morphometric analysis and the differential diagnosis of genetically characterised specimens of the new species have revealed differences in several features in comparison with the type-species, Rh. bufonis. Material of the latter species included voucher specimens from Germany deposited by Hartwich (1972) and other specimens collected from B. bufo in Italy. Among the diagnostic characters, the particular cup-shaped buccal capsule characterising Rh. esculentarum is clearly different from the tear-shaped buccal capsule observed in material of R. bufonis obtained from Berlin Museum and collected in the same geographical area as the green frogs under study. Rh. esculentarum was also found to differ in some measurements and allometric characters from Rh. bufonis (sensu Moravec et al., 1997). The data so far collected appear to indicate a host-preference of Rh. esculentarum for Rana lessonae and R. esculenta, which belong to the R. esculenta hybridogenetic species complex in Italy.