ORIGINAL PAPER Species composition and infection dynamics of ascaridoid nematodes in Barents Sea capelin (Mallotus villosus) reflecting trophic position of fish host Arne Levsen 1 & Michela Paoletti 2,3 & Paolo Cipriani 2,3 & Giuseppe Nascetti 3 & Simonetta Mattiucci 2 Received: 4 July 2016 /Accepted: 19 July 2016 # Springer-Verlag Berlin Heidelberg 2016 Abstract 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 ecosys- tem. 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 feed- ing 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. Keywords Capelin . Mallotus villosus . Nematode parasites . Barents Sea . Trophic position . Life cycle Introduction Capelin (Mallotus villosus) is a small, pelagic schooling fish, attaining a maximum body length of ∼20 cm at 6 years of age (Gjøsæter 1998). It 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 (Gadus morhua), harp seal (Pagophilus groenlandicus) and various cetaceans (Bogetveit et al. 2008; Nordøy et al. 2008; Lindstrøm et al. 2013). Moreover, capelin is the dominant planktivore in the ecosystem with euphausiids (krill), amphipods and Calanus copepods as major prey items. Thus, capelin plays a key role in the energy transfer upwards in the food web and the preda- tion pressure from capelin seems to affect the zooplankton biomass in the area (Gjøsæter 1998; Gjøsæter et al. 2002). The Barents Sea capelin stock undertakes annual migrations between the northern summer feeding grounds and the coast of northern Norway and the Kola Peninsula where spawning takes place in spring (March/April). Capelin has a compara- tively short life span, and due to high spawning mortality— most females seem to die after spawning—the capelin stock consists of only a few year classes (Yndestad and Stene 2002). There is clear sexual dimorphism in capelin, with the males having distinctly longer pectoral and anal fins, and the base of the male anal fin is elevated on a pronounced hump that lacks in females. Additionally, male capelins tend to grow faster and are generally larger than females (Gjøsæter 1998). The stock * Arne Levsen arne.levsen@nifes.no 1 National Institute of Nutrition and Seafood Research (NIFES), P.O. Box 2029 Nordnes, N-5817 Bergen, Norway 2 Department of Public Health and Infectious Diseases, Section of Parasitology, Sapienza - University of Rome, Rome, Italy 3 Department of Ecology and Biological Sciences, Tuscia University, Viterbo, Italy Parasitol Res DOI 10.1007/s00436-016-5209-9