Shedding light on the parasite communities and diet of the deep‐sea shark <i>Deania profundorum</i> (Smith &amp; Radcliffe, 1912) (Squaliform: Centrophoridae) from the Avilés Canyon (southern Bay of Biscay)

Deep-sea elasmobranchs are less resilient to the increasing scale of anthropogenic impacts such as fisheries, owing to their life-history traits. The necessity for proper management measures is hampered by the scant knowledge on these taxa and their biology. Here we provide the first comprehensive insight into the parasite infracommunities and trophic habits of the deep-sea arrowhead dogfish, Deania profundorum (Smith & Radcliffe, 1912) from the Avilés Canyon System in the southern Bay of Biscay (Northeast Atlantic). The study revealed rich parasite infracommunities dominated by cestode species represented by both larval and adult stages, as well as larval nematodes, indicating a rather intermediate position of this shark species in the local food web. Sampling years and sexes did not reveal significant differences in host size. Overall prevalence was 89.7% [confidence interval (CI): 72.6-97.8], with an infracommunity mean abundance of 42.2 ± 71.6 (range: 0-292). The infracommunity parameters mean abundance, richness and diversity were associated with host size. Differences in dominance in parasite communities were explained by the factors, year and sex. The composition and structure of parasite communities revealed differences between sampling years, with a significant effect of host size on interannual community similarity. The abundance of the three key discriminating taxa, adult Deanicola sp., larval Lacistorhynchidae gen. sp. and larval Anisakis sp. (Type I sensu Berland, 1961), increased with host size and revealed a higher parasite burden in larger specimens. Over one third of the sampled specimens revealed empty stomachs. The arrowhead dogfish feeds mainly on bentho- and bathypelagic fishes, crustaceans and cephalopods, but the observed prey composition showed no association with either sampling years, sex, or host size. This study underscores the importance of providing new data to the limited existing knowledge, which is necessary for the potential use of parasites as biological indicators. Such information can help infer long-term feeding niches and elucidate ecological roles within local food webs of hard-to-access species, such as deep-sea sharks.