Eukaryote diets in Arctic marine nematodes across seasons and shelf-to-basin gradients

Marine nematodes dominate the meiofauna of benthic sediments, but few studies have investigated their trophic roles. We studied the eukaryote diet composition of nematodes from surface sediments on the Arctic Barents Sea shelf, shelf break and adjacent Nansen Basin, during four seasons, using prey metabarcoding of the 18S ribosomal RNA gene. Monhysterida (35), Chromadorida (34), Araeolaimida (27) and Enoplida (22) nematodes were most frequently observed across the study area, and diets composed of diverse metazoan, fungal, and protist prey. In contrast to ambient sediment communities, prey followed a strong seasonal pattern, and ordination indicated two distinct seasonal prey clusters. In March and May prey were characterized by high relative abundances of fungi (42% and 48%, respectively). In comparison, August and December compositions had high relative abundances of arthropods (30% and 28%) and peritrich ciliates (11% and 9%, respectively). Other notable protist prey included chlorophytes and dinoflagellates, whereas diatoms – which were highly abundant in the ambient sediment communities, were virtually absent as prey. Nematode taxonomy and trophic groups explained little of the variation in prey, and the latter was only significant when applied at the level of family. Our results indicate that Arctic nematodes are generalists which can feed on a variety of eukaryote items despite differences in morphology. They further indicate that heterotrophs, such as fungi and arthropods, compose important dietary items for nematodes in the Barents Sea. Such trophic tendencies may enable nematodes to fuel continuous growth and reproduction in Arctic sediment communities where food items are seasonally varied.