Dietary plasticity in small Arctic copepods as revealed with prey metabarcoding

Small copepods (<2 mm) compose an important constituent of the Arctic marine food web, but their trophic interactions remain largely unexplored, partly due to methodological limitations.

We here characterize the prey of the abundant cyclopoid Oithona similis, harpacticoid Microsetella norvegica and calanoid Microcalanus spp. from the Arctic Barents Sea and Nansen Basin during four seasons using brute force prey metabarcoding of the 18S rRNA gene.

Chaetognaths were unexpectedly the most consistently identified taxa and composed 47% of all prey reads. Some taxa were seasonally important, including diatoms in April-May (43%), dinoflagellates in December (15%) and March (17%), and urochordates in August (20%). Compositional differences among species were also discernible, and the M. norvegica diet was significantly different from both O. similis and Microcalanus spp. The diets varied nevertheless more with season than species despite the inherent trophic traits that distinguish the ambush-predator O. similis, chemosensoric particle-chaser M. norvegica and current-feeding Microcalanus spp.

Our results thus indicate that dietary plasticity is common in small Arctic copepods, regardless of their behaviors or strategies for finding sustenance. We further hypothesize that such plasticity is an important adaptation in systems where prey availability is highly seasonal.