Polychaete capture by native jellyfish and invasive ctenophore reveals a novel benthic pelagic trophic link

Shallow coastal and estuarine habitats are among the most productive ecosystems, sustained by dynamic benthic-pelagic coupling. While traditionally described through detrital fluxes, living-mediated trophic interactions remain underexplored. Here we present the first field evidence that benthic nereidid polychaetes are preyed upon by gelatinous zooplankton; the native scyphomedusae Aurelia aurita and the invasive ctenophore Mnemiopsis leidyi. These events occurred most frequently in summer, particularly in the inner reaches of a Danish fjord, and were more often associated with A. aurita than M. leidyi. Stable isotope analyses revealed seasonal and species-specific patterns: in summer, polychaetes shared similar isotopic signatures with their gelatinous predators, while in autumn they exhibited significant enrichment, possibly reflecting starvation or decay. Mixing models indicated that polychaetes constitute the second most important dietary component for A. aurita and M. leidyi during summer, after seston and zooplankton, respectively but declined in importance in autumn. These finding uncover a previously overlooked trophic pathway through which benthic prey subsidize pelagic consumers, strengthening benthic-pelagic coupling. Despite their lower observational frequency in M. leidyi, dietary models suggest ctenophores may exploit this resource opportunistically, with potential implications for competitive interactions and invasion dynamics. Our results highlight the need to incorporate living benthic-pelagic interactions into ecological models of energy flow and invasion ecology.