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. Yet living-mediated trophic interactions remain underexplored. Here, we present the first field evidence that benthic nereid polychaetes are preyed upon by gelatinous zooplankton: native scyphomedusae Aurelia aurita and 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 values with their gelatinous predators, while in autumn, they exhibited significant δ 15 N 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 or zooplankton, but declined in importance in autumn. These findings uncover an overlooked trophic pathway through which benthic prey subsidise pelagic consumers, strengthening benthic–pelagic coupling. Despite the lower observations in M. leidyi , dietary models suggest ctenophores may exploit this resource opportunistically, with potential implications for competition and invasion dynamics. Our results highlight the need to incorporate benthic–pelagic trophic interactions into ecological models of energy flow and invasion ecology.