Worms within worms: First identification of parasitic nematodes in nemerteans

We report the first identification of nematode parasitism in a nemertean host (<i>Arctostemma arcticum</i>), representing a novel host-parasite interaction in marine ecosystems. Larvae of the anisakid nematode were discovered within the rhynchocoel of <i>A. arcticum</i> collected from the White Sea, suggesting transmission via ingestion of infected crustaceans - a previously undocumented pathway. The nematode was identified as <i>Phocanema bulbosum</i> through both morphological and molecular analyses.The presence of nematode in nemertean <i>A. arcticum</i> implies trophic transmission through nemertean predation on infected amphipods, supported by hoplonemertean feeding strategy. While some fish occasionally consume nemerteans, their low predation frequency suggests <i>A. arcticum</i> could act as a dead-end host.This study expands the known host range of Anisakidae, though it remains uncertain whether nemerteans serve as competent hosts in the <i>P. bulbosum</i> life cycle. Nevertheless, the findings highlight the need to assess nemerteans' potential influence on parasite dynamics in fisheries and aquaculture, particularly where they coexist with intermediate and definitive hosts.