Insights into the second-known polychaete-asteroid symbiosis, involving a new species of Inermosyllis associated with Luidia maculata

Abstract Marine symbioses involving polychaetes are widespread across the world’s oceans, with Echinodermata as prominent hosts and Asteroidea (sea stars) accounting for approximately 7% of the interactions. The species-rich family Syllidae (Annelida: Polychaeta), with over 1,100 species across 80 genera, is particularly known for its diverse lifestyles, including free-living, commensal, and parasitic forms. Despite a considerable number of syllid species could be expected to live in association with echinoderms, only five have been documented, comprising two living on brittle stars, two on sea cucumbers, and only one on a sea star, Trypanobia asterobia from Japan. During different biodiversity surveys in Nha Trang Bay (south-central Vietnam), over 30 symbiotic polychaete species were documented, including numerous syllids. Most of these were associated with sponges, but an unidentified species of Inermosyllis was found on the asteroid Luidia maculata . Specimens of this symbiotic syllid were extracted, preserved in ethanol, examined, and photographed, leading to the formal description of Inermosyllis asteriakavalaris sp. nov. This new species differs from its four known congeners due to its unique association with an asteroid and its chaetae with bidentate blades, which are partially or almost entirely fused to shafts. Molecular analysis could not clarify its phylogenetic relationships, preventing reevaluating the validity of the genus. However it clearly differs from T. asterobia , thus supporting that symbiosis with asteroids has appeared independently at least twice within the subfamily. This finding represents the fifth-known species of Inermosyllis , the first report of the genus in the tropical western Pacific, and the second-known asteroid-associated syllid.