Hidden diversity in glass sponges: Apodomenia mimica sp. nov., the second known sponge-associated, footless solenogaster (Mollusca)

Glass sponges (Hexactinellida) are key structural components of deep-sea ecosystems, often harboring diverse invertebrates. Yet associations with molluscs, and particularly the vermiform, shell-less Solenogastres (Mollusca, Aplacophora), remain virtually undocumented. The genus Apodomenia is exceptional among solenogasters in completely lacking a foot, a feature that departs from the traditional morphological distinctions separating solenogasters and caudofoveates, the other aplacophoran lineage. Until now, the genus was represented by a single species, A. enigmatica described from the Southern Ocean living in association with the hexactinellid sponge Rossella sp. Here we describe Apodomenia mimica sp. nov., based on the study of two specimens discovered in the hexactinellid Euplectella suberea in the Northwest Atlantic Ocean on the Mississippi-Alabama continental slope (USA). The new species exhibits striking camouflage, mimicking the fibrous skeleton of its host. These findings extend the ecological and geographic range of Apodomenia and indicate that different genera of hexactinellid sponges may harbor specialized but overlooked molluscan associates. Notably, a second solenogaster species was also recovered from the same sponge, suggesting for the first time the co-occurrence of multiple solenogaster lineages within a single host. Taken together, this work highlights glass sponges as reservoirs of overlooked biodiversity.