Integrative Taxonomy as a foundation for understanding host relationships in Solenogastres

Solenogastres (Mollusca, Aplacophora) are small, worm-shaped marine molluscs that are often overlooked in biodiversity studies, despite their widespread distribution across all oceans and depths. Many species exhibit intimate ecological relationships with other invertebrates, particularly colonial cnidarians, but also sponges, which they use as hosts and food sources. Ecological associations often drive adaptation and speciation. For instance, species that rely on obligate symbiosis tend to have smaller populations, limited dispersal, and fragmented habitats, making them more vulnerable to environmental stress and extinction. Thus, studying associations in Solenogastres is essential to understand their evolution and resilience. Moreover, corals and sponges are vital components of benthic ecosystems and are highly sensitive to environmental changes. However, due to challenging taxonomy and historically limited sampling, the nature and specificity of these interactions remain poorly documented. This mirrors a broader lack of data on the biogeography and species documentation of solenogasters. Therefore, building a robust taxonomic framework for Solenogastres is critical, not only to resolve their diversity, but to illuminate the ecological and evolutionary implications of their symbiotic lifestyles. Here, we present a synthesis of what is currently known about solenogaster–host relationships, based on an extensive literature review, alongside preliminary results from ongoing morphological and molecular studies. Our integrative approach combines ecological observations with molecular and morphological approaches to improve species delimitation and host identification. This work lays the foundation for a more comprehensive understanding of Solenogastres ecology and evolution in a changing ocean.