Diversity of Symbiodiniaceae (Dinophyceae) in the sea slug clade Cladobranchia (Nudibranchia)

Abstract In members of Cladobranchia sea slugs, unique adaptations to incorporate cnidocysts and algal symbionts from their cnidarian prey have evolved. However, many aspects underpinning the recognition and maintenance of these stolen cellular components are still unclear. Regarding the algal symbionts, little is known about the exact Symbiodiniaceae species and their abundance and diversity in Cladobranchia. Yet, understanding the diversity of Symbiodiniacee inside the slugs can help better understand the symbionts’ role in establishing and maintaining the symbiosis. We analysed the Symbiodiniaceae diversity across multiple Cladobranchia genera and species, focusing on the genus Phyllodesmium , which contains most of the known cladobranchs in a long-term symbiosis with the algae. Using high-throughput metabarcoding of the Symbiodiniaceae ITS2 region, we found that species of the genus Phyllodesmium harboured primarily Cladocopium , showing a genus-specific Symbiodiniaceae profile. Within a cladobranch genus, we also uncovered species-specific intragenomic variants of the respective symbiodiniacean genus. Our results reveal a previously unexplored diversity of algal symbionts in Cladobranchia and that Cladocopium might be particularly relevant for establishing long-term symbiosis. Cladocopium exhibits enhanced carbon fixation capabilities in symbiosis with corals, which may thus facilitate the symbiosis from the symbionts’ side. Consequently, studying symbiont diversity and abundance in Cladobranchia is essential for understanding the mechanism of symbiosis initiation and maintenance.