More than meets the eye: characterizing the cryptic species complex and Symbiodiniaceae communities in the reef-dwelling nudibranch Pteraeolidia 'semperi' (Nudibranchia: Aeolidioidea) from Singapore

The advent of molecular systematics has revolutionized our knowledge of biodiversity, revealing undiscovered cryptic lineages across the tree of life. Correspondingly, an increasing number of nudibranch species complexes have been found and described through integrative taxonomic approaches. Pteraeolidia 'semperi' (Nudibranchia: Cladobranchia: Aeolidioidea) presently represents a species complex, possessing ambiguous systematics and an underestimated diversity. Sequencing and phylogenetic reconstructions based on the cytochrome c oxidase subunit I marker from 48 samples of P. 'semperi' collected from Singapore's southern waters, as well as morphological analyses including scanning electron microscopy of the radula, suggest that P. 'semperi' comprises several distinct cryptic lineages. We further characterized the Symbiodiniaceae communities of P. 'semperi' from Singapore using metabarcoding of the internal transcribed spacer 2 gene. Pteraeolidia 'semperi' from Singapore was found to predominantly host Cladocopium and Durusdinium symbionts, and was significantly structured by biogeography. This study elucidates the present taxonomic diversity of P. 'semperi' in Singapore and contributes to the growing body of work on the diversity of algal symbionts associated with this enigmatic taxon.