No abstract

Despite the increasing recognition and importance surrounding bacterial and fungal interactions, and their critical contributions to ecosystem functioning and host fitness, studies examining their co-occurrence remain in their infancy. Similarly, studies have yet to characterise the bacterial and fungal communities associated with nudibranchs or their core microbial members. Doing this can advance our understanding of how the microbiome helps a host adapt and persist in its environment. In this study, we characterised the bacterial and fungal communities associated with 46 Pteraeolidia semperi nudibranch individuals collected from four offshore islands in Singapore. We found no distinct spatial structuring of microbial community, richness, or diversity across sampling locations. The bacterial genera Mycoplasma and Endozoicomonas were found across all samples and islands. The fungal genus Leucoagaricus was found with the highest occurrence, but was not found everywhere, and this is the first record of its reported presence in marine environments. The co-occurrence network suggests that bacterial and fungal interactions are limited, but we identified the bacterial family Colwelliaceae as a potential keystone taxon with its disproportionately high number of edges. Furthermore, Colwelliaceae clusters together with other bacterial families such as Pseudoalteromonadaceae and Alteromonadaceae, all of which have possible roles in the digestion of food.

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.