Microbial Associations of the Arctic Nudibranch Mollusk Cadlina laevis (Gastropoda, Nudibranchia) Inhabiting the White Sea Subtidal Environment

Symbioses between invertebrates and prokaryotes are widespread across various environments. This study focused on a stable microbial community localized in the buccal apparatus (teeth of the radula and jaws) of the nudibranch Cadlina laevis found near the MSU White Sea Biological Station. Distinct methodologies have been applied to characterize this microbial community. Electron microscopy revealed a morphologically uniform layer of bacteria consistently associated with the radular teeth of C. laevis. Sequencing of the V4 region of the 16S rRNA gene identified that representatives of the genus Polaribacter accounted for up to 95.2% of the dense biofilm on the buccal apparatus. This finding supports a specific association of Polaribacter representatives with the buccal apparatus, in contrast to the more heterogeneous microbiomes of other mollusk organ samples. Comparative genomic analysis revealed significant differences between the genomes of C. laevis-associated Polaribacter and other available genomes of Polaribacter representatives, indicating that a host-associated lifestyle may require additional adaptations in bacteria. Metagenomic analysis revealed chitinase genes, rare in previously known Polaribacter-affiliated genomes, within the genome of these bacterial associates, suggesting their role in degrading chitin that covers the buccal apparatus. This observation of a stable microbial community in the C. laevis buccal apparatus could suggest that Polaribacter benefits from its ability to degrade chitin within the buccal apparatus, whereas the benefits to the mollusk remain unclear and will be the subject of further studies.