Examining microbial assemblage differences on plants preferred and avoided by threatened Hawaiian land snails to identify important food resources

The Hawaiian Islands, an isolated oceanic archipelago in the Pacific, had one of the world's most spectacular land snail radiations. Unfortunately, only 35% of the 759 Hawaiian snail species remain, with >100 species threatened with imminent extinction. Though many extant Hawaiian snails are arboreal and are known to eat microbes on the surface of plants, our current understanding of which microbes compose arboreal Hawaiian snails' diets remains rudimentary. Here, we examined microbial assemblages (bacterial and fungal) on plants previously identified as preferred and avoided by a diverse assemblage of arboreal snails at three sites in wet and mesic forests on O'ahu, Hawai'i, USA, to identify microbes that may form the foundation of Hawaiian arboreal snails' diets. At Mt. Ka'ala, we also investigated if microbial assemblages change across years. Across sites, microbial assemblages differed on preferred and avoided plants. While differences across sites can complicate the process of identifying key microbes, microbes with elevated abundances on preferred plants across sites are likely important food resources. Using this approach, we identified bacterial and fungal OTUs that we hypothesize form important components of Hawaiian arboreal snails' diets. While we found that microbial assemblages differed among years at Mt. Ka'ala, differences between preferred and avoided plants remained consistent across years. Expanding a snail plant preference-microbiome framework across multiple forest types is a crucial step in understanding arboreal snail diets as microbial taxa enriched on preferred plants across sites may be meaningful components of snail-associated microbial resources. Microbial taxa enriched on preferred plants are not presented here as confirmed dietary components, but as experimentally tractable candidates that should be further evaluated in laboratory trials. This information is foundational to improving captive rearing of critically endangered snails and informing habitat quality assessments for the many threatened Hawaiian snails.