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Synopsis When new land is created, initial microbial colonization lays the foundation for further ecological succession of plant and animal communities. Primary microbial succession of new aquatic habitats formed during volcanic activity has received little attention. The anchialine ecosystem, which includes coastal ponds in young lava flows, offers an opportunity to examine this process. Here, we characterized microbial communities of anchialine habitats in Hawaii that were created during volcanic eruptions in 2018. Benthic samples from three habitats were collected ∼2 years after their formation and at later time points spanning ∼1 year. Sequence profiling (16S and 18S) of prokaryotic and eukaryotic communities was used to test whether communities were similar to those from older, established anchialine habitats, and if community structure changed over time. Results show that microbial communities from the new habitats were unlike any from established anchialine microbial communities, having higher proportions of Planctomycetota and Chloroflexi but lower proportions of green algae. Each new habitat also harbored its own unique community relative to other habitats. While community composition in each habitat underwent statistically significant changes over time, they remained distinctive from established anchialine habitats. New habitats also had highly elevated temperatures compared to other habitats. These results suggest that idiosyncratic microbial consortia form during early succession of Hawaiian anchialine habitats. Future monitoring will reveal whether the early communities described here remain stable after temperatures decline and macro-organisms become more abundant, or if microbial communities will continue to change and eventually resemble those of established habitats. This work is a key first step in examining primary volcanic succession in aquatic habitats and suggests young anchialine habitats may warrant special conservation status. 

Abstract Animals often shape environmental microbial communities, which can in turn influence animal gut microbiomes. Invasive species in critical habitats may reduce grazing pressure from native species and shift microbial communities. The landlocked coastal ponds, pools, and caves that make up the Hawaiian anchialine ecosystem support an endemic shrimp (Halocaridina rubra) that grazes on diverse benthic microbial communities, including orange cyanobacterial‐bacterial crusts and green algal mats. Here, we asked how shrimp: (1) shape the abundance and composition of microbial communities, (2) respond to invasive fishes, and (3) whether their gut microbiomes are affected by environmental microbial communities. We demonstrate that ecologically relevant levels of shrimp grazing significantly reduce epilithon biomass. Shrimp grazed readily and grew well on both orange crusts and green mat communities. However, individuals from orange crusts were larger, despite crusts having reduced concentrations of key fatty acids. DNA profiling revealed shrimp harbor a resident gut microbiome distinct from the environment, which is relatively simple and stable across space (including habitats with different microbial communities) and time (between wild‐caught individuals and those maintained in the laboratory for >2 yr). DNA profiling also suggests shrimp grazing alters environmental microbial community composition, possibly through selective consumption and/or physical interactions. While this work suggests grazing by endemic shrimp plays a key role in shaping microbial communities in the Hawaiian anchialine ecosystem, the hypothesized drastic ecological shifts resulting from invasive fishes may be an oversimplification as shrimp may largely avoid predation. Moreover, environmental microbial communities may have little influence on shrimp gut microbiomes.