Social organization and habitat use shape the gut microbiome of a marine fish.

The gut microbiome hosts diverse bacterial communities that significantly influence individual spatial behaviour in animal societies. However, this relationship remains understudied in marine fish due to the challenges associated with measuring behavioural traits in free-living fish and simultaneously obtaining gut microbiome composition data. In this study, we conducted a field experiment to explore the relationship between space and habitat use, social organization, and gut microbiome composition in marine fish. We used a novel high-resolution acoustic telemetry system to collect 7930 one-day-long movement trajectories from 232 individuals of Xyrichtys novacula of the same population (153 females, 79 males) near the coastline of Mallorca, Spain. A subset of these individuals was recaptured to analyse the diversity of core and non-core gut microbiome, quantified using operational phylogenetic units (OPUs) based on 16S rRNA gene amplicons through Illumina sequencing. Individuals closer to Posidonia oceanica seagrass meadow had higher non-core microbiome diversity, especially larger individuals. Multivariate analysis showed no significant differences in microbiome composition across the tested variables (i.e. body size, territory size, degree, strength, distance to the seagrass meadow, and sex), but males showed a visually greater, but non-significant, variability in core microbiome composition. Core microbiome composition was weakly associated with social harem structure. These findings indicate that gut microbiome composition is primarily shaped by local habitat conditions, while social organization may contribute weakly and indirectly, pending further experimental validation.