Behavioural types correlate with the gut microbiome in juvenile wild and reared gilthead seabream

The gut microbiome influences host behaviour through the gut-brain axis (GBA), a bidirectional network of signalling pathways. Although the GBA has been well studied in humans and other mammals, its role in shaping individual behavioural variation in fish remains largely unexplored. In this study, standardized behavioural tests were conducted on 67 juvenile gilthead seabream (Sparus aurata), consisting of 30 wild and 37 reared individuals, across five major behavioural axes-boldness, aggressiveness, sociability, activity and exploration-to determine their behavioural types using linear mixed models. High levels of repeatability of behaviour and consistent behavioural types were observed along the five studied axes. Gut samples from contrasting behavioural types were analysed for diversity, composition and structure using 16S rRNA sequencing. Statistically significant correlations and differences were found between wild and reared groups in both behavioural types and gut microbiome characteristics. These findings provide novel evidence of associations between behavioural types and the gut microbiome in juvenile marine fish, suggesting that gut microbiome may play a role in modulating fish behaviour. While this relationship could involve GBA interactions, further research is needed to confirm such mechanisms. This work could have translational significance for understanding survival, recruitment and life-history evolution in the early life stages of wild fish, as well as improving conservation management of species in both aquaculture and their natural habitats.