Shifts in turbot (Scophthalmus maximus) larvae bacterial communities from neutrality to non-neutrality: A window of opportunity for microbiome interventions

The early developmental stages of fish larvae are critical for establishing their microbial communities. However, significant gaps remain in our understanding of microbial colonisation and succession during these stages, as well as their role in optimising larviculture practices to enhance larval survival and health. This study investigated the ecology of bacterial communities associated with turbot ( Scophthalmus maximus ) larvae during their first month of development, generating fundamental knowledge that can guide targeted strategies to actively shape the fish bacteriome towards a healthier state. Our results revealed rapid bacterial colonisation and displacement in turbot larvae, marked by significant structural differences between water and larval communities during the initial developmental stages. During this period, larval bacterial communities largely followed neutral model predictions, indicating that stochastic processes governed early bacterial colonisation. As the larvae matured, we observed a convergence in the bacterial community structures of both water and fish, along with increasing deviations from neutral model predictions. This suggests increased niche specialisation and/or competitive exclusion of the bacteriome as the host fish aged. Moreover, this study identified specific bacterial taxa of interest that deviated from neutral model predictions, indicating potential positive or negative host selection. We conclude by discussing the importance of our findings for the development of larviculture microbiome interventions. • Neutral model helps identify critical window for beneficial microbial colonisation. • Vibrio colonisation shows unpredictable patterns, driven by stochastic processes. • Community instability peaks at day 10 with reduced evenness and dominance shifts. • Bacterial communities shift to more deterministic processes as larvae mature.