We recently demonstrated that dietary supplementation with seaweed leads to dramatic improvements in immune responses in S. fuscescens, a candidate species for aquaculture development in Asia. Here, to assess whether the immunostimulatory effect was facilitated by changes to the gut microbiome, we investigated the effects of those same seaweed species and four commercial feed supplements currently used in aquaculture on the bacterial communities in the hindgut of the fish. Since we found no correlations between the relative abundance of any particular taxa and the fish enhanced innate immune responses, we hypothesised that S. fuscescens might have a core microbiome that is robust to dietary manipulation. Two recently published studies describing the bacteria within the hindgut of S. fuscescens provided an opportunity to test this hypothesis and to compare our samples to those from geographically distinct populations. We found that, although hindgut bacterial communities were clearly and significantly distinguishable between studies and populations, a substantial proportion (55 of 174 taxa) were consistently detected across all populations. Our data suggest that the importance of gut microbiota to animal health and the extent to which they can be influenced by dietary manipulations might be species-specific or related to an animals’ trophic level.

BackgroundWe know very little about natural variation in microbiomes of marine herbivorous fish in the wild or in captivity (aquaculture). Understanding how the consumption of seaweed influences intestinal microbial communities will shed light on how such phytobiotics could enhance the health and productivity of farmed fish. Here we screened the effects of supplementing the diets of mottled rabbitfish (Siganus fuscescens), a candidate species for international aquaculture development, with 15 different species of seaweeds and functional supplements currently used in aquaculture, on the bacterial communities that colonised their hindguts. ResultsRemarkably, the second most abundant phylum and the majority (53%) of the bacterial genera were not assigned, highlighting a significant knowledge gap for the field of animal microbiomes. Dietary supplementation increased alpha diversity by up to 23% relative to the control fish. Furthermore, most supplements significantly increased the relative abundance of Firmicutes, with similar trends for Proteobacteria and consistent decreases in Bacteroides. Seaweed supplementation also had important effects at the genus level, including significant increases in Fusobacterium sp. in fish fed seaweed - especially the green Caulerpa taxifolia – and overall trends for reduced levels of Arcobacter sp., a genus that includes fish and human pathogens. When we compared microbiomes in our fish to those from two recently published studies of conspecific populations sampled many thousands of kilometres away, the populations were clearly distinct, however there were 55 ASVs that were shared across the three fish populations, of which 35 were present in 50% of all fish sampled.ConclusionThe identification of a core microbiome suggests that a host organism relies on certain microbes for key functions and our findings suggest that this candidate aquaculture species has a core microbiome in its hindgut which is robust to dietary manipulations and broad geographical and temporal variation. This insight will help guide future work investigating the functional and mechanistic bases of these relationships, as will improvements in microbial taxonomic resolution. Supplementation with seaweeds did have subtle influences on bacteria in the hindgut of Siganus fuscescens which could have important impacts on fish health and should be considered as aquaculture systems are developed for this species.