β-glucan enhances ontogeny and modulates the bacteriome and immuno-stress response of European eel larvae

Closing the European eel ( Anguilla anguilla ) life cycle in captivity is indispensable to ensure aquaculture and conservation plans of this critically endangered species. Advancements in assisted reproduction and hatchery rearing techniques have led to a stable production of offspring reaching the feeding stage. However, larval-bacteria interactions combined with an early undeveloped larval immune system hamper progression during larval culture. Here, the application of a known immuno-stimulant is tested as a prophylactic measure during early larval rearing of European eel. Yolk sac larvae were reared under normal conditions (control) or treated with Yeast β-1,3/1,6-glucan (BG; MacroGard®) added to the rearing water (5 mg/L) from 5 until 9 days post-hatch (DPH). When larvae reached the exogenous feeding stage (10 DPH), both groups of larvae were fed according to the current hatchery protocol until 20 DPH. The experiment was repeated 4 times, using offspring from different parental crosses. Results revealed no effect of BG-treatment on survival or growth. Regarding deformities (pericardial edema, emaciation, neurocranial defects, spinal curvature, and jaw deformities), no significant differences between control and BG-treated larvae were observed at 9 dph. By 20 dph, however, BG-treated larvae showed a higher percentage of normal individuals and a reduced incidence of spinal curvature compared to the control group. Moreover BG-treatment affected larval immune and stress/repair response, as well as the larval and water bacteriome. More specifically, il10 , encoding an anti-inflammatory cytokine and hsp90 , related to cellular stress/repair response, were both downregulated in BG-treated larvae at the end of the treatment period (9 DPH). BG-treatment also altered the alpha diversity of bacteria, resulting in different compositions of bacteriomes in both, larvae and water, where 80 ASVs of the genus Vibrio were more abundant in the control water bacteriome. On the other hand, bacteria of Unassigned taxa (relative abundance of ~69 %) dominated the bacteriome of BG-treated water, suggesting selection towards a specialised (probably K-selected) water bacterial community. In conclusion, treatment of European eel yolk sac larvae with BG (applied in rearing water) proved beneficial as manifested by the significantly downregulated cellular stress/repair response and immunoreaction, as well as healthier bacterial communities of rearing water and larvae through a significant reduction in Vibrio spp. Additionally, β-glucan showed preliminary potential to reduce deformities. Despite these benefits, larval survival and growth were unaffected during the present experimental window and thus, further research is encouraged to refine applications to improve offspring culture of European eel. • BG treatment reduced the occurrence of deformities in eel larvae. • BG treatment improved bacterial communities of eel larvae and rearing water. • BG treated eel larvae downregulated stress/repair and immune related gene expression.