Abstract:
European eel (Anguilla anguilla) is a commercially important species for fisheries and aquaculture
in Europe and the attempt to close the lifecycle in captivity is still at pioneering stage.
The first feeding stage of this species is characterized by a critical period between 20 to 24
days post hatch (dph), which is associated with mortalities, indicating the point of no return.
We hypothesized that this critical period might also be associated with larvae-bacterial interactions
and the larval immune status. To test this, bacterial community composition and
expression of immune and stress-related genes of hatchery-produced larvae were explored
from the end of endogenous feeding (9 dph) until 28 dph, in response to three experimental
first-feeding diets (Diet 1, Diet 2 and Diet 3). Changes in the water bacterial community composition
were also followed. Results revealed that the larval stress/repair mechanism was
activated during this critical period, marked by an upregulated expression of the hsp90
gene, independent of the diet fed. At the same time, a shift towards a potentially detrimental
larval bacterial community was observed in all dietary groups. Here, a significant reduction
in evenness of the larval bacterial community was observed, and several amplicon
sequence variants belonging to potentially harmful bacterial genera were more abundant.
This indicates that detrimental larvae-bacteria interactions were likely involved in the mortality
observed. Beyond the critical period, the highest survival was registered for larvae fed
Diet 3. Interestingly, genes encoding for pathogen recognition receptor TLR18 and complement
component C1QC were upregulated in this group, potentially indicating a higher immunocompetency
that facilitated a more successful handling of the harmful bacteria that
dominated the bacterial community of larvae on 22 dph, ultimately leading to better survival,
compared to the other two groups.
