Exploring bacterial community composition and immune gene expression of European eel larvae (Anguilla anguilla) in relation to first-feeding diets

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.

Effect of Food Amounts on Larval Performance, Bacteriome and Molecular Immunologic Development during First-Feeding Culture of European Eel

“…However, it is worth noting that this second drop in survival was not as pronounced in the present study compared to the earlier-mentioned research. Intriguingly, we also did not observe the shift in larval bacterial community towards domination by potentially harmful or opportunistic bacteria that had previously coincided with the second drop in survival [14]. On other hand, by the time the present experiment concluded (30 dph), the survival was ~10%, which represents an improvement compared to the previously reported survival rate of 4% at 28 dph for similarly captive-reared European eel larvae [9].…”

“…In earlier studies, where the diet used in the current experiment was described [ 9 , 14 ], two significant drops in the survival of European eel larvae were observed during the first-feeding period. The first drop, which we also observed in the current study, occurs within the first few days after initiation of feeding and can be attributed to the challenging transition experienced by the larvae as they switch to exogenous feeding.…”

“…The second drop in survival occurs between 20 and 24 dph. In previous research, where this was even more prominent, this drop was linked to the fact that larvae reached the so-called “point-of-no-return” [ 9 ] and were associated with detrimental bacteria interactions [ 14 ]. However, it is worth noting that this second drop in survival was not as pronounced in the present study compared to the earlier-mentioned research.…”

“…This could lead to a microbially hostile environment, as the immune system of newly hatched fish larvae is generally not fully developed and, thus, highly sensitive to detrimental bacteria, especially under intensive rearing conditions that might cause stress [ 11 ]. In European eel, a sensitive phase was reported during early life, where recently hatched larvae are potentially immuno-compromised [ 12 , 13 ], while nutritional aspects (such as dietary composition) can influence immune gene expression during feeding culture [ 14 ].…”

“…For the European eel, bacterial interference during egg incubation and early larval rearing has been demonstrated, where hatching success [ 17 ] and survival [ 13 ] of non-feeding larvae were negatively affected. Moreover, a drop in survival has been associated with the selection of opportunistic bacteria that dominated the larval bacterial community during the European eel-feeding culture [ 14 ].…”