Because the life cycle of the European eel Anguilla anguilla is not yet closed in captivity, the aquaculture industry depends on wild caught juvenile glass eels that are grown to marketable size. In our facilities, larvae are produced on a regular basis but they die without feeding exogenously. High mortality and deformity rates are often observed during the first week after hatching. Therefore, there is an urgent need to increase egg and larvae quality to produce larvae that can grow, survive and metamorphose into glass eels. In an earlier study (Jéhannet et al., 2021), we compared the transcriptome of larvae at 1 dph, of batches that survived less than 3 days post hatch (dph) vs. batches that survived for at least a week to understand the reasons behind the mortality rates during the first week after hatching. Our results suggested that larvae that die early suffered from microbial infections because the expression of genes related to inflammation and host protection were up-regulated when compared to gene expression in viable larvae. Therefore, in this study, we tested the effects of antibiotics (rifampicin and ampicillin 50 mg.l-1) and egg surface disinfection treatment (povidone iodine 25 ppm), alone or in combination, on hatching success, larvae survival and deformities in larvae from wild and feminized eels.
Materials and methods
Eggs were stripped from wild eels (N=5) and from eels that were feminized during the young elver stage (N=12). Eggs were collected in dry bowls and each individual batch was mixed with the sperm of 6-8 males. Gametes were activated by adding seawater and placed in 5-L beaker containing artificial seawater kept at a water temperature of 18˚C and a salinity of 36 ppt under dark conditions. Within 2 hours post fertilization (hpf), floating eggs were collected from the 5-L beaker with a sieve and weighed. From the weighed eggs, 63 g of eggs were equally distributed over twenty-one 1,800 mL beakers containing the various treatments (Table 1). Eggs were (i) placed in three beakers containing artificial seawater as control (C); (ii) placed in three beakers containing artificial seawater supplemented with antibiotics (A2); (iii) disinfected for 5 mins, rinsed three times and placed in three beakers containing artificial seawater (D2); (iv) disinfected, rinsed and placed in three beakers containing artificial seawater supplemented with antibiotics (AD2) and (v) placed in nine beakers containing artificial seawater until similar treatments but at10 hpf. At 10 hpf, eggs that were previously kept under control conditions were (i) placed in three beakers containing artificial seawater supplemented with antibiotics (A10), (ii) disinfected, rinsed and placed in three beakers containing artificial seawater (D10); (iii) disinfected, rinsed and placed in three beakers containing artificial seawater supplemented with antibiotics (AD10). When females gave less than 63 g of eggs, treatments were applied in the priority order as shown in Table 1. For each treatment, hatching success, larvae survival and the type of deformities observed were recorded.
Results and discussion
Of the twelve feminized eels that were stripped, five of them gave larvae that survived up to 5 dph. All five wild eels that were stripped gave larvae that survived until 21 dph. In the controls for treatment, eggs from the wild eels had significantly higher hatching rates (8 %) than the eggs from the feminized females (1 %). In the controls, larvae from the wild females significantly survived longer (4 ± 1 dph) than the ones from the feminized females (1 ± 2 dph). These results show that wild females produced eggs and larvae of higher quality than the feminized ones. Antibiotic treatment doubled the larval survival period but the effect was only significant vs. eggs that were disinfected, not vs. the controls. As variations were high, we are currently increasing the number of observations. Among the abnormal larvae from feminized and wild eels, larvae were often curved. Larvae that hatched later than 80 hpf were more curved than larvae that hatch before 60 hpf suggesting that curvature is related to delayed hatching. Also other abnormalities such as notochord deformities (Fig. 1), pericardial oedema and head emaciation were frequently observed. Although our results show that antibiotics increase the early larval survival in European eels, sustainable methods aiming for antimicrobial control and increasing larvae survival are being developed.
Jéhannet., P, Palstra, A.P., Heinsbroek, L.T.N., Kruijt, L., Dirks, R.P., Swinkels, W., Komen, H., 2021. What goes wrong during early development of artificially reproduced European eel Anguilla anguilla? Clues from the larval transcriptome and gene expression patterns. Animals. 11, 1710. https://doi.org/10.3390/ani11061710