Ontogeny of the Digestive Tract of First-Feeding European Eel Larvae by using histological and molecular biology techniques.

Even though some research groups can now regularly produce European eel larvae, massive mortality still occurs, particularly during the first weeks after hatching. This occurs especially because suitable start feeds for larval European eel in culture have not been identified yet. Therefore, this study aimed at examining the ontogeny of the digestive tract of European eel larvae by using histological and molecular biology techniques before, during and after the start of the first-feeding window stipulated by Politis and co-workers in 2018. For this, ~5 larvae from 5 egg batches were randomly collected for histology studies from 8 DAH until 22 DAH, and ~15 larvae from 2 eggs batches were collected at each age for molecular studies from 8 DAH until 19 DAH. We found that the most important histological developments occurred in the anterior esophagus. At 8 DAH: the anterior esophagus was a narrow duct with a single-layered pavement epithelium, which sometimes appears with cuboidal cells, and the outer muscle layer was absent. At 12 DAH the anterior esophagus epithelium had changed from cuboidal to squamous cells in all larvae, and the muscular outer layer now was visible. These characteristics are particularly important as they not only indicate the increase of the elasticity of the esophagus but also a food-grinding or crushing function. As for the rest histological findings, the digestive tract morphology was almost the same as that found in European eel leptocephalus larvae. We were not able to properly identify goblet cells throughout the development of the pre-leptocephalus larvae, though we observed some mucous-like cells in the intestine (14 and 18 DAH) and hindgut (22 DAH). For the molecular study, prepro-ghrelin and amylase gene expression remained constant, whereas trypsin and triglyceride lipase significantly increased between DAH 8 and 19 and between 12 to 19 DAH respectively. Another interesting phenomenon was the fact that amylase gene expression was higher than those of triglyceride lipase, in contrast to what was observed in A. anguilla pre-leptocephalus larvae in 2020 and in A. japonica pre-leptocephalus and leptocephalus larvae. This might be explained by the different diets offered to the animals during these studies. In conclusion, the digestive tract of the pre-leptocephalus larvae obtained from adults induced to mature by hormone treatment seems to develop normally, at least, from a histological and histochemical point of view. Whereas, from a molecular view, the wide variability of results between the different studies can be explained by the genetics of the larvae, the health status of the parents, sources of larval stress (physical or chemical) and, of course, the diet.