We analyzed the relationships between the macroparasite community of the European eel and the expression of genes involved in the host physiology during its continental life. The genes studied are implicated in (1) host response to environmental stress, i.e., heat shock protein 70 (HSP70) and metallothionein (MT); (2) osmoregulation, i.e., β thyroid hormone receptor (βTHR) and Na+/K+ATPase; and (3) silvering, i.e., βTHR, freshwater rod opsin (FWO), and deep-sea rod opsin (DSO). All were enumerated by quantitative reverse-transcription polymerase chain reaction. The epizootiological results for 93 yellow eels caught in the Salses-Leucate Lagoon (France) included 11 species: 1 nematode, 2 acanthocephalans, 1 monogenean, and 7 digeneans. The molecular results revealed (1) a significant negative relationship between digenean abundance and the expression level of all the tested genes, except FWO; (2) a significant negative relationship between the abundance of the nematode Anguillicola crassus and the expression level of the Na+/K+ATPase gene; and (3) a significant positive relationship between the A. crassus abundance and the expression level of the MT gene. Eels infected with digeneans had, on average, a lower level of expressed genes. We hypothesize that the parasites may disturb the eel's ability to withstand environmental stress and delay their migration to the Sargasso Sea because of degeneration of the gut. We further propose that the effect of the invasive species, A. crassus, on the gene expression was mainly linked to an increased trophic activity of infected eels. Moreover, it is possible that the parasite may have an effect on the fish's migratory behavior, which is tied to reproductive purposes. Additional work, including an experimental approach, is required to confirm our hypotheses.

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