Swim bladder nematodes (Anguillicoloides crassus) disturb silvering in European eels (Anguilla anguilla).

The introduced parasite Anguillicoloides crassus is thought to play an important role in the decline of freshwater eel (Anguilla spp.) populations. These nematodes are known to negatively affect many fitness-related traits in eels. We used experimental infections to study the effect of A. crassus on the relative size or mass of organs, and the expression of functionally relevant genes (total of 12 parameters) that are involved in the silvering process of Anguilla anguilla. Our results showed that the liver mass, the hemoglobin α-chain, and androgen receptors α expression levels were significantly higher in infected eels, whereas the freshwater rod opsin expression level and the gut mass were significantly lower in infected eels. Our results suggested that infected eels were at a more advanced stage in the silvering process than uninfected counterparts of similar size. These results may be explained by 2 hypotheses. First, A. crassus could trigger physiological mechanisms involved in the silvering process as a side-effect of infection. Second, eels may adjust their life history traits in response to infection. The implications for eel migration and reproductive success may be either negative or positive, depending on whether the response to A. crassus infection results in an additional cost of the parasite or is due to the phenotypic plasticity of the host.

Changes in gene expression in European eels (Anguilla anguilla) induced by infection with swim bladder nematodes (Anguillicola crassus).

The European eel, Anguilla anguilla, is considered an endangered species that is under pressure for many reasons. Among others, the introduced parasite Anguillicola crassus is thought to play an important role in the decline of eel populations. These nematodes have been shown to negatively affect many fitness-related traits in eels, e.g., growth, osmoregulation, and stress tolerance. Nevertheless, there has been little work on the way in which the host-parasite interaction influences the molecular regulation of these key physiological processes. We experimentally analyzed the effect of this nematode on the expression of genes involved in the physiology of European eels during their continental life. Included are genes that are implicated in the eel's somatic growth (insulin-like growth factor 1 and thyroid hormone receptor beta), osmoregulation (Na+/K+-ATPase beta1 and aquaporin 3), and hematopoiesis (hemoglobin alpha-chain). Our results showed the absence of an effect on genes involved in fish growth; the parasite may, however, have an effect on osmoregulation and hematopoiesis. We also noted a differential impact of male and female parasites on the expression of some genes, perhaps owing to the sexual dimorphism in body size of the parasite.

[Isotopic composition of the delta-18O--delta-13C from the otoliths of reef fish from Taiaro (Tuamotu, French Polynesia): isotopic and biological implications]. / Composition isotopique delta 18O-delta 13C des otolithes des populations de poissons récifaux de Taiaro (Tuamotu, Polynésie française): implications isotopiques et biologiques.

Nuclei (larval stage) and outer parts (adult stage) of fish otoliths from the Taiaro closed lagoon (French Polynesia) and adjacent ocean have been analysed for the C-O isotopic compositions. delta 18O values of the nuclei of both populations indicate that isotopic equilibrium is reached. This implies that the lagoonal fish population has done its complete biological cycle in the lagoon and represents an adaptation in a closed system. delta 18O values of the outer parts show a slight isotopic disequilibrium (< 0.2@1000) interpreted in term of vital effect. All the delta 13C values exhibit a strong isotopic disequilibrium related to metabolic activity.