Abstract
Wild fish assemblages that aggregate within commercial marine aquaculture sites for feeding and shelter have been considered as a primary source of pathogenic parasites vectored to farmed fish maintained in net pens at an elevated density. In order to evaluate whether Ceratothoa oestroides (Isopoda, Cymothoidae), a generalist and pestilent isopod that is frequently found in Adriatic and Greek stocks of farmed European sea bass (Dicentrarchus labrax), transfers between wild and farmed fish, a RAD-Seq (restriction-site-associated DNA sequencing)-mediated genetic screening approach was employed. The double-digest RAD-Seq of 310 C. oestroides specimens collected from farmed European sea bass (138) and different wild farm-aggregating fish (172) identified 313 robust SNPs that evidenced a close genetic relatedness between the “wild” and “farmed” genotypes. ddRAD-Seq proved to be an effective method for detecting the discrete genetic structuring of C. oestroides and genotype intermixing between two populations. The parasite prevalence in the farmed sea bass was 1.02%, with a mean intensity of 2.0 and mean abundance of 0.02, while in the wild fish, the prevalence was 8.1%; the mean intensity, 1.81; and the mean abundance, 0.15. Such differences are likely a consequence of human interventions during the farmed fish’s rearing cycle that, nevertheless, did not affect the transfer of C. oestroides.
Highlights
The isopod Ceratothoa oestroides (Cymothoidea, Isopoda) is one of the most detrimental ectoparasites for the European sea bass (Dicentrarchus labrax) fingerlings reared in in-shore aquaculture facilities in endemic marine Adriatic and Greek areas [1]
It needs to be highlighted that the population values of the reared fish were calculated from fish at harvest, so some isopods are likely to have been removed from the population by manual cleaning during vaccination or size-sorting, or because of biological constraints
The population parameters of the Ceratothoa oestroides in the harvested sea bass evaluated in this study, i.e., the prevalence, mean intensity and mean abundance, show low values and mostly uniform patterns across the sampled seasons
Summary
The isopod Ceratothoa oestroides (Cymothoidea, Isopoda) is one of the most detrimental ectoparasites for the European sea bass (Dicentrarchus labrax) fingerlings reared in in-shore aquaculture facilities in endemic marine Adriatic and Greek areas [1]. This generalist species attaches to the fish tongue and grows throughout the oro-pharyngeal cavity, becoming sedentary after its last larval moult. The larval stage (manca or pulli) firstly develops as a juvenile and an adult male, which subsequently transitions into a juvenile and an adult female. The occurrence and aggregation of various C. oestroides-infected wild fish species at aquaculture facilities, either for feeding or sheltering purposes, has been suggested as one of the most important causes of parasite emergence and propagation among farmed fish
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