The key issue to larval health research in Dover sole Solea solea L. : a reliable experimental set-up and challenge model, as exemplified by assessing the protective potential of probiotic candidates

Dover sole (Solea solea) is highly appreciated in quality restaurants and has a high market value, making it a very promising candidate for European aquaculture. Furthermore, due to the increasing importance of the aquaculture sector, diversification in the number of cultured species imposes itself and developing a reliable sole production would reduce fishing pressure on wild populations. However, as for many other fish species, Dover sole production is hampered by amongst others high susceptibility to diseases and larval mortality, justifying the need for more research in this area. Infectious diseases (e.g. vibriosis) are a major cause of larval mortality and various environmentally-friendly prophylactic treatments are currently being identified for marine larvae including pro- and prebiotics, with very limited data available for Dover sole. To remediate this and elucidate the interactions between bacterial pathogens and their host, the availability of experimental infection models is imperative. Nevertheless, only a handful of studies focused on the development of such models for fish larvae and for Dover sole, no such model is available. In this respect, in a first stage, an experimental housing system for Dover sole larvae was pinpointed by keeping the animals individually in 24-well plates for 26 days with good survival rates and initiating metamorphosis. Housing the larvae individually has the advantage that the possible death of one larva has no effect on the other larvae, rendering these experiments more reproducible and ensuring a standardised and reliable experimental set-up. Secondly, the first standardised biotic challenge model for Dover sole was developed, using the bacterial pathogen Vibrio anguillarum. In addition, the protective potential of probiotic candidates (administrated via the water or Artemia nauplii) was evaluated in vitro and subsequently in vivo against V. anguillarum challenge by means of the pinpointed models. In conclusion, the models as described above are to be regarded as powerful tools for investigating the pathogenesis of V. anguillarum infections in Dover sole larvae and evaluating the protective characteristics of probiotics. In addition, the exploration of the impact of other components (e.g. prebiotics, microplastics, PCBs or algal toxins) on larval health is rendered possible