Probiotics as biological control agent in sea bass larviculture

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Fight, flight or freeze : tonische immobiliteit bij haaien

Tonische immobiliteit (TI) is een niet-aangeleerde en omkeerbare toestand van motorische onbeweeglijkheid die bij verscheidene terrestrische en aquatische diersoorten, zoals haaien, kan worden uitgelokt. TI is van oorsprong een natuurlijk gedrag dat bij haaien kan worden uitgelokt door plaatselijke druk uit te oefenen ter hoogte van de vinnen en de dieren op de rug te draaien, waarna ze een verslapping van de spiertonus vertonen en tijdelijk niet meer reageren op omgevingsstimuli. Deze toestand kan mogelijk een effect hebben op de fysiologie van de dieren. TI zou bij haaien de ventilatie-efficiëntie verminderen en bijgevolg verscheidene bloedparameters beïnvloeden, stressrespons induceren en de osmolariteit van het bloed verstoren. Desalniettemin kent TI bij haaien wetenschappelijke en diergeneeskundige toepassingen bij (klinisch) onderzoek, bloedafname en wondbehandeling. Tonische immobiliteit bij haaien is echter een enigmatisch fenomeen en verdient verder interdisciplinair onderzoek.Tonic immobility (TI) is an inherited and reversible status of motoric immobility that can be provoked in a variety of terrestrial and aquatic animal species, including sharks. In sharks, this behavior, which is originally natural behavior, can be induced by putting external pressure on the fins and turning the animal on its back, after which the sharks will display reduced muscle tone and will temporarily not respond to environmental stimulation. It can be expected that this state has an effect on the physiology of the animal. In sharks, it can induce lowered ventilation efficiency with impact on blood parameters, it can cause a stress response and disrupt the osmolarity of the blood. Nonetheless, this TI state in sharks has its applications in scientific and veterinary (clinical) research, blood sampling and wound treatment. However, tonic immobility in sharks remains an enigmatic phenomenon that should be studied in detail using an interdisciplinary approach

Electron microscopic characterization of fertilised Dover sole Solea solea eggs during embryonic development

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Germ-free sea bass Dicentrarchus labrax larval model : a valuable tool in the study of host-microbe interactions

A thorough understanding of host-microbe interactions is crucial for more efficient disease management in the marine larviculture industry. As demonstrated in terrestrial animal research, gnotobiotic systems (involving animals cultured in germ-free conditions or inoculated with known microorganisms) are excellent tools to extend our understanding of the mechanisms involved in host-microbe interactions and allow the evaluation of new treatments for diseases. In this study, we introduce a germ-free European sea bass Dicentrarchus labrax larval model, independent of the continuous addition of antimicrobial agents. This model has an experimental set-up that allows addition of live feed to the larvae without compromising the germ-free status. This model will facilitate and render aquaculture research more effective in terms of mitigation fish larval diseases

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

Development of the first standardized biotic challenge model for Dover Sole (Solea solea) and its validation by assessing the protective potential of probiotic candidates

Dover sole (Solea solea) is gastronomically highly appreciated and has a high market value. The species is therefore considered to be a very promising candidate for European aquaculture. Furthermore, ensuring a reliable supply of sole would reduce fishing pressure on wild Dover sole populations. However, as for many other fish species, 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 are a major cause of larval mortality thereby decreasing survival and having a negative impact on the development of the fish embryo and larva. Severe economic losses due to diseases in larviculture may be linked to vibriosis. Various Vibrio spp. have been cited as causative agents, with Vibrio anguillarum on top of the list. Efforts have been made to reduce infectious diseases, in the past mainly by applying antibiotics. Because of the emergence of acquired antimicrobial resistance, there is a great need for alternative measures to overcome diseases. In this respect, the amount of studies investigating the use and effect of probionts in aquaculture increased drastically. However, the mode of action of these probiotics is largely unknown. In addition, researchers become increasingly interested in studying bacteria-host interactions as understanding these may assist in combatting disease. Reliable experimental models not only enable experiments exploring the mode of action of probiotic candidates but also facilitate research aimed at unravelling the ways pathogens elicit disease and mortality. 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 yet. Our research group developed a standardized biotic challenge model for Dover sole larvae adopting Vibrio spp. as potential pathogens. Five potentially pathogenic isolates were selected: the wild-type V. anguillarum HI610, V. anguillarum Fr and V. harveyi Fr that were both isolated from a disease outbreak in a French sea bass farm, V. tapetis isolated from skin blisters and liver of Dover sole and V. tapetis 2 originating from active skin ulcers of wild dab (Limanda limanda). Each isolate was added to the housing water of larvae at 10 days after hatching in a final concentration of 10^5, 10^6 or 10^7 colony forming units (CFU)/ml. No significant differences in survival were noticed between the larvae inoculated with V. anguillarum wild-type, V. harveyi Fr, V. tapetis, V. tapetis 2 and the control group at any of the concentrations tested. Vibrio anguillarum Fr supplied at a final concentration of 10^6 CFU/ml well water resulted in a larval mortality of 36%, while the non-challenged control group displayed 5% mortality. When this isolate was administrated to the larvae at a concentration of 10^7 CFU/ml, 48% mortality was observed. In addition to proving valuable in many other applications, this model is to be regarded as a powerful tool to evaluate the impact of (a)biotic components on larval health. Different probiotic candidates (administrated via the water and Artemia nauplii) were already assessed for their protective potential against V. anguillarum challenge and several prebiotic components will be evaluated in the future. In a next step, the impact of algal toxins on larval health will be considered amongst others by evaluating the susceptibility to disease agents

Development of the first standardized biotic challenge model for Dover sole Solea solea

The growing importance of the aquaculture sector during recent decades renders a diversification of fish species and products imperative. Flatfish species constitute important and credible new aquaculture candidates as exemplified by Dover sole Solea solea (Linnaeus 1758) exhibiting high flesh quality and important market value, and being the subject of an increasing consumer’s demand. As for other advanced marine teleost species, production has been severely hampered by difficulties in larval rearing which is reflected in low and unpredictable survival rates. Mortality largely may be induced by bacterial diseases (caused by e.g. Vibrio sp.) thereby decreasing survival and influencing the development of the fish embryo and larvae. Because of the emergence of acquired antimicrobial resistance, there is a great need for alternative measures to overcome disease. In this respect, the use of probionts in aquaculture is popular but scientifically poorly documented. Furthermore, the increasing interest of researchers in the interactions between bacteria and their host, either to unravel the mode of action of probiotics or to investigate how pathogens elicit disease and mortality, underscores the need for reliable experimental models. 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 yet. The present study describes a standardized biotic challenge model for Dover sole larvae adopting Vibrio spp. as potential pathogens. Our research group pinpointed a housing system for Dover sole larvae whereby they are kept individually in 24-well plates. This ensures a standardised and reliable experimental set-up in which the possible death of one larva has no effect on the other larvae, rendering experiments adopting such a system more reproducible. In three separate experiments, different pathogenic candidates in varying concentrations were tested. Five pathogenic strains were adopted: the wild-type Vibrio anguillarum HI610 strain, V. anguillarum Fr and V. harveyi Fr that were both procured from a disease outbreak in sea bass larvae in a French hatchery, V. tapetis isolated from skin blisters and liver of Dover sole and V. tapetis 2 originating from skin ulcers of wild dab (Limanda limanda). One of these strains was added to the well water of larvae at 10 days after hatching in a final concentration of 10^5, 10^6 or 10^7 colony forming units (CFU)/ml. No significant differences in survival were noticed between the larvae inoculated with V. anguillarum wild-type, V. harveyi Fr, V. tapetis, V. tapetis 2 and the control group at any of the concentrations tested. Vibrio anguillarum Fr administered at a final concentration of 10^6 CFU/ml well water elicited a larval mortality of 36%, while the control group displayed 5% mortality. When this strain was supplied to the larvae at a concentration of 10^7 CFU/ml, 48% mortality was observed, compared with 5% of the unchallenged larvae. The model as described above is to be regarded as a powerful tool for investigating the pathogenesis of V. anguillarum infections in Dover sole larvae and for evaluating curative or preventive treatments against vibriosis