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ABSTRACT Probiotics are beneficial bacteria able to improve the host welfare and can be supplied to fish through diet or water. They are able to positively act on fish growth and development by modulating innate immune system and digestive system, and perhaps by improving water quality. Probiotics may represent in some cases a valid alternative to the misuse of medicinal drugs widely used in animal husbandry and larviculture. In particular, finfish aquaculture industry broadly developed during last decades and unfortunately antibiotics and chemotherapics have been widely used in order to increase aquaculture productivity. In fact the high mortality that usually occurs during the fish larval stage, leads to high economical losses for aquaculture industry. Thus studies on fish welfare improvement upon probiotic administration are necessary in order to clarify the mechanisms involved in the improvements and at last, to increase aquaculture industry productivity. In this Doctorate thesis we sought to determine the effects of the application of several probiotic species on larval fish survivorship, growth, development and stress resistance. Several fish models (Solea solea, Sparus aurata, Amphiprion ocellaris, Danio rerio GnRH3- EGFP transgenic line) have been used in order to better clarify the effects and the molecular pathways involved in the improvements observed upon probiotic administration. A Bacillus mixture containing B. subtilis, B. pumilus and B. licheniformis was supplied to gilthead sea bream larvae until day 75 post hatch. The control group was fed with a standard diet based on rotifers and Artemia nauplia, in the group 1 the Bacillus mixture was supplied via live prey, while in the group 2, probiotics were delivered through live prey and with addition to rearing water. The Bacillus mixture significantly increased growth in terms of standard length and body weight in both experimental groups, at larval and juvenile stages, 47 and 75 days after hatching, respectively. In addition, the expression of genes involved in growth metabolism (Insulin-like Growth Factors I and Myostatin) and animal welfare (70KDa-Heat shock protein and Glucocorticoid receptor) were analysed. The morphometric analysis was supported by molecular results which clearly evidenced higher expression of IGFI and lower levels of myostatin in groups fed on probiotics. In addition, a better tolerance to farming conditions was also found as evidenced by the lower expression of HSP70 and glucocorticoid receptor, suggesting beneficial effects of the supplied Bacillus mixture on fish welfare. Regarding the application of lactic acid bacteria, the autochthonous probiotic species Enterococcus faecium IMC 511 was supplied at different concentrations (103 and 105 CFU · L-1) and from the mouth opening day or from the Artemia phase to common sole larvae. The effects on larval microbial community has been investigated. Beside, at days 10, 30 and 50 post hatch, the effects on the fish welfare, survival and growth have been evaluated by morphometric approach and by molecular approach through the relative gene expression of 70KDa Heat Shock Protein and Myostatin. Moreover, cortisol levels have been estimated. The results showed that the Vibrio populations of the intestine of S. solea larvae were thwarted in a significant way by the probiotic Ent. faecium IMC 511 which seemed to be predominant by day 50 ph, probably due to a stabilization of the intestinal microflora; this could in turn lead to an improvement of animal wellbeing and growth as also supported by the lower levels of HSP70 and myostatin in the probiotic treated groups. The microbiological data together with the molecular and morphometric results here achieved testimony the importance of administering Enterococcus faecium to common sole larvae ever since the mouth opening day. Another lactic acid bacteria species, Lactobacillus rhamnosus (IMC501TM) was challenged as probiotic in order to improve larval development in the false percula clownfish Amphiprion ocellaris and to determine what molecular responses were observed in the larvae upon probiotic exposure. We supplied the probiotic strain to clownfish larvae from the first day post hatch. We supplied probiotic via live prey and with addition to rearing water (group 2) or exclusively via live prey (group 3). We observed two times higher body weight in both clownfish larvae and juveniles in group 2. In addition, in both groups 2 and 3 development was accelerated with metamorphosis occurring 3 days before than in the control group. Alteration in molecular biomarkers supported the faster growth observation; probiotic treatment significantly increased the gene expression of factors involved in growth and development (Insulin-like Growth Factors I and II, Myostatin, Peroxisome Proliferators Activated Receptor α and β, Vitamin D Receptor α, Retinoic Acid Receptor γ) when probiotics were delivered via live prey and addition to the rearing water. Moreover probiotic treatment lessened the severity of the stress response as exhibited by the lower levels of the Glucocorticoid receptor and 70 kDa Heat shock protein gene expression. Furthermore, positive effects on skeletal head development was observed, with a 10 to 20% decrease in deformities of juveniles treated with probiotic. All data suggest a potent effect through administration of lactic acid bacteria on early larval development of clownfish. These data provide a preliminary molecular entry path into the investigation of mechanisms responsible for probiotic enhancement in fish development. The same probiotic species, L. rhamnosus was supplied to GnRH3-EGFP transgenic zebrafish larvae from the opening mouth day until puberty. Here we sought to determine if 9 weeks L. rhamnosus IMC 501TM treatment could affect Danio rerio early development and gonadal maturation and differentiation. From 6 to 10 dpf we analyzed the effects of probiotics on GnRH3 neurons soma size. Moreover at 3, 6 and 9 weeks post fertilization we observed gonad development by histological sections. By quantitative PCR we analyzed the expression of an array of genes involved in growth and development and stress response, at 6, 10 and 20 days post fertilization. At day 12 post fertilization we checked the ongoing calcification of backbone vertebra. A significant increase of Insuline like Growth Factors I and II, Peroxisome Proliferator Activated Receptor and , Vitamin D Receptor and Retinoic Acid Receptor was found in the probiotic treated larvae with respect to the control ones. The ongoing calcification of vertebral bodies seemed to occur faster in probiotic treated larvae .Moreover probiotic seemed to less the severity of physiological and cellular stress as the lower levels of GR and HSP70 registered in probiotic treated group. Moreover, L. rhamnosus treatment seemed to affect also GnRH3 neurons soma size. More incredibly, 3 weeks old zebrafish treated with probiotic already showed 20% of males. In the control group male appeared only at 9 weeks post fertilization. At the end of the experiment the control group showed a sex ratio of 94% females and 6% males vs. the 55% females and 45% males ratio found in the probiotic group. We hypothesized that probiotic treatment can act on early larval development by delivering organic molecules and increasing the calcium absorption as indicated by the higher calcification of the axial skeleton. The early gonadal development, may be related with the activation of GnRH3 neurons as indicated by the larger soma of treated larvae. These results evidenced that Lactobacillus rhamnosus may improve several cornerstones of fish development. Further studies are necessary in order to deepen these outstanding findings. Finally, for the first time we challenged the potentiality of Magnetic Resonance Imaging (MRI) to depict, with sufficient resolution and satisfactory contrast, the anatomy of clownfish during metamorphosis, without dissecting the larvae. Once the method was optimized, MRI was used to identify internal anatomical changes during metamorphosis climax and by Real time PCR the gene expression trend of TRα and β associated with the metamorphosis process first occur in the inner body, while outer changes, such as the appearance of the typical adult pigmentation, occur soon after. The morphological changes at brain, eyes, swim bladder and digestive tract were associated with higher TRα and β expression levels. Our findings represent the starting point for future applications of MRI to marine fish developmental studies. The present doctoral thesis tried to deep investigate into the signalling pathways involved upon probiotic administration in a number of marine and freshwater fish species. The molecular insights, together with the morphological observations, allowed us to detect some of the molecular mechanisms involved in the host physiological response to beneficial bacteria supply. The results here achieved could provide new insights to the emerging blue biotechnology, for a further increase of productivity and competitiveness of the aquaculture industry and to the field of developmental biology. Next step will be the study of the effects of those beneficial bacteria on the ageing mechanisms. We will seek to determine whether the anticipation of some developmental milestones by probiotic treatment lead in turn to a faster ageing. Moreover, studies on probiotics are important in order to develop a environmental friendly and sustainable aquaculture, together with an increase of productivity.