Simple SummaryOptimization of metamorphosis, a key stage in development where fish undergo several morphological and physiological changes, may assure the efficient mass production of greater amberjack (Seriola dumerili) juveniles. Application of probiotics as feed and/or water additive benefits nutrient utilization through modulation of the digestive enzymes. Therefore, following the addition of biofilters with the probioticPhaeobacter inhibensin the rearing water throughout the early development of greater amberjack, crucial in metamorphosis, cellular pathways were investigated. The probiotic treatment increased the growth of greater amberjack, while hypertrophy was the main growth process in the metamorphosis. Additionally, the rearrangement of structures and tissues may have been facilitated through the observed induced cell death. The present findings are of great importance and may be applied in the aquaculture industry in order to enhance greater amberjack development and growth.AbstractMetamorphosis entails hormonally regulated morphological and physiological changes requiring high energy levels. Probiotics as feed supplements generate ameliorative effects on host nutrient digestion and absorption. Thereby, the aim of the present research was to investigate the impact of the probioticPhaeobacter inhibensas a water additive on cellular signaling pathways in the metamorphosis of greater amberjack (Seriola dumerili). Activation of insulin-like growth factor type 1 receptor (IGF-1R), protein kinase B (Akt), mitogen-activated protein kinases (MAPKs) and AMP-activated protein kinase (AMPK), induction of heat shock proteins (Hsps), and programmed cell death were assessed through SDS-Page/immunoblot analysis, while energy metabolism was determined through enzymatic activities. According to the results, greater amberjack reared inP. inhibens-enriched water entered the metamorphic phase with greater body length, while protein synthesis was triggered to facilitate the hypertrophic growth as indicated by IGF-1/Akt activation and AMPK inhibition. Contrarily, MAPKs levels were reduced, whereas variations in Hsps response were evident in the probiotic treatment. Apoptosis and autophagy were mobilized potentially for the structural remodeling processes. Furthermore, the elevated enzymatic activities of intermediary metabolism highlighted the excess energy demands of metamorphosis. Collectively, the present findings demonstrate thatP. inhibensmay reinforce nutrient utilization, thus leading greater amberjack to an advanced growth and developmental state.