The main objective of this project is to characterize the sexual growth dimorphism in two teleost fish, the Eurasian perch Perca fluviatilis and the Nile tilapia Oreochromis niloticus, in terms of appearance period and amplitude and environmental, social and physiological factors involved in its determinism. This study will mainly determine the influence of sexual steroids on somatic growth and energy allocation over the different target organs as well as the action mode of the steroids through the synthesis and/or release of growth and thyroid hormones. The role of feeding behavior and social interactions, and the influence of environmental factors involved in the control of gametogenesis will also be investigated. Research will be conducted with normal genotypes (XX females, XY males), but also with individuals displaying dissociations between genotype and phenotype (e.g. XX and YY males, XY and YY females), which will be obtained from biomanipulated breeders. 1. Relationship between sexual maturation, reproductive endocrinology and growth dimorphism 2. Mode of action of sexual steroids and assessment of an endocrine relay. 3. Effects of feeding behavior and social interactions.