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International audience; This comparative study of growth performances and sexuel growth dimorphism was conducted with fish obtained from artificial reproduction. Growth was measured weekly. The levels of sexual steroids (11-KT) and thyroid hormones (T3 and T4) was measured after 74 rearing days and at the end of the experiment (103 days). Social interactions were examined with a video camera (three sequences of one hour for each group). #O. niloticus# and #S. melanotheron# achieved the best and worst growth rates, respectively, with their intergeneric hybrides standing in between. Tilapias originating from female #O. niloticus# (pure species and hybrid 1) exhibited a significant sexual growth dimorphism, which contrasted with the situation observed in the two other groups (originating from female #S. melanotheron#), where no significant difference was measured. Male tilapias from the two hybrid groups had similar growth rates, whereas hybrid females originating from female #S. melanotheron# (hybrid 2) grew faster than those coming from female #O. niloticus# (hybrid 1). The levers of 11-KT were highest in #O. nioticus# and lowest in #S. melanotheron#. The levels in the hybrid groups were similar and intermediate between those of the pure species. By contrast, the lever of T4 was highest in #S. melanotheron#. The levels of T3 at 74 days did not differ between groups but between sexes in all four groups. At the end of the experiment, this sex-related difference was still observed in the hybrid 2 group whilst it had vanished in the hybrid 1 group. The analysis of behavioural sequences indicated that aggressive behaviours (face to face or jaw to jaw interactions, attacks with or without poursuit) first emerged among faster growing fish (#O. niloticus# and hybrid 1)

National audience; Il existe chez de nombreuses espèces de tilapia, une différence de croissance entre les mâles et les femelles. La maîtrise de ce dimorphisme sexuel de croissance est essentielle pour la rentabilité des élevages de cette espèce. L'objectif de ce travail est de caractériser ce dimorphisme sexuel de croissance, d'en préciser la période d'apparition, et de rechercher les facteurs endocriniens potentiellement impliqués dans son déterminisme. Les résultats obtenus montrent que la différence de croissance apparaît vers 25-30g et devient significative vers 35 g. L'apparition de cette différence de croissance n'est pas strictement liée à l'ontogenèse gonadique, ni à la reproduction. Elle apparaît après la différenciation sexuelle et avant la maturité sexuelle. Elle n'est donc pas la conséquence de la vitellogenèse, ni de l'incubation buccale des oeufs et des alevins. L'implication des différences de statut des stéroïdes sexuels, oestradiol-17B et des hormones thyroïdiennes (thyroxine (T4) et triiodothyronine (T3)), dans la mise en place et le maintien de la différence de croissance entre les mâles et les femelles, a pu être décomposée expérimentalement en modifiant directement ou indirectement l'environnement endocrinien (gonadectomie, supplémentation, déplétion hormonale).

6 páginas, 4 figuras., [EN] This workexamines the way in which the plasma levels of thyroid hormones of male sea bas(Dicentrarchus labrax L.) are affected by long term exposure todifferent saline environments and under constant elevated temperatures on aseasonal basis. Different osmotic media provoked important changes in theseasonal rhythmicity of the thyroid hormones. Their physiological implicationsare discussed in the light of recent data., [ES] El presente trabajo investiga la variación anual de los niveles plasmáticos de hormonas tiroideas en el macho de la lubina, mantenido a temperaturas elevadas constantes y en diferentes medios salinos. La adaptación a las diferentes salinidades, provocó importantes cambios de la ritmicidad estacional de las hormonas tiroideas. Se discuten las implicaciones fisiológicas de estas hormonas a la luz de los presentes datos y de los aportados por otros autores., This work was supported by a grant from the Commission Interministerial para la Ciencia y la Tecnología (CICYT) (MAR 88-0231).

Trabajo presentado en el 13th Conference of European Comparative Endocrinologists, celebrado en Belgrado (Yugoslavia), del 7 al 12 de septiembre de 1986, The present study investigates the annual effect of constant temperature (19-20”) (except for summer months that follow natural variation) and different salinitiea (0.3%, _ 7%. 1.7%) on 36 adult male \ea bass (Dicrntrarc,hus luhrux) (average. 190 g weight, 27 cm length). evenly distributed in three groups. Periodic samples from each group were taken for assay of cortisol. T,. ‘T,. d’a , and Na and for determination of hematocrit (H~J, condition factor (CF). instantaneous growth rate (G). daily food intake (RJ, and gross conversion efficiency t K). G, R, and K decrease steadily during the reproductive period (December-April) and plasma electrolytes tend to be low at the end of this period. Cortisol, T,. and T? were high especially during January and/or March depending on the osmotic medium considered. Evolution of CF during this time reflects changes in gonad weight and shows different patterns comparing different salinities. Hematocrit levels present an inverse correlation with CF during this period. From May to June CF reaches the lowest values of the year, as do the corti\ol levels in plasma (400 ngiml). During this time (especially for animals adapted to sea water) an increase of’ T,. Ua + , and Na- was noticed, coinciding with a surge of plasma testosterone observed previously CF. Pmt. M. Carrillo, and A. Fostier, 1985 Eur. Sot. Comp. Physiol. Biochem., 7th Conf.. Al.16. University of Barcelona. Spain) for the same species and probably as result of the onset of gametogenesis. A sudden decrease of the forementioned parameters occurs at the end of this period. G, R. and K also increase during this time but continue further on until November. Coinciding with the growth period of the sea bass (S. Zanuy and M. Carrillo. 1985) Ayuac~ullure, 44, ll-25), a steady increase of CF was observed which reflects mesenteric fat accumulation instead of gonadal development. A progressive increase of T,. Ca’ &, and Na+ was observed, which was more evident in sea water-adapted animals. A mechanism of action of hormones and electrolytes are discussed according to the different osmotic medium considered and the time of the year.