EFFECT OF CORTISOL ON LARVAL GROWTH AND DEVELOPMENT IN TILAPIA, OREOCHROMIS MOSSAMBICUS

Abstract

The present study has shown that cortisol is involved in larval growth and development of tilapia, Oreochromi.1· mossambicus. From the profiles of free cortisol and cortisol glucuronide determined during the early development of tilapia, CJ. mossambicus, it is obvious that significant amount of cortisol is present in the fertilized eggs as well as throughout all the stages of early development. The cortisol level declines progressively from fertilization to hatching but increased after hatching suggesting that corticosteroidogenesis began at this time in tilapia larvae. The cortisol glucuronide levels followed a similar trend as the free cortisol during the early development of tilapia. The effect of cortisol on hatching was stage-dependent. The effects varied with the stage of development of the embryo at which the treatment was started and also the duration of treatment. Exogenous supplementation of cortisol just before hatching, when the endogenous cortisol content was not a very low level, accelerated hatching, while treatment just after fertilization delayed hatching. Cortisol treatment of denuded embryos close to hatching stimulated the precocious release of hatching enzyme as evidenced by the increased proteolytic activity. A growth promoting crteet of cortisol was shown in this study. A dose-response relationship was seen with 0.5 ppm showing the best response and 5.0 ppm being detrimental in effect. The standard length, tail length, head width and wet weight showed dose-dependent increases at concentrations of cortisol from 0.05 to 0.5 ppm, with no further increase at 1.0 ppm. However, cortisol at 5.0 ppm retarded the growth of the larvae without causing any morphological abnormality. It is conceivable that supplementation with cortisol during early development of tilapia would be beneficial. Yolk absorption and the onset of free-swimming activity in the larvae were also accelerated by cortisol treatment. The results obtained of free cortisol and cortisol glucuronide profiles after larvae were immersed in cortisol solution for two weeks imply that as the larva grows bigger all the developmental activities are activated and cortisol is being utilized for these activities and metabolized as cortisol glucuronide. The decrease in cortisol during development could be due to the conversion of free cortisol to cortisol glucuronide and cortisol glucuronide elevation is suggestive of cortisol metabolism. Further, it is obvious from this study that cortisol can be successfully administered to fish larvae by immersion. The tilapia larvae could take up exogenous cortisol from the treatment medium according to the concentration of the treatment medium and could metabolize cortisol as cortisol glucuronide. However, the metabolizing capacity of the larvae is limited as they could not metabolize completely the increased exogenous cortisol taken up on prolonged exposure and at higher concentration of 5.0 ppm. The existence of negative feedback mechanism involving hypothalamic- pituitary - interrenal axis, the routes of clearance of cortisol and its metabolites, and the mechanism by which cortisol exerts its effects in the larvae are yet to be investigated. Nevertheless, cortisol could be used to promote growth in fish larval rearing.