Effect of prolonged exposure to thyroxine on growth, puberty timing and ovarian structure in female red tilapia (Oreochromis sp.)

This study explores the influence of exogenous long-term exposure to thyroxine (T4) (0.05 ppm (low) or 0.1 ppm (high)) from hatching until 28 weeks on female red tilapia (O. mossambicus × O. urolepis hornorum). The effect of T4 on ovarian development, puberty percentage and growth indices with special reference to the fine structure of oogenetic tissue was studied throughout the exposure period. The response of red tilapia to the T4 treatment was found to be dose dependent. Long-term exposure to T4 (both low and high doses) significantly decreased the weight, length and gonadosomatic indices in female red tilapia and delayed pubertal development. The percentage of abnormal oocytes increased to 22 ± 2% and 41 ± 2% of the total investigated oocytes exposed to low and high doses of thyroxine, respectively. The fine structure of the abnormal oocytes showed an external atretic thecal layer without thecal cells, and no active organelles were detected in the first and second ovarian layers. The third and fourth zona radiata layers were disrupted and disorganized with a missing cortical alveolar layer. All detected abnormal oocytes had an incompetent nucleus and cytoplasm distorted with hypertrophied follicles, liquefied yolk, and a fragmented and disintegrated zona radiata. In conclusion, long-term thyroxine treatment has a suppressive effect on the growth and maturity of fish species such as red tilapia.