Sexual characteristics of high-temperature sterilized male Mozambique tilapia, Oreochromis mossambicus

INTRODUCTION: In order to clarify the effect of extremely high temperature on gonads of fish, juveniles of the Mozambique tilapia, Oreochromis mossambicus, at three days after hatching (d.a.h.) were reared at a high temperature (37 ± 0.5 °C) for 50 days. The heat-treated fish were then cultivated at a normal water temperature for over six months. RESULTS: The testes of all individuals heat-treated for 50 days were sterile. Histological analysis revealed the complete absence of all stages of spermatogenic germ cells in the testes of the heat-treated males; however, structures within a layer of epithelial cells lining the efferent ducts were observed to actively secrete sperm fluid into the ducts, as in the mature testes of normal males. Clusters of cells immunopositive against P450scc and 3β-hydroxysteroid dehydrogenase were observed in the sterilized testes. Leydig cells had developed smooth endoplasmic reticulum and several mitochondria with tubular cristae indicating active steroidogenesis. The sterilized males displayed male nuptial coloration, actively dug spawning nests, and mated with normal mature females. However, females mated with these males initially brooded their eggs normally but released them prematurely at 4–5 days. All the released eggs were unfertilized and dead. CONCLUSION: Heat-sterilized male tilapia matures endocrinologically but completely lacks spermatogenic germ cells.