Morphological and coloration changes during breeding
A knowledge of the external characteristics during breeding is an important additional tool for sexing and selection of sexually active, healthy broodstock.
Juvenile tilapias are generally silvery in colour. During sexual maturation and breeding their body undergoes both morphological as well as coloration changes. The extent of these changes depends on the sexes and species.
Both male and female of T. zillii and T. rendalli undergo coloration changes of similar intensity. In the genus Sarotherodon, for example S. galilaeus where both parents orally rear eggs, the differences in colour and their intensity is minimal whereas in Oreochromis, for example O. niloticus, O. aureus and O. mossambicus, there is a marked sexual difference between males and females. The breeding coloration of some commercially important species is given in Table 10.3.
A notable feature of the mouth-brooders such as O. placidus, O. mossambicus and O. hornorum is the enlargement of the upper and lower jaws of males which are used in jostling to assert male dominance.
During breeding, the genital papillae of both sexes become enlarged (Fig. 10.3). The papilla of Tilapia spp. is mottled grey and conical, the males generally having a longer and more pointed papilla. In Sarotherodon spp. the papilla is simple and conical. In both these groups the sexes are not easily distinguishable.