Principles of Genetics and Breeding

4.1.8.1 Secondary sexual characteristics

All the secondary sexual features, i.e. those that relate to courtship and other aspects of mating, develop after sex differentiation and can be permanent or temporary attributes of fish. In the latter case the sexual characteristics develop before reproduction and disappear afterwards.

• All of the secondary sexual characteristics seem to be controlled by steroid hormone produced in the gonads.
• Males typically develop more extreme characteristics of shape, colour and aggressive behavior, and all of these traits reported to be production of androgens by the testis and can be influenced by the artificial administration of sex hormones such as the manufactured steroid methyltestosterone.
• Female accessory secondary sexual characteristics are less obvious distinguishing feature across a wide range of species in their large size. E.g. Marine flat fish, many cyprinid fish (common carp), eels and sturgeons. A few groups of fish, such as the cichlids (tilapia), show male predominance in size, often in conjunction with positive sexual roles such as nest building and egg care.
• The secondary characteristics are of very significant economic importance. The brilliance of colour patterns and elegance of fin extensions are of obvious value to the ornamental fish breeders whilst growth rate is of considerable interest to fish farmers. For these reasons, the control of sex ratio and of sexuality itself are important in fish genetics.