In males of Mormyridae the anal fin is a sexual character, (male fish ) though most probably only during the spawning season, which is highly dependent on local hydrographic conditions. Between the spawning seasons the bases of the fin-rays seem to show peculiar enlargements, but only visible on X-rays. If fully developed, the sexual character shows up at the anal fin: its base is bent inward, so is the outer contour, whereas in females the anal fin has a straight base line. This makes the anal fin in males look crippled, and perhaps some collectors refused specimens with a 'crippled' anal fin. This may be a reason that most museum specimens seem to be females, if not they got caught outside their spawning season. In nature at spawning season the sex ratio is 1 to 1. The physiological meaning of the fin structure in males is unknown. It may be worth mentioning that in Mormyridae and in Gymnarchus the spermatozoa do not have a flagellum – aflagelllate spermatozoa –, and that there is only one rather small testicle, a feature shared with other taxa of the order Osteoglossomorpha.
In many other fishes sexual dimorphism is manifested as often striking differences in coloration and / or shape of dorsal or anal fins etc. In general sexual dimorphism his is a feature often met with in the animal kingdom which quite often has lead to regard the different sexes as different species even.