From Ancient Greek δι- - two, μορφή - form.

Sexual dimorphism - sexual differences that distinguish males and females, manifested in the shape and size of the body, fins, coloration, spawning rash, etc. In the vast majority of cases, these names take place in males, after spawning, all neoplasms disappear, but in Pacific salmon and some other fish they are irreversible, and after spawning these fish die.

Fish exhibit sexual dimorphism only during mating. According to one theory, sexual dimorphism is more pronounced the more different the contributions of the two sexes to offspring care are. It is also an indicator of the level of polygamy.

Polyandry, in which a female mates with several males, occurs in fish. In this case, reversal of sexual dimorphism is often observed (females are larger than males, brighter colored, males build the nest, incubate eggs and take care of the brood, there is no struggle for females). For example, this phenomenon is present in water bugs Phoreticovelia disparata.