In many species of osteichthyes (bony fish), sperm and eggs are developing in separate individuals - males and females. Fertilization is usually external. In some cases, fertilization is internal.
Males and females may show different or they could not be distinguished, in terms of size, colors, external reproductive organs, head or body shape characteristics.
The reproduction in fish bone is generally cyclical, but it can be influenced by certain factors such as changes in temperature, moon phases or changes of location of spawning areas.
Fish bone have at least three types of development of the embryo: the spawning (oviparous fish, egg-laying), egg retaining (ovoviviparous) and the birth of live young fish (viviparous).
In some ovoviviparous species, the offsprings are developing in enormous eggs where they are fed by the egg yolk until birth. At birth, they already have 30 cm and they are exact miniature copies of their parents.
Although the reproduction methods used by species of fish are heterosexual, some species of fish are hermaphroditic - individuals develop both ovaries and testes.