An organism with male and female characteristics. Widespread among leeches, bristleworms, gastropods, fish, etc.

Clownfish (genus Amphiprion) are colourful reef fish that live in symbiosis with sea anemones. Typically, an anemone contains a "harem" consisting of a large female, a smaller reproductive male and even smaller non-reproductive males. If the female is removed, the reproductive male will change sex and the largest of the non-reproductive males will mature and become reproductive. It has been shown that fishing pressure can change when males switch to females, as fishermen usually prefer to catch larger fish. Populations tend to change sex at smaller sizes through natural selection.

Gubans (family Labridae) are a group of reef fish in which protogyny is common. Gubans also have an unusual life strategy called diandria (literally, two males). These species have two male morphologies: an initial phase male and a terminal phase male. Initial phase males do not look like males and spawn in groups with females. They are not territorial. They may be imitators of the female sex (which is why they are found swimming in groups with females). Terminal stage males are territorial and have a distinctly bright colouration. Individuals are born male or female, but if they are born male, they are not born terminal phase males. Initial phase females and males can become terminal phase males. Usually the most dominant initial phase male or female will replace any terminal phase males as they die or leave the group.

Lythrypnus dalli (family Lythrypnus) is a group of coral reef fish that undergoes bidirectional sex change. Once a social hierarchy is established, the fish changes sex according to its social status, regardless of its original sex, based on a simple principle: if the fish exhibits subordinate behaviour, it changes sex to female, and if the fish exhibits dominant or non-dominant behaviour of superiority, it changes sex to male.