All clownfish are born male. When they become adults, the largest, most dominant fish becomes a female, and the second largest becomes the breeding male. If the breeding female disappears, the breeding male (now the largest of the group) will become a female, and so on. In other species of tropical fish, when the ratio between males and females becomes imbalanced, dominant females may become males. 

Source: ohscience

6animals, biology, clownfish, fish, gender, ocean, science, sex, trans, transfish?, tropical fish, story ideas,