Clownfish are famous for changing sex from male to female within their social groups.
Clownfish live in hierarchical groups among the tentacles of sea anemones. The largest individual is the breeding female, and the next-largest is the breeding male. Smaller fish remain nonbreeding and occupy lower positions in the hierarchy.
If the breeding female dies, the breeding male changes sex and becomes the group’s female. The largest remaining immature fish then matures into the breeding male. This pattern is called sequential hermaphroditism, specifically protandry because an individual functions first as a male and later as a female.
The clownfish–anemone relationship benefits both partners. The fish receives shelter among the anemone’s stinging tentacles, while its movements and waste can provide food or nutrients to the host. Not every fish called a clownfish has identical social details, but this sex-change pattern is well documented in anemonefish.