ClownfishesAmphiprioninae

Every clownfish that matures does so as a male. The big fish in the anemone, the female, was one too.

A group in a single anemone is ranked by size: a breeding female at the top, one breeding male below her, and a line of smaller fish that breed with nobody. When the female dies, her partner changes sex and takes her place, permanently, and the next fish moves up into his. Nothing arrives from outside and nothing is contested. That is protandry, and it runs the opposite way to the sex change on the wrasse page, where fish start out female and the largest becomes male.

Both systems answer the same question, which is which sex gains most from being big, and the answers differ because the lives do. A large male wrasse holds a territory and gets more matings out of it. A clownfish cannot hold a territory; it has one anemone, one partner and nowhere else to be, so extra size buys it nothing as a male and buys it a great deal as a female, since the number of eggs a fish can carry climbs steeply with its body. The largest animal in the group should therefore be the one laying, and it is.

The fish below them keep themselves deliberately small, staying a clear step under the fish immediately above, because challenging that fish means being driven out of the anemone, and a clownfish in open water is not a clownfish for long. The anemone itself, and what each side gets out of the arrangement, is on the sea anemones page.

It also means the honest version of the film is a harder watch: with the mother gone, the father becomes the mother. Three species are listed for this sea, and the one nearly every diver here meets belongs to the 🌊 Red Sea and the 🌊 Gulf of Aden and to nowhere else on the planet.