Which fish can change from male to female?

Clownfish live in small groups in a single anemone, and the group is a strict queue. One large female breeds, the largest male is her partner, and the rest are smaller non-breeding males waiting. If the female dies, her partner changes sex and takes her place, and the next male in line moves up.

The change is not a costume. The fish's gonads reorganise, and it is permanent - a clownfish can go from male to female but never back. This makes the plot of a certain film about a clownfish father searching for his son rather more complicated than it appeared, since the biologically likely outcome of the mother's death is that the father becomes the mother.

Sequential sex change is common in fish and runs both directions. Wrasses and parrotfish generally go the other way, female to male, when the dominant male dies. The pattern usually follows whichever sex benefits more from being large: if only big males get to breed, start female and switch up; if a big female can carry far more eggs, do the reverse - which is exactly what the clownfish, locked in one anemone with nowhere to go, does.