The bluehead wrasse is a fish that lives in small social groups in coral reefs in the Caribbean. Only the male has a blue head — signalling his social dominance over a harem of yellow-striped females. If this male is removed from the group, something extraordinary happens: the largest female in the group changes sex to become male. Her behaviour changes within minutes. Within ten days, her ovaries transform into sperm-producing testes.
Fish FAQ - Can fishes change sex?
Why do some fish change sex? - OpenLearn - Open University
Millions of people saw a dramatic example of this in the first episode of Blue Planet II, in which a ten-year-old female kobudai also known as an Asian sheepshead wrasse, Semicossyphus reticulatus changes into a male. After many months, the transformed male emerges from its lair larger than before, bearing testes, a huge bulbous forehead, and an aggressive nature. Now even larger than the existing dominant male it had previously mated with when female, the new male defeats the aged alpha in a violent battle for dominance. The footage is remarkable — but the transformation is actually not terribly unusual. Some like the kobudai change routinely from female to male. Others, like the clownfish, do the opposite, from male to female. Still others can switch back and forth depending on the circumstance, such as a variety of coral-dwelling gobies.
For nearly fish species, including the clownfish in Finding Nemo , the great divide between sexes is more like a murky line: If circumstances call for it, the fish can swap their sex, with females turning into males in some species and males turning into females in others. People think of sex as being fixed, said biologist Erica Todd from the University of Otago in New Zealand, "but there are so many fish that can push it in the other direction. Scientists have known for decades about the sex trades, but they've had limited understanding of how the exchange happens.
Sex change is a process by which a person or animal changes sex — that is, by which female sexual characteristics are substituted for male ones or vice versa. Sex change may occur naturally, as in the case of the sequential hermaphroditism observed in some species. Most commonly, however, the term is used for sex reassignment therapy , including sex reassignment surgery , carried out on humans.