Miranda Dyson explains the biological process Copyright: BBC In the first episode of Blue Planet II we met the Kobudai — the Asian sheepshead wrasse - that was introduced as a female, behaved like a female and looked like a female. And then, as we watched she slowly but surely morphed into a male — displayed male behaviours and developed male characteristics. It may seem strange but in fact it's common among fish. Known as sequential hermaphrodism, sex change is a common and usual adaptive part of the life cycle. It is documented in at least 27 families of fish, spread across nine orders and displays three patterns: changing from female to male, known as protogyny; changing from male to female, known as protandry; and serial bidirectional sex change.

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.

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.

Some species of fishes such as sharks and rays are born one sex and stay that sex throughout life. For others, such and the wrasses Family Labridae and parrotfishes Family Scaridae sex reversal is the norm. For species that change sex there may be up to three colour phases.