The fish you have in the image is the red Anthia, which lives and feeds between the corals of the Red Sea. In this species, females are much more abundant than males. The proportion is approximately one hundred.
When one of the males of this species dies, one of the largest females changes from sex to male. This image, taken by Michael Mckinnon in Arabia, analyzes in a book that collects this news the ecology and strange changes that occur in the sea near Arabia.