Some Animals that Change Gender

Prawns change from male to female after 1-2 years

In Clown fish couples when the female dies the male becomes female

Reed Frogs turn from female to male for breeding as needed

The Spotlight parrotfish turns from female to male after age 3

The Bluestreak wrasse changes from female to male (the largest female changes when one male in the school dies)

Source: "World's oddest & most wonderful Mammals, Insects, Birds & Plants" (Jeanne Hanson & Deane Morrison, HarperCollins, 1991)