Some fish have the ability to change their sex—nifty trick, huh? Unfortunately they can't just wake up and decide to be male one day, though, because it's all regulated by size and number.
For the Indo-Pacific reef-living cleaner fish, Labroides dimidiatus, individuals gather in all-female groups with only the largest fish in the group male. If he dies or disappears, the largest female becomes the sole male.
And the Indian Ocean anemone fish, Amphiprion akallopisos, lives in all-male groups, with only the largest fish in the group female.
Bet you didn't know fish had such an interesting life!