So a cool thing that most of you have probably already seen me talk about is the sexual hierarchy of clownfish. Every shoal of clownfish has exactly one female, usually the biggest fish of the bunch. When the female dies, the dominant male becomes the new female and the secondary male assumes its place. Femininity is only achieved by first dominating the male hierarchy.

That is strictly sex, though. In guppies, the sub-dominate male will sometimes take on the outward appearance of a female without any change to its reproductive systems. Their gonopods (fish dicks) will shrink, they will lose their color and they will take on female behavioral traits. They trade their ability to successfully reproduce for the ability to live peacefully among the females without the constant stress of being bullied by the other males. And once they've done this, the other males begin to treat the fish just like any other female. While I'd hesitate to go so far as to call this gender, they do change in behavior, physical appearance and social interactions without actually changing their sex.

Idk, I just think it's neat.