In the clownfish world, size does matter. But, it's not what you think. For clownfish, the females are larger than the males. If you have a large clownfish or a clown that has been alone for a long period of time, chances are that it is a female. Female clowns are much more dominant than the males. They also try to eat most of the food that gets into the tank which is one reason the males don't get as big. They will often aggressively protect their territory which can be an anenome, a nesting site, a clay pot or even the entire fish tank. If it attacks your hand when you're trying to clean the glass or it won't let any other fish near its spot, you most likely have a female clownfish.
All clowns are born gender neutral. They are not male nor female at birth and as juveniles. The other thing that makes sexing Ocellaris Clownfish difficult is that they can become male and then female. This means that a juvenile clownfish can turn into a male, and the same clownfish can later change sexes to become a female. Once they become female they cannot change back to male.
This is how the social hierarchy works for clownfish . . . . There is one dominant female clownfish, partnered with a male, who is the next most dominant fish. The female is larger - much larger than the male. Next are non-mating males and juveniles. They are usually smaller still than the dominant male. If the male mate were to die, the next dominant male would become the mate if the female accepts him. If the female were to die, then the dominant male would change to female and the next male in line would become the mate.
If you introduce two fish at the same time then one will likely become the female and the other male. This does not guarantee that they will mate. You will be able to tell which is the female because she will be much more aggressive. Often, the female will barely let the male eat and sometimes will almost control where in the tank the male is allowed to go.
