There are different variaties of clown fish.
Nemo & Marlin are True Percula Clown.s - [Amphiprion percula]
According to National Geographic Kids :
Clownfish actually change gender as they grow! When the head female of a clownfish group dies, the biggest male of the group turns into a female to take her place!
Dory is a "Blue Tang" (aka Palette Surgeon) - [Paracanthurus hepatus]
Paracanthurus hepatus Fact Sheet
