This is the fact that clownfish can change sex depending on their social situation. Immediately the comedic image of Nemo revealing his complete change of sex to his dad came to mind, of which I have thumbnailed on my whiteboard.
|Dad: "Nemo are you in there?", Nemo: "Hi dad, I'm Nimi now."|
|A useful shot from Finding Nemo (2003) [source]|