Maybe it is the size of the fins or the color or something. Maybe my dad can answer that!
That reminds me of a time I was at a reptile museum, and the lady asked if we had anymore questions. And so I spoke up, and said "How do snakes reproduce?" and the women goes "well your parents should explain that to you missy" and so then I go "No I mean, where are the ...parts.. and the lady kicked me out . Well I read up on it and I found out. It was so embarassing for the lady to kick me out!