i had a pair - but they now reside in my freezer - had a mishap of epic proportions years ago
the difference i noticed beside the male caving hard and the female not.... was male had a bigger broader longer head than the female... wasnt a great difference in body shape... mine were about 17-18cm.... and the other noticeable difference was the male had a slightly bigger dorsal fin than the female.... but if you have a pair you will notice different behaviour in the male/female.... unmistakeable really.....
having said that - im only guessing saying it looks like a male/female/female not in that order of course....