Over at Pharyngula PZ has a nice post on sexual selection in Swordtails (Xiphophorus helleri). By a strange coincidence, I bought a couple of male neon swordtails yesterday for my tank and was reminded why the staff in many pet stores know nothing about their stock. Male swordtails have an elongated caudal fin – hence the name – and I asked the clerk for two male fish. He disappeared off to the front of the store, came back after a minute, only to announce to me that “We can’t tell which ones are male.” Ummm. So I set him straight, showing him that the tank contained two visiblly different forms of fish. I should perhaps have just hit him over the head with a copy of Darwin’s Descent of Man – the book features an illustration and brief mention of sexual dimorphim in the taxon (Fig 30):
In the male of an allied form, the Xiphophorus Hellerii (fig. 30 ), the inferior margin of the caudal fin is developed into a long filament, which, as I hear from Dr. Günther, is striped with bright colours. This filament does not contain any muscles, and apparently cannot be of any direct use to the fish. As in the case of the Callionymus, the males whilst young resemble the adult females in colour and structure. Sexual differences such as these may be strictly compared with those which are so frequent with gallinaceous birds.
And as an aside, the species was described by Ernst Haeckel in 1848. You can find out more here.