From this story on Allvoices.com:
_A recent study of the Mexican male molly fish shows they grow a type of "moustache" to draw their mates, and apparently, size does matter. The molly fish is reasonably common species in Mexico. They live in freshwater lakes, ponds, creeks, rivers, and sometimes brackish waters. Unique to the Mexican molly fish is their courtship ritual. Unlike most species the female mollys are fertilized internally rather externally. According to a recent article on PhysOrg.com, what scientists are calling the male molly's "mustache" is actually a series of epidermal outgrowths from the edge of the scales on the fish's top lip. Apparently these growths appear to have no sensory function, and the size of these protrusions are not linked to the size of the fish.