Poecilia velifera, the Yucatan molly, lives in coastal waters of the Yucatan peninsula. These live-bearer (Poeciliidae) fish are particularly well known for both the extreme size variation among males, and the sexual dimorphism between males and females in both body shape and behavior. If the males spread their dorsal fins in display, these have a distinct fan or trapezoid shape, with the upper edge being distinctly longer than the lower. The height of the dorsal fin, measured at the posterior edge, is a bit larger than the height of the tail.