 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.Some names of the Yucatan molly - such as "Segelkärpfling" or the specific name "velifera" - contain an element signifying "sail", aggravating the confusion with the sailfin molly.