commonly found under floating vegetation or near structures in the water, minimizing their chances of being observed by potential predators. The sailfin molly is a tolerant species, as it can exploit the thin film of oxygen-rich surface water with its upturned mouth, so is able to survive oxygen-depleted habitats. A euryhaline species, the sailfin molly may be found in a variety of saline environments, tolerating salinities as high as 87 ppt and breeding in brackish waters. Biology Distinctive features The body of the sailfin molly is essentially oblong. The head is small and dorsally flattened, with a small, upturned mouth. The caudal peduncle is broad and the caudal fin is large, rounded, and sometimes tipped with black. The pelvic fins originate at a point anterior to the dorsal fin. In mature males, the dorsal fin is greatly enlarged (it is this feature that gives the species its common name) and the caudal fin is similarly colourful; these conspicuous secondary sexual features play a role in female mate choices. Females tend to be larger and more plainly colored, a different characteristic to the Poeciliidae. (See: Sexual selection) It is a smaller fish than the Yucatan molly (P. velifera), though that species often does not grow to full length if bred in an aquarium. The dorsal fins are the most distinctive character: Those of the sailfin molly have less than 15 fin rays, counting where the fin meets the back, whereas the Yucatan molly has 18-19 (intermediate numbers may indicate hybrids). If a male spreads his dorsal fins in a display, in this species it forms a trapezoid, with the posterior edge being shortest. The height of the dorsal fin, measured at the posterior edge, is a bit less than the height of the tail. The male molly is more aggressive than the females. Coloration The body is generally light grey, although breeding males may be greenish-blue. Several rows of spots occur along the sides, back, and dorsal fin. Often, these spots blend together, forming stripes. Aquarists have developed many color variations in this species (variation occurs naturally in the wild), with melanistic, leucistic, albino, and speckled forms known. Sailfin and black mollies An entirely melanistic form exists called the midnight molly, or nondescriptly, "black molly". The latter originally refers to melanistic breeds of Poecilia sphenops. Midnight mollies actually originated from hybrids between that and the present species. As hybridization, like in