Sailfin molly

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.

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.

A male midnight molly

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 most Poecilia, is easy between these two species and due to the more spectacular appearance of P. latipinna, such sailfin-black molly hybrids, with males' conspicuous, large, yellow-rimmed dorsal fins, are often seen. Due to genomic recombination, F1 hybrids often display novel and bizarre fin shapes. Otherwise, hybrids can be bred among themselves, or with a higher rate of success with their parent species. They often have a somewhat decreased lifespan, but not as much (in healthy fish) as the deformed "balloon" molly breeds of P. sphenops.

The natural lifespan of sailfin mollies is short, particularly in the case of the males, which may live less than a year after achieving sexual maturity. Depending upon environmental conditions, sailfin mollies may become reproductive in less than a year. Sailfin mollies are small fish. At one year of age, males typically range in size from 0.5 to 3 in (13 to 76 mm) SL, while mature females are likely to be 0.5 to 2.5 in (13 to 64 mm) inch SL. The size of adult males is directly correlated with population density. The greater the population, the smaller the average size of males. The maximum recorded size for this species is 150 mm (5.9 in) TL.[6]

Sailfin mollies feed primarily upon algae and other plant materials, although they will consume a number of aquatic invertebrates, including the larvae of mosquitoes.

Fertilization is internal, and is accomplished by means of highly modified fin elements within the anal fin of males that form a structure known as the gonopodium. Sailfin mollies produce broods of 10-140 live young, depending on maturity and size, and females may store sperm long after the demise of their relatively short-lived mates. The gestation period for this species is about three to four weeks, depending upon temperature, and a single female may give birth on multiple occasions throughout the year. Although sex ratios of the broods are balanced, adult populations tend to be largely female, as males appear to suffer higher rates of mortality due to a greater susceptibility to predators and disease as a consequence of their brighter colours and a life devoted to frenzied breeding. No parental care is exhibited by this species.