The sailfin molly, Poecilia latipinna, is a species of fish, of the genus Poecilia. They inhabit fresh, brackish, and coastal waters from North Carolina to Texas and the Yucatán Peninsula of Mexico. Preferring marshes, lowland streams, swamps, and estuaries, the sailfin molly is very common in peninsular Florida. Non-indigenous populations are established in New Zealand, in the western U.S. and in Hawaii. Sailfin mollies introduced to California have caused a decline in populations of the federally protected and endangered Desert Pupfish (Cyprinodon macularius).
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 and colourful (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 coloured, a difference 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 which should be avoided). If a male spreads his dorsal fins in 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 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 colour variations in this species (variation occurs naturally in the wild), with melanistic, leucistic, albino, and speckled forms known.
Sailfin and black mollies
There exists an entirely melanistic form 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 hybridisation, 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. This can include a grotesquely elongated gonopodium in males; such hybrids are usually unable to breed. Otherwise, hybrids can be bred among themselves, or with 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. Like the sailfin molly, they require a bit more attention of the fishkeeper than the extremely hardy “true” black mollies. The female has a fan-shaped anal fin and male has a pointed anal fin.
Size, age, and growth
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 15–68 mm SL, while mature females are likely to be 19–53 mm 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 TL.
Sailfin mollies feed primarily upon algae and other plant materials, although they will consume a number of aquatic invertebrates, including the larvae of mosquitoes.
Fertilisation 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 upon 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. There is no parental care exhibited by this species. A ratio of three females to one male is preferred, as with all live bearers, because the females are harassed by males to the point of exhaustion, and having more females gives the others a rest.
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