Taxonomy & naming
Poecilia latipinna was described by the French naturalist Charles Alexandre Lesueur in 1821, from material collected on the North American coast. For much of the twentieth century the sailfin molly and its relatives were placed in the genus Mollienesia, and a great deal of older aquarium and ichthyological literature refers to the fish as Mollienesia latipinna. Subsequent revision folded Mollienesia into Poecilia, the large New World genus that also contains the guppies and the other mollies, and Eschmeyer's Catalog of Fishes treats Poecilia latipinna (Lesueur, 1821) as the valid name within the family Poeciliidae, subfamily Poeciliinae.
The family Poeciliidae unites the guppies, mollies, platies, swordtails and mosquitofishes — small, internally fertilising, mostly live-bearing toothcarps of the Americas. Within Poecilia the sailfin molly belongs to a closely knit group of mollies that intergrade and hybridise readily; it is most often confused with the related Poecilia velifera (the Yucatán sailfin molly) and with P. latipinna's own domestic derivatives. The familiar solid-black aquarium "black molly" is not pure P. latipinna at all but a selectively bred hybrid of sailfin and short-finned molly stock, a point worth keeping straight when matching shop fish to the wild species.
Morphology
The sailfin molly is a stout, laterally compressed livebearer with a small upturned mouth set for surface grazing. Males reach about 6 in in total length and females roughly 4 in, though most aquarium and field specimens are considerably smaller; common length is only a few centimetres, and males mature from around 2–3.5 in. Base coloration runs from greenish and grey to nearly jet black, frequently overlaid with rows of speckles and spots along the flanks, and many populations and aquarium lines show heavy melanism or speckled patterning.
The defining feature, and the source of the common name, is the enormous dorsal fin of the mature male — a broad, high "sail" that can be raised and spread in display. Females and immature males carry an ordinary, modest dorsal, so the sail is both a sex marker and a courtship signal. As in all poeciliids, the male's anal fin is modified into a gonopodium, a slender intromittent organ formed from elongated and fused fin rays and used to deliver sperm to the female internally. Sexual dimorphism is therefore strong: large-sailed, gonopodium-bearing males versus larger-bodied, plain-finned females.
Habitat
Poecilia latipinna is a fish of warm, shallow, slow-moving coastal waters. Its native range follows the Atlantic and Gulf coast drainages from the Cape Fear system of North Carolina south through Florida and the Gulf states to Veracruz, Mexico. Through most of that range it is tied to the coast, but in Florida, Louisiana and Texas it penetrates well inland. Typical habitats are marshes, ponds, sloughs, swamps, drainage ditches, sluggish creeks, vegetated backwaters and estuaries, very often among dense aquatic plants near the surface.
The species is remarkable for its physiological breadth. It is benthopelagic in still or gently flowing water at subtropical temperatures of roughly 68–82 °F, and it tolerates a wide span of salinity — from pure freshwater through brackish tidal ditches to near-marine coastal water — as well as low dissolved oxygen, which it can offset by skimming the oxygen-rich surface film. This tolerance of warmth, salt and poor water is exactly what makes the sailfin molly both a successful coastal native and a readily established introduction: it has been moved widely within the United States (Arizona, California, Colorado, Nevada, New Mexico, Texas, Hawaii and elsewhere) and to Puerto Rico and many other countries, largely through aquarium releases and abortive mosquito-control stockings.
Feeding
In the wild the sailfin molly is largely herbivorous, a grazer of algae and plant matter with a low trophic level near 2.0. It works the surface and submerged surfaces of vegetation, rasping algal films and consuming diatoms and detritus, and supplements this plant diet with small animal prey — rotifers, copepods and ostracods, other small crustaceans, and aquatic insect larvae. Its strongly algal diet is one reason early attempts to use the species for mosquito control largely failed: it simply does not target mosquito larvae the way a dedicated predator such as Gambusia does.
In the aquarium that herbivorous bias should be respected. Sailfin mollies do best on a diet rich in vegetable and algal matter — spirulina-based flakes and pellets, blanched vegetables, and a well-lit tank that grows some grazable algae — rounded out with small live and frozen foods such as Daphnia and brine shrimp for protein and breeding condition. A diet too heavy in protein and too poor in plant matter is a common cause of digestive trouble in captive mollies, so vegetable content and frequent small feedings suit them better than rich, infrequent meals.
Mating
Reproduction in the sailfin molly is internal, as in all poeciliids. The male transfers sperm to the female using the gonopodium, the modified anal fin that swings forward and acts as an intromittent organ. Males are persistent and use the great sail as their principal display, flaring it broadside to females and to rival males in a sigmoid courtship posture; as in guppies, males also attempt rapid "sneak" copulations that bypass female choice. Females can store viable sperm internally and produce a succession of broods from earlier matings.
Female choice and male display make the sailfin molly a well-known subject in studies of sexual selection and of the mating dynamics of mollies, including the curious case of the all-female Amazon molly (Poecilia formosa), a clonal hybrid that depends on sperm from males of sexual species such as P. latipinna to trigger development of its eggs. Larger males with bigger sails generally enjoy a mating advantage, which selects for the exaggerated dorsal that gives the species its name. Whether sailfin mollies practise superfetation — carrying overlapping broods at different developmental stages simultaneously, as some poeciliids do — is not clearly established for this species in the cached sources, so it is best treated as not demonstrated here rather than asserted.
Breeding
The sailfin molly is a viviparous livebearer: after internal fertilisation the female carries the developing embryos and gives birth to free-swimming young rather than laying eggs. Gestation lasts roughly four weeks (about 28 days), and brood size is large and variable — commonly on the order of 10 to 100 young, with reported fecundity spanning roughly 10 to 300 depending on the size and condition of the female. With multiple broods possible across a season and high reproductive resilience, a healthy group breeds readily and steadily.
The newborn fry are fully formed miniature fish that swim and feed immediately, and they need cover at once: adults, including the mother, will eat fry given the chance, so dense planting, floating cover, or separating the young raises the most. Fry grow well on finely powdered foods, newly hatched brine shrimp and grazable algae, and given the species' herbivorous leaning, vegetable matter belongs in their diet early. Brackish or slightly hardened, alkaline water tends to suit molly fry and helps stave off the soft-water disease problems to which the species is prone.
In the aquarium
The sailfin molly is a handsome, active, generally peaceful aquarium fish, but it is not the no-thought beginner livebearer the guppy is — its size, its grazing appetite and its taste for hard, alkaline (often slightly brackish) water all need accommodating. Mature males in particular grow large by livebearer standards and display constantly, so a roomy tank with open swimming space, good light to encourage algae, and dense planting suits a group best. As with most mollies, keeping more females than males spreads out male attention and reduces harassment.
Water chemistry is the crux of keeping this fish well. Sailfin mollies want warm water in the low-to-high 20s Celsius and firmly favour hard, alkaline conditions; many keepers add a little marine salt, and the species is fully capable of life in brackish or even near-marine setups. Soft, acidic water and protein-heavy diets are the usual roads to the molly's characteristic ailments. They are peaceful community fish but should not be mixed with fin-nippers or kept in conditions that ignore their salinity and vegetable-matter needs.
Conservation
Poecilia latipinna is assessed as Least Concern on the IUCN Red List, an assessment dated 3 February 2019. The species is abundant, adaptable and widespread across a broad native range in the south-eastern United States and north-eastern Mexico, tolerates a wide range of temperature, salinity and water quality, and faces no significant species-level threat. As with the guppy, the conservation concern runs the other way.
Because of its hardiness and live-bearing fecundity, the sailfin molly has been moved far beyond its native coast — introduced across the western United States, to Hawaii (first brought from Texas in 1905 for mosquito control, a failure given its herbivory), to Puerto Rico and to numerous other countries, mostly through aquarium releases. In some places it is a recognised problem: it has been implicated in the decline of the endangered desert pupfish (Cyprinodon macularius) in California and of endemic Megalagrion damselflies on Oahu. The responsible position for an aquarist is therefore the usual one for a superbly adaptable livebearer — never release unwanted mollies into local waters, where they can establish all too easily.