Taxonomy & naming
Poecilia velifera was described by the British ichthyologist Charles Tate Regan in 1914, and the species belongs to the family Poeciliidae — the New World livebearers — within the order Cyprinodontiformes. Eschmeyer's Catalog of Fishes treats the name as valid, placing it in the genus Poecilia alongside the guppy and the other mollies. In the older and hobby literature the sailfin mollies are sometimes sorted into the subgenus Mollienesia, a grouping for the algae-grazing, sail-finned Poecilia of Central America and the Gulf coast, but the species sits squarely in Poecilia as currently circumscribed.
The genus Poecilia is large and has been repeatedly revised; phylogenetic work on the tribe Poeciliini (for example Rodríguez, 1997, in Copeia) has clarified relationships among the mollies and their relatives. P. velifera is one of three large sailfin mollies, the others being the Atlantic sailfin molly P. latipinna of the southern United States and Gulf coast and the smaller P. petenensis of the Petén region. The three are similar in body plan and freely hybridise in captivity, which is one reason commercial "sailfin" mollies in the trade are often of mixed ancestry rather than pure P. velifera.
The common names follow the fish directly: "sailfin molly" for the towering dorsal of the male, and "Yucatan molly" for its native peninsula. The species epithet velifera is itself descriptive, from the Latin for "sail-bearing," and is the most diagnostic single feature of the adult male.
Morphology
The Yucatan sailfin molly is the largest of the sailfin mollies, reaching about 6 in in total length, with females growing larger and deeper-bodied than males. The body is stout and laterally compressed, silvery to olive or blue-grey, often overlaid with rows of small iridescent spots that catch the light along the flanks; domestic strains add gold, black (melanistic "black molly") and marbled forms far beyond the wild colouration.
Sexual dimorphism is strong and centres on the dorsal fin. The adult male carries a greatly enlarged, sail-like dorsal of numerous soft rays — characteristically around eighteen in P. velifera, more than in the other sailfins — that he can erect into a tall, spotted banner during display. Females have a normal, modest dorsal and a fuller abdomen, with a dark gravid patch visible when carrying young. As in all poeciliids, the male's anal fin is modified into a gonopodium, a slender intromittent organ built from elongated fin rays and used to transfer sperm internally.
Beyond the fins, the species shows the broad head and upturned, fine-toothed mouth of a surface- and substrate-grazing molly, adapted to rasping algae and aufwuchs from hard surfaces. The robust build and large size make a mature male sailfin in full display one of the more imposing of the small livebearers.
Habitat
Poecilia velifera is endemic to the Yucatan Peninsula of Mexico, where it occupies the coastal fresh and brackish waters of the Yucatán and Quintana Roo region and the lower Usumacinta and Laguna de Términos systems. It is a benthopelagic fish of lagoons, coastal marshes, estuaries, cenotes and slow lowland waters — habitats shaped by the peninsula's limestone geology, which makes the water characteristically hard and alkaline.
The species is notably euryhaline, living comfortably across a gradient from fresh through brackish water and tolerating outright marine salinities at the seaward end of its range. FishBase characterises its waters as tropical, roughly 77–82 °F, with a high pH around 8.0 and hardness on the order of 13–19 dH — firmly hard and alkaline. This salinity tolerance is part of why the fish has been moved beyond its native range; peer-reviewed records (for example Elbaraasi et al., 2022, in BioInvasions Records) document P. velifera as an introduced, non-native fish outside the Yucatan.
The restricted, coastal nature of the native range is central to the species' conservation profile: a fish tied to a narrow band of hard-water lagoon and estuarine habitat on a single peninsula is more exposed to local habitat change than a wide-ranging generalist, even one as hardy in captivity as this.
Feeding
Like the other sailfin mollies, P. velifera is fundamentally an omnivore with a strong herbivorous bent. In the wild it grazes algae and aufwuchs — the film of diatoms, micro-algae and associated microfauna on submerged surfaces — and also takes worms, small crustaceans, insects and other invertebrate prey, giving it a trophic level around 3.0 in FishBase's reckoning. The fine-toothed, upturned mouth is well suited to rasping growth from rocks, wood and plants.
In the aquarium the species is undemanding but does best on a diet that respects its grazing nature: a good-quality flake or pellet with substantial vegetable or algal content (spirulina-based foods suit it well), supplemented with blanched vegetables and grazing on natural algae, plus periodic small live or frozen foods such as Daphnia and brine shrimp for protein and condition. A diet too heavy in animal protein and too poor in plant matter is a common cause of digestive trouble in mollies.
Because sailfin mollies are large, active and more or less continuous grazers, several modest feedings suit them better than one heavy meal, and a planted, algae-tolerant tank provides useful supplemental grazing throughout the day.
Mating
Reproduction in P. velifera is internal, as in all poeciliids. The male fertilises the female by transferring sperm through the gonopodium, the intromittent organ formed from the modified rays of the anal fin; there is no external spawning and no egg deposition. Courtship is driven by the male's most conspicuous asset — he positions himself before a female and raises the great sail-like dorsal fin into a vertical, spotted display, the structure that makes the sailfin mollies a textbook case of a sexually selected ornament.
Males court more or less continuously and, as in related livebearers, may alternate showy display with quicker, less conspicuous mating attempts. Females can store viable sperm internally for an extended period after mating, so a single insemination can fuel several successive broods without further contact with a male. Sexual dimorphism is therefore not only in size and the dorsal fin but in role: the male invests in display and pursuit, the female in carrying and producing the young.
The large dorsal sail is metabolically and hydrodynamically costly to carry, and its size and spotting are exactly the sort of honest signal that female choice tends to favour — the same dynamic, scaled up, that makes the sailfin display one of the most striking courtship structures among the small livebearers.
Breeding
Poecilia velifera is viviparous (more precisely lecithotrophic — the developing young are nourished largely by the egg yolk while retained inside the female), and gives birth to free-swimming, fully formed fry rather than laying eggs. Gestation in sailfin mollies typically runs on the order of three to six weeks, around four weeks at warm temperatures, after which the female releases a brood of well-developed young. Brood size scales with the size and condition of the female, and because P. velifera is a large molly the broods of a mature female can be substantial — commonly tens of fry, and larger still in big, well-fed females. There is no evidence of superfetation (the simultaneous carrying of broods at different stages) in this species; that trait is found in other poeciliids such as Heterandria and Poeciliopsis rather than in the sailfin Poecilia.
The newborn fry are comparatively large and capable, able to swim and feed immediately, and they take finely powdered foods, infusoria and newly hatched brine shrimp. As with most livebearers, the adults — including the mother — will eat fry given the chance, so dense planting, floating cover or separation is needed to raise good numbers.
Breeding the species true is more demanding than breeding guppies, partly because of the male's slow development of the full sailfin and partly because hard, alkaline, ideally slightly brackish water and a vegetable-rich diet are needed to bring fish into good condition and produce strong fry. Soft, acidic water tends to yield poor results and unhealthy adults.
In the aquarium
The Yucatan sailfin molly is a long-established aquarium fish, sold commercially and the wild source of many of the hobby's big sailfin and fancy molly strains. It is a peaceful, active, mostly midwater-and-surface community fish, but it is a large livebearer: a mature male approaching 6 in with a full sail needs real space, and FishBase suggests a minimum tank length around 23.5 in as a floor, with more being far better for a group and for the male to display.
Water chemistry is the decisive husbandry point. This is a hard-water, alkaline fish that does poorly in soft, acidic conditions; aim for clearly hard water with a stable pH around 7.5–8.5 and measurable carbonate hardness. Many keepers find the species more reliable with a little added salt, reflecting its naturally brackish habitat, though it can be maintained in pure freshwater if the water is genuinely hard and alkaline. Warm temperatures, roughly 77–82 °F, suit it. As with other mollies, a vegetable-rich diet and clean, well-oxygenated water reduce the digestive and bacterial problems to which soft-water-kept mollies are prone.
Temperament is generally peaceful, making it suitable for a spacious hard-water community, but its size, salinity preference and prolific breeding all need planning. Keeping several females per male reduces harassment of any one fish, and as with all livebearers, population control becomes the standing management task.
Conservation
Poecilia velifera is assessed as Vulnerable on the IUCN Red List (assessment ID 191755), under criteria B2ab(ii,iii,iv,v). The listing reflects the species' restricted distribution on the Yucatan Peninsula and concerns over decline in its area of occupancy, the quality of its habitat, and the number of locations and subpopulations — the classic profile of a range-restricted fish exposed to local habitat change rather than a widespread generalist.
The pressures on its coastal, hard-water lagoon and estuarine habitat — development, water-quality decline and habitat alteration along the Yucatan coast — fall directly on a species that cannot simply retreat to a larger range. This is a useful counterpoint to the fish's abundance in the hobby: the commercially traded sailfin molly is genetically a mix of farmed and hybrid stock, and its ubiquity in shops says nothing about the security of the wild Yucatan populations.
There is also an invasive dimension. Like other hardy, salt-tolerant livebearers, P. velifera has been introduced outside its native range and recorded as a non-native fish elsewhere, where it can establish feral populations. The responsible position for aquarists is twofold: support the conservation of the wild Yucatan habitat that the species depends on, and never release sailfin mollies into local waters, where this adaptable, brackish-tolerant fish can take hold all too readily.