Taxonomy & naming
Xiphophorus hellerii was described by the Austrian ichthyologist Johann Jakob Heckel in 1848, from material collected in Mexico. The species is the type that gives the genus its character: Xiphophorus, the swordtails and platies, is a well-studied New World genus within the family Poeciliidae. The valid spelling of the epithet — hellerii, with the double 'i' as originally published — is retained by Eschmeyer's Catalog of Fishes, which treats Xiphophorus hellerii Heckel, 1848 as the valid name; the simplified spelling "helleri" is a common but technically incorrect variant seen throughout the older and hobby literature.
Within Xiphophorus the swordtails (those whose males bear a caudal sword) and the platies (those that do not) form a single genus rather than two, and the boundary between them is biological rather than tidy: swordtails and platies hybridise readily, both in the laboratory and where ranges meet, and this interfertility is the basis of the genus's celebrated use in cancer and pigment genetics. Xiphophorus hellerii is itself a variable, wide-ranging species, and several distinct regional populations exist across its range, some of which have at times been treated as subspecies or separate forms.
Morphology
The green swordtail is an elongate, fairly large poeciliid: wild females reach roughly 4.5–5.5 in in total length excluding any sword, with males somewhat smaller in body but bearing the eponymous tail extension. The wild-type body is a slender, streamlined green to olive, with a brassy or greenish sheen and a reddish, brownish or darker lateral stripe running along the flank; the back is darker, the belly paler.
The male's defining feature is the sword — a ventral prolongation of the lower caudal-fin rays into a slender, often yellow-to-orange blade edged in black, which in a fine male can approach the length of the rest of the fish. As in all poeciliids, the male also carries a gonopodium, the rod-like modified anal fin used for internal fertilisation. Females are larger-bodied, lack the sword, and have a normal rounded anal fin and tail. Beyond the wild green form, more than a century of selective breeding has produced a wide palette of domestic strains — red, orange, black, "pineapple," wagtail, tuxedo and lyretail swordtails among them — many of them carrying pigment from hybridisation with platies; the genetics behind these patterns are precisely what made the genus a laboratory staple.
Habitat
Xiphophorus hellerii is native to the Atlantic (Gulf and Caribbean) slope of Middle America, ranging from Mexico — including the Río Pánuco and other Veracruz and Gulf-slope drainages — south through Belize and Guatemala to Honduras. It lives in flowing fresh water: the swordtail is a fish of warm streams, rivers, spring runs and their associated pools, and is characteristically found in well-vegetated, moderately to swiftly flowing reaches, including spring-fed headwaters, rather than in stagnant water.
The species favours warm, clear, hard and alkaline water — broadly in the low-to-high 20s Celsius — with a preference for current and dense plant growth, where it forages and shelters. Like the guppy it is tolerant and adaptable, and that adaptability has a downside: the green swordtail has been introduced well beyond its native range through the aquarium trade and is now established as a feral and sometimes invasive fish in various warm regions of the world, from parts of the United States to islands and highland streams elsewhere, where it can compete with native species.
Feeding
The swordtail is an omnivore with a notably herbivorous leaning. In the wild it grazes heavily on algae and the layer of algae-and-micro-organisms (aufwuchs) on submerged surfaces, supplementing this with detritus, small aquatic invertebrates, insect larvae and other small drifting items. Its relatively long gut reflects this plant-rich diet.
In the aquarium it is an easy, eager feeder. A good staple is a quality flake or pellet with a meaningful vegetable component, supplemented by algae, blanched vegetables or spirulina-based foods, and by small live and frozen foods such as Daphnia, brine shrimp and bloodworm for condition and colour. Because of its grazing habit, a swordtail kept in a planted, algae-bearing tank will browse contentedly through the day; as with other small active livebearers, modest, frequent feeding suits it better than infrequent large meals.
Mating
Swordtail reproduction follows the poeciliid pattern of internal fertilisation, but with an added layer of male competition that the sword itself helps mediate. Males are active and often combative, displaying to females and sparring with rival males; dominant males establish a loose hierarchy, and the sword functions in both contexts — as a signal in courtship and as a flag in male-male assessment. Males court by displaying laterally and backing toward the female, and transfer sperm via the gonopodium; females can store sperm and produce successive broods from a single mating.
The sword has been a focus of sexual-selection research because females of swordtail species often prefer males with longer swords, a preference that appears to predate the sword's own evolution in the genus and has become a classic case study in how mate choice can drive the elaboration of male ornaments. A further curiosity of swordtail biology is that some females can, late in life or under certain conditions, develop male-like traits including a partial sword and functional male behaviour — a form of sex change reported in the group that has long intrigued aquarists and biologists alike.
Breeding
The green swordtail is among the easiest aquarium fishes to breed and is often a beginner's first deliberate spawning success. It is viviparous: after internal fertilisation the female carries the developing young and gives birth to free-swimming fry, with gestation lasting roughly four weeks depending on temperature, and a single insemination can produce several broods because the female stores sperm.
Broods are large for the fish's family — a well-grown female may drop from a few dozen up to well over a hundred fry at once. The fry are large, well-developed and able to swim and feed immediately, taking finely powdered foods, infusoria and newly hatched brine shrimp and growing rapidly. As with other livebearers the adults will prey on their own young, so dense planting, floating cover or a separate rearing space is needed to raise good numbers. Males develop the sword and gonopodium as they mature; in domestic strains, the timing and length of sword development is part of what breeders select for.
In the aquarium
The swordtail is a hardy, active, mostly peaceful community fish, but it carries two husbandry demands beyond those of the smaller livebearers. First, it is a strong, restless swimmer that grows reasonably large and produces current-loving fish, so it wants a longer tank with open swimming space and some water movement rather than a small, still aquarium. Second, males can be territorial and pushy toward one another and toward females; keeping a single male, or several males in a group large enough to spread the aggression, together with a majority of females, gives the calmest result.
Water should reflect the species' hard-water, flowing-stream origins: neutral to alkaline, medium-hard to hard water suits it best, in temperatures through the 20s Celsius, with good filtration and oxygenation. Swordtails are accomplished jumpers and a covered tank is essential. They mix well with other robust, similarly active community fishes that share their water preferences — other livebearers, peaceful barbs, larger tetras and rasboras — but pair poorly with very timid or long-finned tankmates that may be outcompeted or harassed. Given space, current and a planted tank, the green swordtail is an undemanding and rewarding fish, and the wild-type green form in particular is a handsome, underrated alternative to the gaudier domestic strains.
Conservation
Xiphophorus hellerii is assessed as Least Concern on the IUCN Red List. The wild species remains widespread and generally common across its native Atlantic-slope range from Mexico to Honduras, occupies a broad range of stream and river habitats, and is not considered globally threatened, though localised populations can be affected by habitat alteration, pollution and the introduction of non-native fishes within parts of its range.
As with the guppy, the more pressing conservation issue attached to the swordtail is its role as an introduced species elsewhere. Its hardiness, livebearing reproduction and tolerance of varied conditions have allowed it to establish feral populations well outside its native range, where it can compete with and pressure native fishes, and it is regarded as invasive in a number of warm regions. The keeper's responsibility is therefore straightforward: enjoy the fish, but never release surplus swordtails or their fry into local waterways, where this adaptable livebearer can take hold readily.