Taxonomy & naming
Xiphophorus multilineatus was described in 1990 by Mary Rauchenberger, Klaus Kallman and Donald Morizot, who separated it from the closely allied X. nigrensis on the basis of pigmentation, allozyme and morphological evidence. The two are members of the "northern swordtail" clade of the genus Xiphophorus — a group of small, sword-tailed livebearers from the Atlantic drainages of north-eastern Mexico that has become one of the most intensively studied species assemblages in poeciliid biology. Eschmeyer's Catalog of Fishes treats Xiphophorus multilineatus Rauchenberger, Kallman & Morizot, 1990 as the valid name, placed in the genus Xiphophorus within the family Poeciliidae, the New World livebearers.
The genus name Xiphophorus refers to the male's caudal sword — the lower rays of the tail fin drawn out into a coloured, sword-like extension — while the epithet multilineatus ("many-lined") describes the series of dark horizontal stripes along the flanks of the males. The swordtails sit within the same family as the platies, guppies, mollies and mosquitofishes, and the genus as a whole has a long history in both the aquarium hobby and the genetics laboratory, where Xiphophorus hybrids have been used in cancer and pigment-pattern research for the better part of a century. X. multilineatus itself, however, is a wild-type fish of scientific rather than ornamental fame, and is rarely encountered in general aquarium commerce.
Morphology
Xiphophorus multilineatus is a small, slender swordtail. Males reach roughly 2 in and females about 2 in in total length, the females being the larger and plainer sex. The common name striped swordtail comes from the several dark longitudinal lines that run along the male's body — the "many lines" of the epithet — overlying a pale to olive ground colour, with the lower caudal rays produced into the genus's characteristic sword, edged in yellow and black.
What sets the species apart morphologically is that its males are not all the same. Body size at maturity is polymorphic and heritable, so a population contains males spanning a wide size range, from small individuals that mature early to large individuals that mature late. The larger males carry longer, more developed swords and are the ones that perform courtship; the smaller males are comparatively drab and swordless or short-sworded. This size variation is tied to a Y-linked locus controlling the onset of maturation, and it is expressed as a continuous-to-discrete spread of adult male phenotypes rather than the single male form typical of most livebearers.
As in all poeciliids, the males bear a gonopodium — an intromittent organ formed from modified rays of the anal fin — used to deliver sperm internally to the female. Females lack the sword and the gonopodium, are deeper-bodied, and develop a darkened gravid region at the rear of the abdomen when carrying young.
Habitat
The species is a narrow endemic of eastern Mexico, recorded from the Río Coy, a tributary within the Río Pánuco basin in San Luis Potosí. This is a clear, warm, fast-flowing river habitat, and the fish are found preponderantly among stands of aquatic plants or beneath overhanging bank vegetation, where the current is broken and cover is available. FishBase characterises the species as freshwater, benthopelagic and tropical, living in the warm waters typical of the region.
Reported temperatures for the species sit in a narrow tropical band of roughly 73–77 °F. The restriction to a single river system within the larger Pánuco drainage is a defining feature of the species' biology — it is a localised fish rather than a widespread one, and this narrow range is part of why its conservation status has not been confidently resolved. Detailed published figures for pH, hardness and depth specific to X. multilineatus are sparse; the fish should be understood as an inhabitant of clean, oxygenated, flowing tropical fresh water rather than of still pools.
Feeding
Like its swordtail relatives, Xiphophorus multilineatus is a small omnivore, taking a mixed diet of algae, organic detritus, small aquatic invertebrates and insect larvae from among the plants and stream margins it favours. Its trophic level is estimated at around 3.1, consistent with a fish that combines plant and animal matter rather than specialising narrowly on either. Specific dietary studies for this species are limited, and the general swordtail pattern is the best guide.
In an aquarium setting the species would be expected to behave as other northern swordtails do: accepting good-quality flake and small pellets readily, but doing best with a varied diet that includes a share of vegetable or algal matter alongside small live and frozen foods such as Daphnia and brine shrimp. As a constant grazer in flowing water, it suits several small feedings rather than infrequent large ones.
Mating
Mating in Xiphophorus multilineatus is internal, and the species is one of the best-known natural examples of alternative reproductive tactics in fishes. The population contains two behaviourally distinct kinds of male tied to body size. Larger "courter" males, which mature later and carry the developed sword, attract females with active courtship displays; smaller "sneaker" males, which mature earlier, largely forgo courtship and instead attempt sneak or coercive copulations that bypass female choice. The tactic a male uses is closely associated with his genetically influenced adult size, and the behaviour of the large courting males is essentially fixed rather than facultative.
This size-and-tactic polymorphism is governed in large part by a Y-linked "P" (pituitary) locus that controls when a male matures, making body size at maturity heritable. Sneaker males trade lower success per mating attempt for the advantage of maturing early and reproducing sooner; courter males invest in size, sword and display to win matings through female preference. The coexistence of these strategies within one population — and the question of how selection keeps both in play rather than letting one drive the other extinct — is exactly what has made X. multilineatus, together with X. nigrensis, a classic model system for the evolution and maintenance of alternative reproductive tactics and male size polymorphism in poeciliids. As in other swordtails, the male transfers sperm via the gonopodium, and females can store sperm internally to fuel successive broods.
Breeding
Xiphophorus multilineatus is viviparous: females are internally fertilised and give birth to free-swimming young rather than laying eggs. According to FishBase, gestation lasts about 24–28 days, after which a female produces a brood of roughly 10–15 young, occasionally up to about 20. As with other poeciliids, stored sperm allows a single mating to fuel more than one brood, so a female may continue producing young over time without remating.
The newborns are fully formed miniature fish, able to swim and feed from birth, and as in livebearers generally they need cover from the adults, which will eat fry given the chance. The heritable male size polymorphism means that, as broods mature, males sort into the early-maturing small phenotype and the later-maturing large phenotype — a developmental fork that is itself part of what makes the species scientifically valuable. Superfetation, the carrying of overlapping broods at different developmental stages, is documented in some poeciliids but is not a established feature of Xiphophorus, and there is no reliable indication that X. multilineatus practises it; the species is best described simply as a standard internally bearing swordtail.
In the aquarium
Xiphophorus multilineatus is uncommon in the general hobby and is kept mainly by livebearer specialists and researchers rather than offered as a community-tank ornamental. Its needs follow from its habitat: clean, well-oxygenated, gently flowing fresh water in the warm tropical range of about 73–77 °F, with planted cover that reproduces the plant stands and overhanging vegetation it occupies in the Río Coy. Stability and cleanliness matter more than chasing precise chemistry values, which are not well documented for this species.
As a small, active swordtail it is best kept in groups, and — because males pursue females persistently and some males use coercive mating tactics — a sex ratio skewed toward females helps spread out that attention. It is a peaceful fish suited to keepers who value the species for its biology and want to maintain a genuine wild-type strain rather than a fancy ornamental. The most important husbandry point for a rare, narrow-range species like this is the integrity of the maintained line: keeping it unhybridised with other Xiphophorus and breeding from healthy stock preserves both the fish and the male-polymorphism that makes it interesting.
Conservation
Xiphophorus multilineatus is assessed as Data Deficient (DD) on the IUCN Red List, with the assessment dated 28 March 2018. The Data Deficient listing reflects genuine uncertainty rather than confirmed safety: as a narrow-range endemic known from a single river system within the Río Pánuco basin, the species has not been evaluated with enough information on population size, trend and threats to place it confidently in a threat category. It is not listed under CITES or CMS.
A fish restricted to one river is, by its nature, vulnerable to local pressures — water extraction, pollution, habitat alteration and the introduction of non-native species — even where no formal threat assessment has been completed, and the Data Deficient status should be read as a flag for further survey work rather than as reassurance. For aquarists, the species' scientific importance as a model for the evolution of alternative mating strategies adds a particular reason to maintain it carefully: well-kept, accurately identified, unhybridised captive populations preserve a fish whose wild status remains formally unresolved.