Taxonomy & naming
Xiphophorus signum was described in 1969 by Donn Eric Rosen and Klaus D. Kallman, working from material collected in the Chajmaic River basin of Alta Verapaz, Guatemala. The species was originally proposed within the swordtail complex centred on Xiphophorus helleri, and it circulated for a time under the subspecific combination Xiphophorus helleri ssp. signum before being treated as a full species. Eschmeyer's Catalog of Fishes — the authority for valid names — places Xiphophorus signum Rosen & Kallman, 1969 in the genus Xiphophorus within the family Poeciliidae, the New World livebearers, and the order Cyprinodontiformes.
The genus name Xiphophorus is built from the Greek xiphos, "sword," and pherein, "to carry," a reference to the sword-shaped lower extension of the caudal fin that the males of most species in the genus bear. The specific epithet signum is Latin for "sign" or "mark," and points to a distinctive comma-shaped marking at the base of the caudal sword that separates this species from its relatives. Among Spanish-speaking communities in Guatemala the small livebearers of the region are loosely called olominas, and signum shares in that vernacular.
The species belongs to the southern swordtail lineage of Xiphophorus, a group whose internal relationships have been worked out in detail through molecular phylogenetics — Kang and colleagues (2013) used the swordtails as a case study in the evolution and loss of the sword across the genus. Its closest affinities lie with the helleri-group swordtails of Central America rather than with the platyfish lineage of the same genus.
Morphology
Xiphophorus signum is one of the larger swordtails. Males reach about 3 in in body length, to which the caudal sword adds a further three to four centimetres, so that a fully grown male measured tip to tail is a strikingly long fish for the genus. Females are deeper-bodied and lack the sword, reaching a comparable body size. The body is elongate and laterally compressed in the typical swordtail manner, built for life in flowing water.
The feature that gives the species its name is a dark, comma- or sign-shaped mark at the base of the caudal sword, set off against the paler ground colour of the tail. The sword itself — the produced lower rays of the caudal fin — is edged and marked in the manner of the helleri-group swordtails, and in healthy males the body carries lateral striping along the flank. The overall impression is of a robust, clean-lined swordtail rather than a brightly saturated one; wild signum are coloured for a forest stream, not for the show tank.
As in all poeciliids, the species shows clear sexual dimorphism beyond the sword. The male's anal fin is modified into a gonopodium, the slender intromittent organ used to fertilise females internally, while the female retains an ordinary fan-shaped anal fin and, when carrying young, develops the dark gravid patch at the rear of the abdomen characteristic of livebearing toothcarps.
Habitat
Xiphophorus signum is endemic to Guatemala and, within Guatemala, to a single small drainage. It has been collected only from the Salba River, a headwater creek of the Chajmaic River, which is itself a tributary of the Río de la Pasión within the larger Usumacinta drainage, in the department of Alta Verapaz. The IUCN assessment records the species from three localities treated as a single location, with both the extent of occurrence and area of occupancy estimated between roughly 12 and 45 mi² — an exceptionally small wild range.
The habitat is a small creek of fast, clear water running through relatively dense forest cover, with lateral pools along its course. FishBase records the species from fast-flowing, usually densely vegetated waters at around 77 °F, and notes that it occasionally co-occurs with another livebearer, Scolichthys iota. Reported water conditions are warm and hard: temperatures of about 75–82 °F, pH around 7.2–8.0, and hardness in the range of 10–20 dH, consistent with the limestone karst geology of the region.
This is a fish of moving, oxygen-rich forest streams rather than still ponds, and its dependence on clear, fast water with intact riparian forest is central to its conservation profile. The original description was based on a large type series — some 928 paratypes were collected — and the species was sampled for the Xiphophorus Genetic Stock Center in Texas as early as 1963, with further wild collections documented in 2015 and 2018.
Feeding
Detailed dietary studies of wild Xiphophorus signum are sparse, but its biology is that of a typical small swordtail of flowing water, and its trophic level is estimated at about 3.1 — placing it as an omnivore that leans toward small animal prey. In its forest-stream habitat such a fish would graze on aufwuchs — the film of algae, diatoms and associated micro-organisms on submerged surfaces — while taking aquatic invertebrates, insect larvae and drifting items carried on the current.
The dense vegetation of its habitat would supply both plant matter and the invertebrate fauna that lives among it, and a swordtail in fast water is well placed to pick food from the drift as well as from the substrate. As with its relatives, the balance of plant to animal matter is likely opportunistic, shifting with what the stream offers through the seasons.
In captivity the species is undemanding at the table, as swordtails generally are: it accepts good-quality flake and micro-pellets readily, takes small live and frozen foods such as Daphnia and brine shrimp, and benefits from a meaningful share of vegetable or algal matter to reflect its grazing habits in the wild. A varied diet supports both colour and breeding condition.
Mating
Like all poeciliids, Xiphophorus signum practises internal fertilisation. The male's anal fin is modified into a gonopodium — a mobile, rod-like organ formed from elongated and specialised anal-fin rays — through which packets of sperm are transferred to the female's genital opening. There is no external spawning and no egg-laying; fertilisation and the early development of the young both take place inside the female.
Male swordtails court using the sword and body displays, and the long sword of the helleri-group species is thought to function partly in female assessment and partly in male–male signalling. As in other Xiphophorus, males are expected to combine display-based courtship with persistent pursuit of females, and the sword itself is the kind of exaggerated male trait that sexual selection acts upon — the broader swordtail genus is a classic model for the evolution of such ornaments. Specific mate-choice data for signum are limited, so finer details of its courtship are best described conservatively.
Female poeciliids are commonly able to store viable sperm internally for extended periods, allowing a single mating to fertilise more than one successive brood. This trait, well documented across the family, means that an isolated female swordtail may continue to produce young for some time after the last contact with a male.
Breeding
Xiphophorus signum is viviparous — a true livebearer that gives birth to free-swimming young rather than laying eggs. FishBase, citing Wischnath's Atlas of Livebearers, records that a female gives birth roughly every four to five weeks, producing broods of about 20 to 40 young. The fertilised eggs are retained and develop within the female, and gestation is paced largely by temperature, as it is throughout the family.
The newborn fry emerge fully formed and able to swim and feed at once. In the wild the dense streamside vegetation of the species' habitat would provide the cover that newly born livebearers need, since adult poeciliids — including the parents — will readily take small fry given the chance. Whether signum shows the superfetation seen in some other poeciliids (the simultaneous carrying of broods at different developmental stages) is not well documented for this species, so it should not be assumed; the regular four-to-five-week birth interval reported is consistent with discrete, sequential broods.
In captivity the species breeds in the manner of other large swordtails: given warm, clean, well-oxygenated water and dense planting or a separate nursery, broods can be raised in good numbers, with the fry growing readily on finely powdered foods and newly hatched brine shrimp. An ex-situ inbred strain has been maintained for decades at the Xiphophorus Genetic Stock Center, where the species serves genetic research.
In the aquarium
Xiphophorus signum is a large, active swordtail best suited to aquarists who can give it room and current. It is a strong swimmer from fast water, so a long tank with brisk, well-oxygenated flow and a secure, well-fitted lid suits it — swordtails are notorious jumpers, and hobbyist keepers stress a tight cover above all. Hobby discussion suggests it is maintained across roughly 64–82 °F and around neutral to slightly alkaline pH, broadly in line with the wild figures of 75–82 °F and pH 7.2–8.0, with hard water reflecting its limestone-stream origins.
As with other swordtails, males can be pushy toward one another and toward females, so a group skewed toward females reduces harassment, and enough space and visual breaks help diffuse aggression. Dense planting both calms the fish and gives any fry a chance, while the same planting and current mimic the forest-creek conditions the species evolved in. It is not a difficult fish to keep for an aquarist already comfortable with livebearers, but its size and need for flow make it a poorer fit for a small, still community tank.
This is also a species where keeping it carries a faint conservation weight: with such a tiny wild range, captive-maintained stock has genuine value, and aquarists who breed it responsibly help preserve a fish whose natural population could be erased by a single bad year in one creek. Sourcing healthy, well-bred stock and keeping good records of its provenance is worthwhile.
Conservation
Xiphophorus signum is assessed as Vulnerable on the IUCN Red List under criterion D2 (version 3.1), an assessment dated 17 October 2019 and published in 2020 by Elias, Quintana and Lyons. The D2 listing reflects a species with a very small range and a plausible threat capable of driving it to a more threatened category or to extinction in a short time: it is known from a single location with an extent of occurrence and area of occupancy each estimated at only 7–45 mi², and it occurs in no protected area. An earlier 2006 regional report had listed it as Critically Endangered, and it carries Category 1 status on Guatemala's national red list (CONAP 2009).
The principal threats stem from human activity along its one creek. Small-scale agriculture and the rural communities lining the Salba River already affect the catchment, and the assessment flags potential future palm-oil plantation development as a serious risk — bringing erosion and sedimentation that would cloud the clear water the species depends on, surface-water abstraction, and runoff of herbicides and pesticides. Because the fish needs fast, clear, forested streamflow, degradation of the riparian forest strikes directly at its habitat.
A measure of security comes from ex situ stock: an inbred strain has been maintained at the Xiphophorus Genetic Stock Center in San Marcos, Texas, descended from collections begun in the 1960s, and the species was sampled again in the wild in 2015 and 2018. But an inbred laboratory line is no substitute for a healthy wild population, and the conservation of Xiphophorus signum ultimately depends on protecting the forest and water quality of the small Guatemalan drainage that is its only home.