Taxonomy & naming
The species was described by Cloudsley Rutter in 1896 as Characodon eiseni, in his "Notes on the Fresh Water Fishes of the Pacific Slope of North America" (Proceedings of the California Academy of Sciences). The description rested on four specimens — the largest only 1.5 in total length — collected by Gustav Eisen and F. Vaslit in November 1894 from a branch of the Río Grande de Santiago at Tepic, Nayarit, the type locality. The species epithet honours Eisen, a Swedish-born American polymath who collected the type series. The genus Xenotoca was later erected by Hubbs and Turner (in Turner, 1937) around Characodon variatus, and eiseni was moved into it, giving the combination Xenotoca eiseni (Rutter, 1896) — the authority parenthesised because the species was first placed in a different genus.
The genus name comes from the Greek xenos, "strange," and tokos, "birth" — an apt label for a family whose mode of reproduction sets it apart from other livebearers. Eschmeyer's Catalog of Fishes governs the valid name; the species sits in the family Goodeidae, subfamily Goodeinae, the live-bearing splitfins endemic to the Mexican Plateau and Pacific slope.
The taxonomy of this group is unsettled. In 2016 Domínguez-Domínguez and colleagues split what had long passed as a single wide-ranging "Xenotoca eiseni" into several species, describing X. doadrioi and X. lyonsi as eiseni's closest relatives. Some workers, including the Goodeid Working Group, regard true eiseni as not actually belonging in Xenotoca at all — placing the name in quotation marks pending a formal generic revision. For the aquarist the practical consequence is real: much of the fish sold for decades as "Xenotoca eiseni" may belong to one of the segregate species, so locality data matters.
Morphology
Xenotoca eiseni is a small, deep-bodied and somewhat chunky goodeid. Published maximum size is about 3 in standard length (Miller et al., 2005); FishBase lists roughly 2.5 in total length for males and up to about 3 in for females, the females running the larger of the two sexes. The body is laterally compressed with a rounded head and a small, terminal mouth set with bifid (two-pointed) outer teeth and conical inner teeth — the dentition of an omnivore that grazes plant matter as well as taking animal prey.
The sexes are strongly dimorphic and the males are the show. A mature male develops a swollen, brilliant red-orange caudal peduncle and caudal fin — the "redtail" of the common name — set against a body that turns iridescent blue-black to turquoise over its rear half, with an enlarged dorsal fin. Females and juveniles are far plainer: brownish to brownish-grey, marked with a dark longitudinal midline and, in gravid females, a prominent dark abdominal spot.
The family's diagnostic male feature is the andropodium — not the true gonopodium of poeciliids but a different modification. In goodeids the first six or seven anal-fin rays are shortened and slightly separated from the rest of the fin, producing the notched, divided look that gives the "splitfin" its English name. This crowded front section of the anal fin is the structure used to transfer sperm during internal fertilisation.
Habitat
The redtail splitfin is endemic to a small part of Nayarit on Mexico's Pacific slope, historically the springs, spring-fed creeks and pools of the upper Río Grande de Santiago drainage near Tepic — including the Río Mololoa and the Laguna de Tepetiltic, and the headwaters of the Río Santa Rosa — together with the Arroyo Compostela and Laguna de San Pedro in the separate Río Huicicila drainage, about 16 mi south of Tepic. Two evolutionarily significant units are recognised, one in the Santiago basin near Tepic and one in the Compostela/Huicicila system. These waters lie only some 19–25 mi from the Pacific.
Within that range the fish occupies a variety of small waters: springs, spring-fed pools, lakes, creeks and even turbid, muddy rivers. It tolerates conditions from clear to silty and turbid, over substrates of mud, silt, sand, rubble and rock, in current ranging from still to moderate. Aquatic vegetation may be sparse or absent — recorded plants include green algae, water hyacinth (Eichhornia), duckweed (Lemna), Potamogeton, Typha, Nasturtium and Ceratophyllum — and the fish favours shallow water, generally less than a metre deep.
FishBase characterises the species as temperate and demersal, citing a broad temperature span of about 59–90 °F, a pH range near 6.0 to 8.0, and hardness around 9 to 19 dH. Several of the surviving springs are cool and well-oxygenated, and the natural seasonal cooling of these waters appears to structure the fish's breeding year.
Feeding
In the wild Xenotoca eiseni is an omnivore that takes the greater part of its diet as plant material (Fitzsimons, 1972). The bifid outer teeth are suited to scraping and cropping algae and soft vegetation, while the gut — roughly equal to or a little longer than the body length — reflects a diet with a substantial herbivorous component. Alongside plant matter the fish eats worms, small crustaceans, insect larvae, aquatic insects and detritus, making it a flexible feeder across the springs, creeks and turbid rivers it inhabits.
In the aquarium the species is undemanding and accepts a wide range of foods, but a vegetable-rich diet keeps it in best health and colour. Good-quality flake and pellets with algal or spirulina content, blanched greens, and the usual small live and frozen foods — Daphnia, brine shrimp, bloodworm — all suit it. As with many vegetable-leaning fishes, a diet too heavy in rich animal protein is better avoided; offering plant matter regularly mirrors the natural balance and supports the long-lived, slow-breeding habit of a goodeid.
Mating
Like all goodeids, Xenotoca eiseni reproduces by internal fertilisation, but the mechanism differs from that of the more familiar poeciliid livebearers. Rather than the elongate, mobile gonopodium of a guppy or molly, the male uses the andropodium — the crowded, shortened front rays of the anal fin — to transfer sperm to the female. Courtship in goodeids is comparatively elaborate, the bright-tailed male displaying his red-orange caudal region and iridescent flanks to gravid and receptive females.
Unlike poeciliids, female goodeids do not store sperm for months to fuel a long string of broods from a single mating; each brood generally follows its own fertilisation, so a continued presence of males is needed for sustained reproduction. Sexual dimorphism makes pairing easy to manage in captivity — the red-tailed, blue-bodied male is unmistakable against the plain, dark-lined female with her gravid spot. Data on detailed wild mating behaviour are sparse, but the strong male ornamentation points to active female choice as in other goodeids.
Breeding
The redtail splitfin is viviparous in the full sense unique to the Goodeidae: the developing embryos are nourished inside the female not by a stored yolk but through trophotaeniae, ribbon- or rosette-like external structures that trail from the embryo's vent and absorb nutrients from the mother's ovarian tissue — a placental analogue. The young are therefore born as large, well-developed free-swimming fish, often bearing the remnant of the trophotaeniae for a short time after birth. Broods are modest compared with poeciliids, typically a few to a few dozen fry rather than the "millions-fish" output of a guppy, and gestation runs on the order of weeks.
In the wild young are produced at least in March and April and probably over a longer season. In captivity, at warm temperatures of about 75–81 °F, broods can appear year-round, but breeders find that imposing a winter rest — cooling the tank below 68 °F for two to three months — halts fry production and appears to mimic the natural seasonal cycle, improving the long-term health and fertility of the stock. FishBase notes females can produce young roughly every 60 days.
Because the fry are large and fast, they are easier to rear than poeciliid fry and are not as ruthlessly predated by the adults, though cover and good water quality still help. The combination of large, advanced young, modest brood size and a true matrotrophic placenta makes the species a favourite of goodeid specialists and a frequent subject of reproductive-biology study.
In the aquarium
Xenotoca eiseni is hardy and adaptable, but it is not a small, gentle community fish in the guppy mould — it is an active, robust goodeid that needs room and clean, oxygen-rich water. The Goodeid Working Group recommends a large, low-and-wide aquarium of at least 65 US gal, with moderate current and high dissolved oxygen (8 mg/l or better), maintained by large weekly water changes of 60 to 80 percent. The fish does best kept below 77 °F for much of the year, with a cooler winter rest.
Temperament is a genuine consideration: the species can be a fin-nipper, so long-finned tankmates are a poor match, and a roomy tank with a group of its own kind diffuses aggression better than a cramped one. Hard, neutral-to-alkaline water suits it, in keeping with its spring-fed origins, and its tolerance of a wide temperature and pH span makes it forgiving of ordinary tap conditions provided the water stays clean and well-aerated.
The redtail splitfin was among the first goodeids to reach hobbyists outside Mexico, imported to Europe in the early-to-mid 1970s — the American Livebearer Association records it from 1974 — and it remains the goodeid most aquarists meet first. Keeping it well, with locality data preserved where possible, is now part of a deliberate conservation effort, because captive lines are a hedge against the species' decline in the wild.
Conservation
Xenotoca eiseni is assessed as Endangered on the IUCN Red List (species 191717), under criteria B1ab(i,ii,iii,iv)+2ab(i,ii,iii,iv), with the assessment dated 14 March 2018. The listing reflects a very small range fragmented into a handful of localities, with continuing decline in extent, area, habitat quality and number of subpopulations. In Mexico the species is also listed under NOM-059-SEMARNAT-2010 as P (en peligro de extinción).
The threats are the familiar pressures on Mexican Plateau goodeids: extraction and diversion of spring and creek water for human use, outright destruction and pollution of habitat, and the introduction of non-native fishes that compete with and prey on the native fauna. Lyons and colleagues (2019), reviewing the Mexican Goodeidae, classed eiseni as endangered and declining, with both evolutionarily significant units endangered and many historical populations already lost. The species is now thought to survive in the wild at perhaps only a spring at Seis de Enero in the Río Santa Rosa drainage and at the Arroyo Compostela.
This is the rare case where the aquarium hobby is part of the conservation answer. Coordinated captive maintenance of locality-documented stock — the work of groups such as the Goodeid Working Group — preserves the genetic lineages of a fish whose wild habitat may not survive. For the aquarist, that reframes the redtail splitfin from a pretty livebearer into a small ark: keeping pure, well-recorded populations, and never releasing fish into local waters, is the most useful thing a hobbyist can do for it.