Taxonomy & naming
Xenotoca doadrioi was described in 2016 by Omar Domínguez-Domínguez, Dulce Bernal-Zuñiga and Kyle Piller in Zootaxa (4189(1):81–98), splitting off populations from the endorheic region of Etzatlán in Jalisco that had previously been lumped with the redtail splitfin, Xenotoca eiseni. The holotype, an adult male of 1.5 in standard length catalogued as CPUM-9589, was collected on 17 June 2010 together with thirty paratypes from a spring-fed pond at Hacienda San Sebastián, north of Etzatlán. The species belongs to the family Goodeidae — the goodeids or splitfins — and within it to the subfamily Goodeinae, the live-bearing splitfins endemic to the Mesa Central of Mexico. Eschmeyer's Catalog of Fishes treats Xenotoca doadrioi Domínguez-Domínguez, Bernal-Zuñiga & Piller, 2016 as the valid name.
The genus name combines the Greek xenos, "strange," with tokos, "birth" or "offspring," a reference to the unusual reproductive biology of these matrotrophic livebearers. The specific epithet doadrioi honours Dr. Ignacio Doadrio of the Museo Nacional de Ciencias Naturales in Madrid, for his contributions to the study of Mesoamerican freshwater fish diversity. The English common name is San Marcos Redtail Splitfin and the Mexican name Mexclapique de San Marcos.
The generic placement is provisional. Molecular and morphological work indicates that the cluster of species around Xenotoca eiseni — including X. doadrioi — does not sit neatly within Xenotoca as currently defined, and several authors write the name as 'Xenotoca' to flag that the species likely belongs to an as-yet-undescribed genus. Pending a formal revision the species is kept in Xenotoca, and that is the name carried by the Catalog of Fishes.
Morphology
This is a small, deep-bodied splitfin. FishBase records a maximum of about 1.5 in standard length for males and unsexed fish and 2 in SL for females, and the Goodeid Working Group gives a maximum known size of 2 in SL — so females run distinctly larger than males, as is usual in the goodeids. Meristics from the original description include 12–14 dorsal soft rays, 13–15 anal soft rays, 12 pectoral rays, and about 32 scales in the lateral series.
The species is strongly sexually dimorphic in colour. Males are described as the most colourful of all the 'Xenotoca' — an orange-to-red caudal peduncle (the "redtail" of the common name) set against iridescent blue and green flanks, a golden saddle-like band, a yellow-to-orange belly, and an enlarged dorsal fin. Females are far plainer: brownish-grey, with a broken row of midlateral blotches, clear unmarked fins, and a dark gravid spot when carrying young.
The diagnostic feature shared with other male goodeids is the andropodium — the "split fin" that gives the family its English name. The first roughly seven anal-fin rays are shortened and set off from the rest of the fin by a distinct notch, forming a modified structure used in internal fertilisation. Unlike the rod-shaped gonopodium of the poeciliid livebearers, the goodeid andropodium is a notched, only partly separated front section of the anal fin.
Habitat
Xenotoca doadrioi is endemic to the endorheic — internally draining — region of Etzatlán in the state of Jalisco, western Mexico, within the Laguna de Magdalena basin. The type locality is a permanent, spring-fed pond of turbid, shallow water no more than about 5 ft deep, with a bottom of mud and gravel and no rooted aquatic plants. The species also turns up in springs, reservoirs and seasonally affected streams across this small drainage.
Following the distributional survey of Lyons and colleagues (2019), the known range comprises a scatter of separate waters: the springs of Almoloya and San Sebastián, dams within the endorheic Laguna de Magdalena basin, the headwaters of the Río Chiquito near San Marcos (Arroyo Tezontete), and the Presa Limoncito (the Oconahua dam, an affluent of the Río Ameca); it probably also occurs in the Río Salado, another Río Ameca affluent roughly 9 mi west of Guadalajara. These add up to only about three inferred subpopulations spread across hydrologically separate drainages.
The species shares its waters with a characteristic Jaliscan fish assemblage. Recorded sympatric natives include Xenotoca melanosoma, Goodea atripinnis and Poeciliopsis infans, alongside several introduced fishes — Xiphophorus variatus, an Oreochromis tilapia, bluegill (Lepomis macrochirus) and common carp (Cyprinus carpio) — whose presence is itself part of the species' conservation problem.
Feeding
Direct dietary study of Xenotoca doadrioi is sparse, but the available signals point to an omnivore that leans toward plant and detrital material. FishBase places the species at a low trophic level of about 2.1, consistent with a diet weighted toward algae and detritus rather than animal prey.
By analogy with its close relative Xenotoca eiseni, the species is taken to be omnivorous with plant matter forming the greater volume of the diet — grazing on algae and aufwuchs (the film of algae, micro-organisms and detritus on submerged surfaces) and supplementing this with small crustaceans and insect larvae. The turbid, plant-free type-locality pond suggests much of its foraging is over mud and gravel and on the biofilm of hard surfaces rather than among vegetation.
In the aquarium the species accepts a wide range of prepared and frozen foods, but its natural bias toward herbivory matters: a diet too rich in protein and fat is poorly suited to a fish built to process algae and aufwuchs. Keepers therefore favour a varied diet with a substantial vegetable and algal component — spirulina-based flakes and pellets, blanched greens, and grazing on natural algal growth — rounded out with small live and frozen invertebrates for condition.
Mating
Like all goodeids, Xenotoca doadrioi reproduces by internal fertilisation and gives birth to live young. The male uses his andropodium — the notched, partly separated front portion of the anal fin — to transfer sperm to the female. This differs from the poeciliid livebearers (guppies, mollies, platies), whose males possess a true rod-like gonopodium and whose females can store sperm for many months; goodeids lack that long-term sperm storage, so each brood generally follows its own mating.
Mating in the genus is bound up with the males' conspicuous colour. In Xenotoca doadrioi the males are exceptionally bright — orange-red tails over iridescent blue-green bodies — and this colour functions in courtship and competition, with females the plainer, choosing sex. Reproduction is seasonal: in this species the breeding period is reported to resemble that of Xenotoca eiseni, with fry appearing in late spring after a winter cool-down.
Goodeids do not show superfetation — the carrying of two or more broods at different stages of development simultaneously — which does occur in some poeciliid livebearers. A female Xenotoca doadrioi therefore carries a single cohort of embryos at a time, all developing and being born together rather than in overlapping waves.
Breeding
The defining feature of goodeid reproduction is matrotrophy: the embryos are not provisioned by a yolk but are nourished directly by the mother throughout development, by way of trophotaeniae — ribbon- or rosette-like structures that trail from the vent of the developing embryo and serve as a placenta-like organ, absorbing nutrients from the female's ovarian fluid. This makes splitfins like Xenotoca doadrioi genuinely placental livebearers, and the young are born comparatively large and fully formed; the smallest field-collected specimens measure around 1.5 in total length.
Breeding is seasonal and temperature-cued. In keeping with the wild pattern, captive breeders report that a winter cooling below about 68 °F for two to three months conditions the fish; spawning resumes as temperatures rise to roughly 68–70 °F and shuts down again above about 77 °F, with fry produced into late spring. Brood sizes are small relative to the prolific poeciliids — a feature of the energetically costly matrotrophic strategy — though precise counts for this newly described species are data sparse.
The newborn fry are large, well developed and able to feed at once, sometimes still bearing the remnant of the trophotaeniae for a short time after birth. Their size means they take small prepared foods, finely crushed flake and newly hatched brine shrimp from the outset, and they are large enough that predation pressure within a well-planted tank is lower than for tiny poeciliid fry. As with most livebearers, providing cover and ample food reduces losses to adults.
In the aquarium
Xenotoca doadrioi is a fish for the dedicated keeper and the conservation-minded hobbyist rather than the casual beginner, and much of its captive population is maintained through coordinated efforts such as the Goodeid Working Group. The recommended setup is a roomy, low and broad aquarium of at least about 40 US gal with moderate current, reflecting the active, open-water habits of the genus. Splitfins are demanding of water quality: keepers aim for high oxygen levels — on the order of 8 mg/l or more — and large weekly water changes of 60–80 % to keep nitrogen compounds low.
Temperature management is central to keeping the species healthy and in breeding condition. Unlike tropical livebearers, this is a subtropical highland fish that benefits from a seasonal cool period: a winter drop below 68 °F for two to three months, with the working range around 68–70 °F for spawning and an upper limit near 77 °F, above which reproduction ceases and the fish are stressed. The species does well in outdoor tubs from spring through autumn wherever daytime water temperatures stay above roughly 63 °F, which also encourages strong colour and natural grazing on algae.
The males' boldness has a behavioural cost: like many goodeids they can be fin-nippers and somewhat boisterous, so tankmates and stocking should be chosen with that in mind, and a group with enough females helps spread male attention. Above all, this is a species whose responsible keeping carries an explicit conservation dimension — captive stocks should be kept genetically clean and never released, and the aquarium serves in part as an ark for a fish that is vanishing in the wild.
Conservation
Xenotoca doadrioi is assessed as Critically Endangered on the IUCN Red List (species 130988902; assessment 130988927), under criteria B1ab(i,ii,iii,iv), reflecting its very restricted endorheic range in Jalisco, severe fragmentation into a few isolated subpopulations, and continuing decline in extent of occurrence, area of occupancy, habitat quality, and the number of localities at which it survives.
The drivers of that decline are the familiar ones of the Mexican Mesa Central. Water diversion and intensive groundwater pumping are eliminating the springs and spring-fed streams on which the species depends, drying out habitat outright in a basin that has no outlet to replenish it. On top of this, the fish faces displacement by non-native species — the twospot livebearer Pseudoxiphophorus bimaculatus is singled out as a competitor, and tilapia, bluegill and carp are established in its waters. Earlier work (Pedraza-Marrón, 2011) found the species had already been extirpated from more than half of its historical localities, and Lyons and colleagues (2019) classed it as endangered and declining. Notably, the species currently carries no risk category under Mexico's national standard NOM-059-SEMARNAT-2010, a gap between its formal national status and its dire IUCN listing.
For this species, captive husbandry is not a side note but a conservation strategy. Goodeids are a flagship for ex-situ hobbyist conservation, and organised breeding networks maintain genetically documented stocks against the loss of wild populations. The aquarist's role is correspondingly serious: keep lines pure and well recorded, support coordinated breeding programmes, and never release fish into local waters — the same restraint that protects native ecosystems elsewhere here also keeps a captive ark viable for a fish running out of wild habitat.