Taxonomy & naming
Allotoca catarinae was first described by the Spanish-Mexican ichthyologist Fernando de Buen in 1942, not as a full species but as a subspecies, Neoophorus diazi catarinae, recognising its close kinship to the better-known Allotoca diazi. De Buen deposited no holotype, an awkwardness that has followed the taxon ever since. The form was raised to species rank by Alvarez del Villar in 1959, who treated it as Neoophorus catarinae; subsequent revision of the goodeid genera folded Neoophorus into Allotoca, yielding the combination Allotoca catarinae (de Buen, 1942) recognised today. Eschmeyer's Catalog of Fishes governs the valid name, and the parenthetical authority reflects the original placement in a different genus.
The species sits in the family Goodeidae, subfamily Goodeinae — the splitfins, a group of viviparous toothcarps endemic to the highlands of central Mexico. Its closest relative is Allotoca diazi, to which it is genetically very similar; molecular work by Corona-Santiago and colleagues (2015) raised the possibility that A. catarinae represents a human-mediated transfer of A. diazi-like stock into the upper Balsas drainage within roughly the last thousand years, a hypothesis that underlines how shallow the genetic divergence between the two forms is. For now the species is maintained as distinct, with no recognised subpopulations or evolutionarily significant units — all known fish belong to a single lineage.
The common name Catarina allotoca, and the Mexican vernacular Tiro Catarina, both derive from the type locality. The epithet catarinae is a toponym honouring the Presa Catarina (Laguna de Santa Catarina) at Uruapan, the reservoir from which de Buen's original material came.
Morphology
Allotoca catarinae is a small, stoutly built goodeid. The maximum known size is around 3 in in standard length (Miller et al., 2005), and FishBase records females reaching about 3 in total length against roughly 2 in for males — females being both longer and bulkier, with a broader, somewhat blunt head. The body is typical of the genus: moderately deep, robust, and adapted to life among dense aquatic plants and in moderate-to-swift current.
The sexes are readily told apart. Males develop the diagnostic splitfin feature — an andropodium, a modified anal fin in which the first six or seven rays are conspicuously shortened and set off by a notch, forming the structure used to deliver sperm. Males also tend to carry a larger, more prominent dorsal fin. Females are plainer and larger overall, occasionally showing a yellowish wash on the anal fin, and lack the anal-fin modification.
Colouration in this species is muted rather than showy, in keeping with its streamliving habits; the fish relies on its build and behaviour rather than bright pattern. As with all goodeids, the absence of a true gonopodium — the rod-like organ of the unrelated poeciliid livebearers — is a defining structural point: the splitfins evolved internal fertilisation independently, and the male's andropodium is anatomically a quite different solution to the same problem.
Habitat
The species is endemic to a single river system in Michoacán, in west-central Mexico. It inhabits the Río San Antonio — also recorded as the Río Santa Bárbara or Río Santa Catarina — an affluent of the Río Cupatitzio, within the upper Río Balsas drainage (the Tepalcatepec–Infiernillo basin, Cupatitzio subbasin). Its entire world range lies in and around Uruapan, on the southern flank of the Mexican Mesa Central. There are no separate subpopulations: every known fish comes from this one connected drainage.
Within that system A. catarinae occupies moderately sized streams and associated reservoirs of clear water flowing over rock, sand, mud and silt. It favours stands of dense submergent vegetation in shallow water, typically less than a metre deep, in moderate-to-swift current with high dissolved oxygen — figures of at least 8 mg/l have been recorded. A 2017 survey at the Presa Caltzontzin found the fish at around 19.5 in depth over silt, mud and sand strewn with algae- and aufwuchs-covered rocks, shaded by overhanging willows, sharing the water with the goodeid Ilyodon whitei. The water is warm but not hot — FishBase places the species in the 75–81 °F band — reflecting its highland-stream origins.
This extreme range restriction is the heart of the species' conservation problem: a single drainage near a growing city leaves it acutely exposed to local water extraction, pollution and habitat change.
Feeding
The diet of Allotoca catarinae is somewhat contested. The fish has been described as grazing on aufwuchs — the film of algae, microorganisms and detritus coating submerged surfaces — and sifting organic material from the silt, which would make it a detritivore and micro-grazer in the manner of many splitfins. Its trophic level is estimated low, around 2.2, consistent with a substantially plant- and detritus-based intake.
However, observers of the genus note that the species' dentition and mouth structure point toward a more predatory habit than the aufwuchs story alone suggests. On that reading, much of its real intake is small animal prey: insect larvae, freshwater shrimps, water fleas (Cladocera) and worms taken from among the vegetation and substrate. The likely truth is an opportunistic omnivory weighted toward small invertebrates, with algae and detritus taken alongside.
In practice this is a fish well suited to picking through dense plant growth for whatever small living and dead matter it can find — a foraging style that fits its preference for heavily vegetated, well-oxygenated stream margins. Captive observations support a readiness to take small live and prepared foods readily.
Mating
Allotoca catarinae is viviparous, with internal fertilisation — the reproductive mode shared by the entire family Goodeidae. Unlike the poeciliid livebearers, which fertilise via a true gonopodium, the male splitfin uses an andropodium: the shortened, notched front rays of the anal fin, which channel sperm to the female during mating. The structural distinction matters because it reflects the goodeids' independent evolution of live-bearing, and it is the principal external clue to a male's sex.
Reproduction in the wild is seasonal but extended. De Buen's original observations and later fieldwork indicate a long reproductive period: gravid females have been found from late February into mid-April, with embryos recorded as late as early August. This protracted breeding window is typical of goodeids in their warm but seasonally varying highland habitats.
Sexual selection in the species is expressed through the males' enlarged dorsal fin and their courtship of females among the plant stands rather than through bright nuptial colour. As in other splitfins, mating brings the andropodium into play directly, and females carry the resulting embryos internally for the long gestation that follows.
Breeding
Goodeid reproduction is among the most specialised of any livebearing fish, and Allotoca catarinae shares the family's signature feature: matrotrophy. Rather than relying on a yolk, the developing embryos are nourished directly by the mother through trophotaeniae — ribbon-like outgrowths from the gut that function as a placenta-like connection, absorbing nutrients from the maternal tissue throughout development. This makes goodeid gestation a genuine investment of maternal resources, not merely the brooding of yolked eggs, and the young are born comparatively large and well developed.
Gestation is long for a fish of this size: FishBase gives roughly 55 days, far exceeding the three-to-four-week pregnancies of guppies and their poeciliid kin. Brood sizes are correspondingly modest — on the order of 12 to 20 young. De Buen's original description recorded a clear example: a single female of 2 in total length carrying thirteen embryos each about 0.5 in long, already substantial fish at birth. There is no superfetation reported in this species; unlike some poeciliids, goodeids generally carry a single brood at a time rather than overlapping pregnancies.
The combination of low fecundity, long gestation and high per-offspring investment is a double-edged trait. It produces robust, capable fry, but it also means slow population turnover — a vulnerability that compounds the threat from the species' tiny range and leaves little reproductive slack to absorb habitat loss.
In the aquarium
Allotoca catarinae is a fish for the dedicated goodeid keeper rather than the casual livebearer enthusiast, and it is maintained chiefly within conservation-minded networks — the Goodeid Working Group and studbook programmes such as OVVO's Allotoca register (Allotoca – Mesa Central) exist precisely to keep ark populations alive against the species' wild decline. Husbandry guidance reflects its stream origins: a tank of at least 25 US gal, ideally long and wide rather than tall, with moderate-to-swift water movement and scrupulous water quality, including large weekly water changes on the order of 60–80%.
Temperature management is the key to good breeding. Keepers report that the species spawns best at around 68–70 °F and effectively stops reproducing above roughly 77 °F. A deliberately cool winter rest — dropping to below 68 °F, and tolerating 59–61 °F — is recommended to give breeding females a recovery period, mimicking the seasonal rhythm of its highland habitat. High oxygenation and clean, flowing water matter more to this fish than to most aquarium livebearers.
For anyone keeping the species, the central point is responsibility. This is a Critically Endangered fish whose captive populations may outlast its wild ones; maintained stock should be kept pure, well-documented and ideally tied into a studbook, so that aquarium fish remain a genuine conservation reservoir rather than a dead end.
Conservation
Allotoca catarinae is assessed as Critically Endangered on the IUCN Red List (species ID 191694), under criteria B1ab(i,ii,iii,iv), with the assessment dated 7 March 2018. The listing rests on the species' extraordinarily restricted range — a single drainage in the upper Cupatitzio River near Uruapan, Michoacán — which leaves it exposed to habitat degradation, water extraction and pollution associated with a growing urban area. Under Mexico's national framework, NOM-059-SEMARNAT-2010, the fish is likewise classified as P, En Peligro de Extinción (in danger of extinction).
The picture is not uniformly bleak: a 2019 review by Lyons and colleagues found the species persisting at around ten locations across the upper Cupatitzio drainage, all of which had held populations since 2000, and judged the situation locally stable on that evidence. Even so, the overarching constraint — total dependence on one small, increasingly pressured river system — keeps the species at high risk, and the IUCN's Critically Endangered listing stands.
Allotoca catarinae is one face of a wider catastrophe. Nearly three-quarters of Mexico's roughly forty goodeid species are now listed as Critically Endangered or Endangered, making the family one of the most threatened freshwater fish groups on Earth, as documented in the 2023 Action Plan for the Conservation of Mexico's Goodeid Fishes. For this species, the conservation strategy runs on two tracks at once: protecting what remains of its single wild drainage, and sustaining the captive studbook populations that may prove to be its insurance against extinction.