Taxonomy & naming
Allotoca zacapuensis was described in 2001 by Meyer, Radda and Domínguez-Domínguez in the Annalen des Naturhistorischen Museums in Wien (103B: 453–460), based on material from Lago de Zacapú in Michoacán, Mexico. The holotype — a young male of just 1 in standard length, catalogued as NMW-94584 — was collected on 14 February 2001 from the type locality. Eschmeyer's Catalog of Fishes treats Allotoca zacapuensis Meyer, Radda & Domínguez-Domínguez, 2001 as the valid name, placing it in the genus Allotoca within the family Goodeidae, the splitfins or goodeids of the Mexican highlands.
The Goodeidae are a small family of livebearing toothcarps of the order Cyprinodontiformes, largely endemic to the interior plateau of central Mexico. The subfamily Goodeinae — to which Allotoca belongs — is distinguished from the New World poeciliids (guppies, mollies, swordtails) by a fundamentally different mode of viviparity and by the male's andropodium rather than a true gonopodium. The genus Allotoca was erected by Hubbs and Turner in the late 1930s; the name combines Greek allos ("other, different") and tokos ("birth, offspring"), a reference to the unusual maternally nourished young of these fishes.
The specific epithet zacapuensis honours the Laguna (Lago) de Zacapú basin, the type locality and the heart of the species' tiny range. The place-name Zacapú itself derives from Nahuatl (zacapú, "place of stones"). The fish carries the Mexican common name Tiro de Lago de Zacapú and the English-language name Zacapu allotoca; within the goodeid conservation community it is tracked as the single evolutionarily significant unit "Altza1."
Morphology
Allotoca zacapuensis is a small fish with a fusiform body, reaching a maximum standard length of about 2 in (2 in) according to Miller et al. (2005), the figure preferred by the Goodeid Working Group; FishBase lists a smaller 1 in SL maximum drawn directly from the original description, with first maturity around 1.5 in. The fin formula is typical of the genus: roughly 18–20 dorsal soft rays, 12–13 anal soft rays, 30–32 vertebrae and no fin spines.
The ground colour in both sexes is greyish-brown, overlaid with blue, pink and green reflections along the midline and venter. The most conspicuous markings are 10–14 broad, dark brown V-shaped vertical bars along the flanks, most strongly developed on the rear half of the body, with a darker reticulum picked out on the scales of the upper body. The dorsal fin is light orange, the anal and caudal fins a light yellow-orange, and the paired fins are hyaline. Adult males develop a dark brown area in and just in front of the genital region.
The diagnostic feature of mature males is the andropodium — the goodeine analogue of the poeciliid gonopodium. In Allotoca zacapuensis the first roughly seven anal-fin rays are shortened and the rays behind them (rays 2–7 in the original description) are thickened basally and curved anteriorly, producing a notch that forms the male's copulatory structure, sometimes termed a spermatopodium. Males also carry a larger dorsal fin and darker, more contrasting mottling with a golden sheen; females are slightly longer, with a blunter head and a yellow base to the anal fin.
Habitat
Allotoca zacapuensis is endemic to the state of Michoacán in west-central Mexico, in the flat former paleolake of Zacapu. The basin lies within the Lerma–Santiago hydrographic region, in the upper Río Lerma–Chapala drainage and specifically the Río Angulo subbasin of the Río Lerma. The species is known from spring areas within Lago/Laguna Zacapú and — following a discovery in 2013 — from a tiny spring at the village of Jesús María feeding the Canal Patera and the headwaters of the Río Angulo near Villa Jiménez, roughly 22 mi east-north-east of Zacapu.
The historical habitat has been catastrophically reduced. What was once a ciénega (marsh-lake) of some 33,500 hectares was drained for agriculture; the remaining open spring-fed lake covers only about 33.5 hectares. At the collection points the water is 2–3 ft deep over a mud bottom, ranging from translucent greenish to turbid, with little current and partial shade. The fish prefers well-vegetated areas, hiding beneath aquatic plants — Chara, Potamogeton, Ceratophyllum and green algae — and among the submerged roots of willow (Salix). Adults are reported from turbid water over mud bottoms.
Water conditions at the type locality are warm-temperate rather than tropical: FishBase records a temperature range of roughly 63–68 °F and a pH near 7.0. The species lives alongside a characteristic assemblage of Mesa Central goodeids and relatives, including Skiffia lermae, Xenotoca variata, Goodea atripinnis, Hubbsina turneri and Zoogoneticus quitzeoensis.
Feeding
Allotoca zacapuensis is essentially carnivorous. Its dentition and relatively short gut are typical of the genus and indicate a predatory rather than herbivorous habit, in contrast to some of the more algae-grazing goodeids. In the wild it feeds mainly on small to medium aquatic invertebrates — bloodworms and other insect larvae prominent among them — taken from among the dense vegetation it shelters in. Its trophic level is estimated at about 2.5.
In captivity the species takes a wide range of foods readily. Goodeid keepers report success with live and frozen invertebrate foods, brine shrimp nauplii for fry and adults alike, and prepared diets including flake, granulate and tablet foods. As a predatory, somewhat shy fish it does best with regular feedings of varied, protein-rich foods that match its natural invertebrate diet.
Mating
Like all goodeids, Allotoca zacapuensis practises internal fertilisation, but the mechanism differs from that of the poeciliid livebearers. Instead of an elongate gonopodium, the male transfers sperm using the andropodium — a structure formed from the shortened and modified front rays of the anal fin, with a characteristic notch after the first several rays. This split anal fin is the visible sign of a mature male and the source of the family's common name, "splitfins."
Field data suggest a female-skewed wild population. A study running from May 2019 to March 2020 (Ramírez-García et al. 2021) reported a sex ratio of roughly two females to one male, set against an extremely small overall population. Males are the more strongly marked sex, with the enlarged dorsal fin, darker golden-tinged mottling and the dark abdominal region, features consistent with sexual selection in a small livebearing fish where females are the more numerous and choosier sex.
Reproductive activity is strongly seasonal and temperature-cued. In culture, spawning begins in spring as temperatures rise to about 68–70 °F and ceases when the water warms above roughly 77 °F or turns cold — a pattern that mirrors the cool, spring-fed character of the natural habitat and that breeders deliberately exploit to time and pause reproduction.
Breeding
Allotoca zacapuensis is viviparous in the full goodeid sense: the female does not merely retain fertilised eggs but actively nourishes her developing young through the gestation period. After internal fertilisation the embryos develop within the ovary and are fed via trophotaenia — ribbon- or rosette-like extensions of the embryonic gut that absorb nutrients directly from maternal tissue, functioning as a placental analogue. This matrotrophic mode is the defining reproductive feature of the genus, and indeed the trophotaeniae of the fry are what inspired the genus name Allotoca, "different offspring." Goodeids do not exhibit the sperm-storage superfetation seen in many poeciliids; broods develop as a single synchronous cohort rather than as overlapping batches.
Gestation in goodeids of this size typically runs on the order of two months, after which the female gives birth to a relatively small number of comparatively large, well-developed fry — far fewer than the dozens a guppy produces, but each better provisioned at birth. Newly born young still bear remnants of the trophotaeniae for a short time before these are absorbed.
In captivity, breeders manage the species around its temperature-driven cycle. Spawning is initiated as spring warming brings the tank to around 68–70 °F, and a deliberate cool winter rest below 68 °F (down to 59–61 °F is tolerated) for two to three months is used to pause fry production and let adults recover. Sustained warmth is counterproductive: temperatures held above about 77 °F halt reproduction, and prolonged exposure above 72–75 °F weakens the fish and shortens their lives. The species is maintained in the ÖVVÖ/Goodeid Working Group studbook as part of the Allotoca–Mesa Central / Banded Allotoca conservation programmes.
In the aquarium
Allotoca zacapuensis is a specialist fish kept chiefly by goodeid enthusiasts and conservation breeders rather than the general hobby, and its needs reflect its cool, well-oxygenated spring habitat. Keepers recommend an aquarium of at least 25 US gal — low and broad rather than tall, with about 10 in of water depth being sufficient — structured with rocks, roots and wood and densely planted in the corners to give this somewhat shy, predatory fish the cover it seeks.
Water quality demands are high. The fish wants moderate to swift current and high dissolved oxygen (on the order of 8 mg/l or more), supported by large weekly water changes of 60–80%. Temperature management is the crux of successful husbandry: the species should not be kept warm. Sustained temperatures above roughly 72–75 °F cause the fish to weaken and age prematurely, and a cool winter rest below 68 °F — down to 59–61 °F — for two to three months is beneficial. This is firmly a temperate-leaning subtropical fish, not a tropical community species.
Feeding is straightforward for a predatory livebearer: it takes invertebrate foods, brine shrimp nauplii, and prepared flake, granulate and tablet foods. It is on the shy side and is best kept as a species group where its behaviour and breeding can be observed and managed. Because wild populations are so reduced, captive lines maintained in coordinated studbooks are an important reservoir for the species.
Conservation
Allotoca zacapuensis is assessed as Critically Endangered on the IUCN Red List (criteria B1ab(i,ii,iii,iv)+2ab(i,ii,iii,iv); species ID 191699). The listing reflects an extremely restricted range: the fish is known from only about two spring-fed sites and is absent from roughly 95% of Lake Zacapu, even though its numbers at the surviving sites were reported as relatively stable (Domínguez-Domínguez et al. 2005). Under Mexico's NOM-059-SEMARNAT-2010 it carries no category of risk, a gap between national and international assessments common to many micro-endemic Mesa Central goodeids.
The principal threat is habitat loss and degradation. The Zacapu paleolake has been drained from a 33,500-hectare ciénega down to a remnant of about 33.5 hectares of open spring-fed water, a reduction that has eliminated the great majority of the species' historical habitat. Surviving populations cluster at a handful of springs, leaving the species acutely vulnerable to any further water extraction, pollution, or introduction of non-native fishes.
Given the precariousness of the wild population, ex-situ conservation is central to the species' survival. Allotoca zacapuensis is maintained in coordinated studbooks by the Goodeid Working Group and the Austrian society ÖVVÖ, under the Allotoca–Mesa Central and Banded Allotoca projects, so that a managed captive stock backs up the few remaining wild sites. For aquarists, the species is less a casual livebearer than a participation opportunity in organised conservation breeding.