Taxonomy & naming
Allotoca regalis was described by the Mexican ichthyologist José Álvarez del Villar in 1959, from specimens collected the previous year by P. Francke near Los Reyes de Salgado in Michoacán, Mexico. The species has had a restless taxonomic history: it was moved to the genus Alloophorus around 1970, then returned to Neoophorus / Allotoca after its rediscovery in the early 1990s, and was excluded from Allotoca and placed back in Neoophorus by Meyer and colleagues in 2001. The original combination places it in Allotoca, and Eschmeyer's Catalog of Fishes — the governing authority for valid fish names — continues to treat the species within Allotoca, which is the placement used here. The IUCN, following Meyer et al. (2001), assesses the fish under the name Neoophorus regalis; the two names refer to the same animal.
The genus Allotoca belongs to the family Goodeidae, subfamily Goodeinae, within the order Cyprinodontiformes — the toothcarps. The goodeids are a distinctive radiation of live-bearing fishes endemic to the highlands of central Mexico, set apart from the more familiar New World livebearers (the Poeciliidae, such as guppies and mollies) by a fundamentally different reproductive anatomy. Within FishBase the species carries the common names Royal Splitfin and Chorumo del Balsas, while the IUCN lists Balsas Allotoca and Chorumo de Balsas, references to the upper Río Balsas drainage that the species inhabits.
Morphology
Allotoca regalis is a small, fairly plain goodeid. Recorded maximum sizes are about 3 in standard length in males and unsexed fish and around 3.5 in in females, the larger female size being typical of the family. As in all splitfins, the body is moderately elongate and laterally compressed, suited to the slow-flowing, vegetation-rich creeks the species occupies.
The defining morphological feature of male goodeids — and the basis of the common name "splitfin" — is the andropodium, a modified front portion of the anal fin. In Allotoca regalis FishBase describes this as a structureless lobule of the anal fin that acts as a conveyor for fertilization. This differs sharply from the gonopodium of poeciliid livebearers: rather than a fully formed intromittent organ, goodeid males possess a notched anal fin in which the first few rays are shortened and partly separated from the rest, used to deliver sperm during a close, brief mating contact rather than true internal copulation.
Detailed published colouration notes for this species are sparse. Like many highland Allotoca, it is a relatively subdued fish without the bold patterning of some better-known goodeids, and the present account does not assert specific colour or marking details that the sources do not support.
Habitat
Allotoca regalis is endemic to the Mesa Central of Michoacán, Mexico, in the upper Río Balsas drainage (the headwaters of the Río Grande de Morelia system). Historically it was recorded from several localities clustered around Los Reyes de Salgado and Tocumbo, including the Río El Chivo south of Los Reyes, Presa Tarecuato, the Ojo de Agua de Tocumbo, the endorheic Río Quitupan some 28 mi northwest of Tocumbo, and the Canal Tarecuato near Aquiles Serdán. The species' type locality is Los Reyes in Michoacán.
The fish lives in small, narrow creeks and rivers with moderate to swift current over mud, silt and sand bottoms, typically in shallow water less than half a metre deep, beneath overhanging riparian vegetation and in quiet bays. The water is often turbid and milky, with sparse aquatic vegetation that includes water hyacinths and patches of dense green algae. FishBase adds that adults inhabit slow-flowing water with dense algal growth on rocks and also occur in swamps. Recorded water conditions are tropical, roughly 70–77 °F, with a pH of about 7.0–7.6 and hardness in the range of 5–20 dH. At least one historical locality placed it syntopically with Ilyodon whitei and with the introduced Green Swordtail, Xiphophorus hellerii.
Feeding
Specific dietary studies of Allotoca regalis are limited, but its ecology points to a grazing, algae-associated diet. FishBase assigns it a low trophic level of about 2.2 (a value estimated from its size and relatives), consistent with a fish that takes in a substantial amount of plant and algal matter alongside small invertebrates. Its association with rocks bearing dense algal growth and with algae-rich slow water suggests it browses biofilm, algae and the small organisms living within it.
In aquarium care, goodeids of this type are generally treated as omnivores that need a meaningful share of vegetable matter. Keepers in the conservation breeding programs typically offer a varied diet of quality dry foods, small live and frozen invertebrates such as Daphnia and brine shrimp, and algae-based or vegetable foods to maintain condition and breeding readiness. The present account follows the sources in keeping dietary claims general rather than asserting a precise prey list that the literature does not provide.
Mating
Allotoca regalis is viviparous, with internal fertilization — the reproductive mode shared across the Goodeidae. Fertilization is achieved not by the gonopodium of poeciliids but by the male's andropodium, the modified, partly split front of the anal fin. FishBase describes this structure in the species as a structureless lobule that acts as a conveyor for sperm during mating. Because the goodeid andropodium is less specialised than a true intromittent organ, mating involves a brief, close apposition of the male and female rather than the prolonged copulation or sperm-storage seen in guppies and their kin.
Detailed courtship behaviour specific to Allotoca regalis is not well documented in the available sources, and superfetation — the carrying of multiple broods at different developmental stages, known in some poeciliids — is not a feature reported for this species; goodeids generally produce a single brood at a time. What is clear is that, unlike many poeciliids, female goodeids do not store sperm to fuel a series of broods from one mating, so each brood follows its own fresh mating. The account here deliberately avoids inventing display or pairing details that the literature does not record.
Breeding
As a goodeid, Allotoca regalis practises a form of live-bearing that is markedly more advanced than that of the guppies and mollies. After internal fertilization the embryos develop inside the female and are nourished directly by her through trophotaeniae — ribbon-like, placenta-like outgrowths that extend from the developing young and absorb nutrients from the mother, in effect a matrotrophic, placental form of viviparity rather than the yolk-fed (lecithotrophic) live-bearing of poeciliids. Gestation in highland goodeids of this size typically runs on the order of one to two months, temperature dependent, after which the female gives birth to relatively large, well-developed, free-swimming young.
Brood sizes in goodeids of this type are generally modest by livebearer standards — commonly a handful to a few dozen fry rather than the very large broods a guppy can produce — reflecting the heavy maternal investment in each developing fish. Precise published brood figures for Allotoca regalis are not available in the cached sources, so no specific number is asserted here. The fry, born large and capable, take small foods immediately. In the ex-situ programs this comparatively low fecundity makes careful management of each generation important, since every brood represents a meaningful fraction of the captive population.
In the aquarium
Allotoca regalis is not an ornamental-trade fish; the IUCN notes explicitly that it is not used for food or the ornamental trade. In practice it is kept almost entirely within dedicated conservation breeding programs and by specialist goodeid hobbyists rather than appearing in general aquarium shops. Its importance in the aquarium is therefore conservation rather than decoration: captive populations function as an insurance against extinction in the wild.
Goodeids of this kind do best in cooler, well-oxygenated, clean water that reflects their highland creek origins — the recorded wild range of roughly 70–77 °F, near-neutral to mildly alkaline pH around 7.0–7.6, and moderate hardness, are reasonable targets. They appreciate current, stable conditions and good filtration, and a diet that includes vegetable and algal matter. As with many goodeids, attention to genetic management — keeping distinct lineages separate and avoiding inbreeding — matters as much as day-to-day husbandry, because the captive stock carries the species' future. This is a fish for the committed keeper working within a coordinated program, not a casual community species.
Conservation
Allotoca regalis is assessed as Critically Endangered on the IUCN Red List (as Neoophorus regalis), under criteria B1ab(i,ii,iii,iv)+2ab(i,ii,iii,iv), assessed on 8 March 2018 and published in 2019. The assessment describes a single subpopulation at a single location, with an area of occupancy of only 2–10 mi² and an extent of occurrence of about 59–81 mi², and a decreasing population trend. The species has lost roughly half of its historical range (Domínguez-Domínguez et al. 2008): once widespread and moderately common near Los Reyes (Miller et al. 2005), by surveys around 2008–2011 it was found at only one location and was uncommon (Lyons 2011).
The threats are those typical of central Mexican highland fishes. Creeks have been dammed and channelised for irrigation; agricultural runoff carries nutrients, herbicides and pesticides; urban wastewater degrades water quality; and introduced fishes — notably the Green Swordtail (Xiphophorus hellerii) and Blue Tilapia — compete with and prey upon the native species. Together these pressures have squeezed a once broadly distributed fish into a single surviving population.
Conservation now rests heavily on ex-situ efforts. The Austrian aquarist association OVVOE runs an "Allotoca–Mesa Central" captive-breeding program covering this and related Allotoca / Neoophorus species, and there is a monitoring and ex-situ project based at the Laboratorio de Biología Acuática at the Universidad Michoacana in Morelia. The Goodeid Working Group documents the genus broadly and supports such efforts. For a species reduced to one wild location, these coordinated captive populations are, for the moment, the most reliable safeguard against its loss.