Taxonomy & naming
Chapalichthys peraticus was described by the Mexican ichthyologist José Álvarez del Villar in 1963, in a regional paper on the fishes of San Juanico and Tocumbo ("Ictiología Michoacana 3," Anales de la Escuela Nacional de Ciencias Biológicas). In the very same publication Álvarez also named Chapalichthys pardalis, from a spring-fed pool at Tocumbo a few kilometres away; the two were distinguished largely on locality and slight differences in pattern and proportion. Eschmeyer's Catalog of Fishes records C. peraticus Álvarez del Villar, 1963 with the type material from the Presa San Juanico, a dam near Cotija in Michoacán.
The taxonomy did not stay settled. When Miller, Minckley and Norris reviewed the freshwater fishes of Mexico (2005), and again when Piller and colleagues examined goodeid relationships with molecular data (2021), the differences between peraticus and pardalis proved insufficient to uphold two species. The current consensus — followed by the Goodeid Working Group and by the IUCN — is that C. peraticus is a junior synonym of C. pardalis, the Polkadot Splitfin, so that the San Juanico population and the Tocumbo population represent a single, variable species. This atlas keeps a page under the historical name peraticus for the San Juanico form, while pointing the reader to pardalis as the name under which the combined taxon is now assessed and conserved.
The genus Chapalichthys was erected by Seth Eugene Meek in 1902 and named for Lake Chapala, the great lake on the Jalisco–Michoacán border from which its type species comes; the name combines "Chapala" with the Greek ichthys, "fish." The genus belongs to the Goodeidae, the splitfins or Mexican livebearers, and within it to the subfamily Goodeinae — a radiation of viviparous fishes almost entirely endemic to the interior highlands of Mexico.
Morphology
Chapalichthys peraticus is a small, deep-bodied goodeid reaching about 3.5 in in standard length in large females; males are smaller. Álvarez's holotype was a female of roughly 2.5 in standard length, with the largest measured fish in later surveys around 3.5 in. The body is silvery-grey overlaid with a yellow-green sheen and scattered with irregular dark blotches that are heaviest on the lower flanks, where they tend to coalesce into a broken, irregular lateral band — the "polkadot" or "pintito" ("little spotted one") pattern that gives the fish its common names.
The sexes differ in size, finnage and colour. Males are smaller and more slender, with a larger, more developed dorsal fin and a bright yellow terminal band on the caudal fin; females are deeper-bodied, plainer, and often show a yellow belly. As in all goodeines, the males lack the rod-like gonopodium of the New World poeciliids. Instead the first six or seven rays of the anal fin are shortened and set off from the rest by a distinct notch, forming the structure called the andropodium — the diagnostic "split fin" of the family, used to deliver sperm rather than to lock onto the female.
Internally the fish is built for a herbivorous life: it carries bifid (two-pointed) teeth suited to scraping algae and a long, convoluted intestine for digesting plant matter, both typical of the algae-grazing goodeids.
Habitat
The species is endemic to a very small area of the Michoacán highlands, in the upper Río Balsas basin — specifically the Río Itzícuaro subbasin within the Río Tepalcatepec drainage. Historically it was known from only two waters a short distance apart: the spring-fed pool at Tocumbo (the type locality of pardalis) and the Presa San Juanico, a reservoir near Cotija that is the type locality of peraticus. The two were linked by a canal draining toward the Río Grande and Río Itzícuaro, but each population has long been treated as its own evolutionarily significant unit.
The habitats are warm, vegetated still and slow waters. At Tocumbo the spring outflow runs from about half a metre to two metres deep over a bottom of silt, mud, gravel and rock, fringed with green algae and a little water hyacinth, with spring temperatures measured around 70–75 °F. The San Juanico reservoir holds pools and ponds thick with hornwort (Ceratophyllum) and water hyacinth (Eichhornia) and reed-lined shores; one February measurement near the surface read about 75 °F, while the main body of water sat below 68 °F, reflecting the marked seasonal and depth-related temperature swings of these upland Mexican waters.
That tiny, two-point range is the heart of the conservation problem. A fish confined to a single reservoir and a single spring has no refuge if those waters are altered, polluted, or invaded by non-native fishes — and both have been.
Feeding
Chapalichthys peraticus is predominantly herbivorous. Its dentition and gut anatomy point to a diet built around algae and aufwuchs — the film of algae, diatoms, micro-invertebrates and detritus that coats submerged surfaces — which the fish grazes from rock, plants and substrate. The bifid teeth scrape this material loose, and the long, coiled intestine extracts nutrition from a fibrous, plant-heavy diet. FishBase places the species at a low trophic level consistent with this algae-leaning ecology.
The diet is not strictly vegetarian: like many grazing goodeids the fish will also take small insects and other animal items from the surface and water column when they are available, supplementing its plant food opportunistically. In aquaria the species accepts a varied diet but does best with a strong vegetable and algal component — spirulina-based foods, blanched greens and natural algal growth — alongside small live and frozen foods such as Daphnia and brine shrimp to round out condition.
Mating
Like all goodeids, Chapalichthys peraticus reproduces by internal fertilisation, but it does so without the gonopodium of the poeciliid livebearers. The male's sperm-transfer organ is the andropodium — the notched, shortened front portion of the anal fin — and mating involves the male manoeuvring alongside the female to deliver sperm rather than the brief, often forced couplings of guppies and their kin. Goodeid courtship tends to be more deliberate, with males displaying their finnage and colour to females.
The species is a seasonal but protracted breeder. Surveys have turned up small, recently born young (0.5 in) across a wide spread of the year — September, January, February, April and late May — indicating a long reproductive season rather than a single tight pulse. Unlike many poeciliids, goodeids do not store sperm for serial broods from one mating; each brood follows its own fertilisation, and superfetation (carrying overlapping broods at different stages) is not a feature of this fish, so successive litters are produced sequentially rather than simultaneously.
Breeding
Goodeids are true viviparous livebearers, and their reproduction is among the most specialised of any freshwater fish. The developing embryos are nourished inside the female through trophotaeniae — ribbon-like rosette structures extending from the vent that function as a kind of umbilical connection, drawing nutrients directly from the mother across gestation. This matrotrophic (mother-feeding) strategy contrasts with the largely yolk-fed development of poeciliids and produces relatively large, well-formed young.
The trade-off is fecundity: goodeids give birth to small broods of large fry rather than the dozens of tiny young a guppy can produce. Chapalichthys peraticus has low overall fecundity — modelled at fewer than ten young per brood for the combined taxon — and a correspondingly slow population recovery rate, which is part of why it is so vulnerable. Gestation runs for several weeks, after which the fully formed fry are born swimming and feeding.
In captivity the species breeds reliably as a colony, and encouragingly the adults rarely eat their own fry, so well-planted tanks can yield steady recruitment without elaborate fry-rearing setups. Keepers report best spawning around 68–70 °F, with a cool winter rest below 68 °F for two or three months appearing to set up the breeding season; reproduction stops above about 77 °F. This pattern of seasonal cooling mirrors the natural temperature cycle of the highland waters the fish comes from.
In the aquarium
Chapalichthys peraticus is kept almost entirely as a conservation-minded hobby fish rather than a commercial ornamental — its survival in aquaria is itself part of the story. It is not a difficult fish, but it has particular needs that reflect its cool, well-oxygenated highland origins. Keepers recommend a roomy tank with a low, wide footprint (on the order of 40 US gal or more), strong aeration and high dissolved oxygen, and large, frequent water changes — commonly 60–80% weekly — to keep these messy grazers in clean water.
The key husbandry detail is temperature cycling. The fish wants a genuine winter cool rest below 68 °F for a couple of months, then spawning temperatures around 68–70 °F; held warm year-round (above ~77 °F) it stops breeding and does poorly. This is the opposite of the steady tropical warmth most community livebearers want, so the species is best kept as a single-species colony, or with other cool-tolerant goodeids, rather than in a mixed tropical community.
Given its herbivorous bent, plenty of algal grazing surface, robust plant cover and a vegetable-rich diet keep the fish in good order. Kept as a flock, with the cool seasonal rhythm and clean, oxygen-rich water it needs, C. peraticus breeds dependably — which is exactly why hobbyist colonies have become a meaningful refuge for a fish that is disappearing from its handful of wild homes.
Conservation
The combined taxon — assessed by the IUCN under the name Chapalichthys pardalis, into which peraticus is synonymized — is listed as Critically Endangered. There is no separate IUCN assessment for C. peraticus, precisely because it is no longer recognised as a distinct species; the San Juanico fish is covered by the pardalis assessment. The species is endemic to just two localities in the upper Balsas basin of Michoacán, and both are in trouble.
The Tocumbo population may already be extinct in the wild: its spring pool has been concreted and overrun by non-native fishes — among them Goodea, Ilyodon, Poeciliopsis, Poecilia and Xiphophorus species — and no fish have reportedly been seen there since the mid-2010s. The San Juanico population survives but is small and confined to the reservoir's margins. With only two sites, low fecundity, slow recovery and mounting pressure from habitat alteration and invasive fishes, the outlook in the wild is precarious; under Mexico's NOM-059-SEMARNAT-2010 the fish carried no risk category, but international assessment places it among the most endangered of its family.
For this species, captive maintenance is not a luxury but a lifeline. The Goodeid Working Group and allied aquarists keep flock-bred colonies expressly as an insurance population against loss in the wild, and the species' willingness to breed in well-managed tanks makes that strategy viable. The single most important thing a keeper can do is maintain healthy, genetically documented stock and never release aquarium fish — and, where possible, support the coordinated conservation breeding that may yet keep this little spotted splitfin from vanishing altogether.