Taxonomy & naming
Chapalichthys pardalis was described by the Mexican ichthyologist José Álvarez del Villar in 1963, in his study of the fishes of San Juanico and Tocumbo (Ictiología Michoacana 3; Anales de la Escuela Nacional de Ciencias Biológicas). The holotype, an adult female of about 2 in standard length, was collected at Tocumbo in September 1961. In the same work Álvarez named a second form, Chapalichthys peraticus, from the nearby Presa San Juanico; this was later synonymised, and phylogenetic work (Piller et al., 2021) confirmed that both names refer to a single species, with Chapalichthys pardalis as the valid name under the Catalog of Fishes.
The genus Chapalichthys was erected by Meek in 1902 and means "fish of Lake Chapala," combining the lake's name — itself derived from the Aztec chief Chapalac — with the Greek ichthys, fish. The specific epithet pardalis comes from the Latin pardus and Greek pardos, meaning panther or leopard; as an adjective it means "mottled" or "spotted like a panther," chosen for the fish's disjunct, blotchy colour pattern. The species belongs to the family Goodeidae, subfamily Goodeinae — the splitfins — a group of viviparous toothcarps endemic to the highlands of central Mexico and named for the modified anal fin of the males.
Morphology
The polka-dot splitfin is a small, fairly deep-bodied goodeid. FishBase gives a maximum length of around 2.5 in total length for males and about 3 in for females, while Miller and colleagues (2005) report fish up to roughly 3.5 in standard length, so the species can reach a slightly larger size than the commonly cited aquarium figures. The body is silvery-grey with a yellowish-to-greenish sheen, overlaid by irregular dark blotches that are strongest on the lower flank, where they tend to coalesce into a broken, panther-like lateral band — the pattern that gives the fish both its scientific and common names.
The sexes differ in fin shape and colour. Males are recognised by the notched, or "split," anal fin: the first several rays are shortened and set off by a notch, forming the andropodium, the goodeine mating organ. Males also carry a larger dorsal fin and a bright yellow terminal band across the caudal fin, edged by a dark line, while females are more slender and may show a yellowish belly. The operculum and fins are otherwise unspotted, the dark mottling being confined to the body.
Habitat
Chapalichthys pardalis is endemic to a very small area of Michoacán, in the upper part of the Río Balsas basin. It is known from just two localities about 12–16 mi apart: the Ojo de Agua / Balneario de Tocumbo, a spring-fed pool, and the Presa San Juanico near Cotija. The two are linked by a lateral canal and drain via the Río Grande / Río Itzícuaro into the Río Tepalcatepec, a western tributary of the Río Balsas. The species occurs nowhere else.
At Tocumbo the fish historically lived in a clear spring and its outflow creek, since converted into a concrete bathing pool; the fish favoured shallow water around half a metre deep (occurring down to one or two metres) over silt, gravel, rocks and boulders, among green algae and sparse water hyacinth, at spring temperatures around 70–75 °F. The San Juanico locality consists of pools and ponds with dense Ceratophyllum and Eichhornia and reed-fringed shores; surface temperatures near 75 °F have been recorded, though the waterbody as a whole sits at or below about 68 °F. This is a subtropical highland fish of clear, well-oxygenated, moderately hard, alkaline spring water (recorded pH roughly 7.2–8.0).
Feeding
The polka-dot splitfin is predominantly herbivorous. Its anatomy reflects this: it has bifid (two-cusped) teeth for rasping and a long, convoluted gut suited to processing plant material. In the wild it grazes on algae and aufwuchs — the film of algae, diatoms and small organisms coating submerged surfaces — and is also drawn to insects falling on the water surface, taking occasional animal food. FishBase places it at a low trophic level of about 2.0, consistent with a largely plant-based diet.
In the aquarium it should be offered a diet weighted toward vegetable and algal matter, supplemented with small live and frozen foods; keepers note that the fish takes live food readily and benefits from it for conditioning. As with most grazing goodeids, a steady supply of algae or spirulina-based foods, rather than a heavily protein-rich diet, suits its physiology best.
Mating
Like all goodeids, Chapalichthys pardalis practises internal fertilisation, but the goodeine mechanism differs from that of the more familiar poeciliid livebearers. Instead of a gonopodium formed from a fully elongated anal fin, the male carries an andropodium: the first roughly seven rays of the anal fin are shortened and separated from the rest by a distinct notch, producing the "split fin" for which the family is named. This modified fin is used to transfer sperm to the female.
Males court and compete through colour and fin display, the larger dorsal fin and the bright yellow caudal band serving as signals. The breeding season is protracted: juveniles of 0.5 in have been collected across many months from January to September, indicating that reproduction continues over much of the year rather than in a single brief pulse. Unlike many poeciliids, goodeines such as this species do not store sperm for long-term superfetation; each brood follows a discrete mating, and the female nourishes the developing young directly.
Breeding
Chapalichthys pardalis is viviparous, giving birth to fully formed, free-swimming young rather than laying eggs. As in other goodeids, the embryos are matrotrophic — nourished by the mother during gestation through trophotaeniae, ribbon-like placental structures that absorb nutrients, rather than relying on a yolk supply as poeciliids do. FishBase reports broods of about 10–20 offspring, with several broods produced per year over the long breeding season.
In aquarium care the species is reported as peaceful and, unusually for a livebearer, not strongly inclined to eat its own fry, so good numbers can be raised in a well-planted tank. Keepers recommend a winter cool rest below about 68 °F for two to three months to condition the fish; spawning occurs at around 68–70 °F and tends to stop above 77 °F. The fish is said to do well kept outdoors from spring to autumn where daytime water temperatures stay above about 59 °F, which mimics the seasonal cycle of its native springs.
In the aquarium
The polka-dot splitfin is a peaceful, undemanding aquarium fish that is best regarded as a conservation-minded keeper's species rather than a casual community fish. Keepers recommend a long, fairly low aquarium of at least about 40 US gal (water height around 10 in) furnished with rocks and roots and patches of dense vegetation, with little to moderate current. Because the fish comes from clear, spring-fed water it demands high oxygen levels (around 8 mg/L or more) and good water quality, with large weekly water changes on the order of 60–80%.
Water should be moderately hard and alkaline, reflecting its native spring chemistry, and the tank benefits from the seasonal temperature swing described above — a cool winter rest followed by warmer spawning conditions. The species is harmless to tankmates and has no special aggression to manage, but its sensitivity to water quality and its need for a seasonal cycle make it a fish for keepers prepared to maintain those conditions. Given its conservation status, much of the husbandry interest in this species comes from networks such as the Goodeid Working Group, which maintain ex-situ stocks as insurance against loss in the wild.
Conservation
Chapalichthys pardalis is assessed as Critically Endangered on the IUCN Red List (assessment dated 8 March 2018; criteria B1ab(i,ii,iii,iv)+2ab(i,ii,iii,iv)). The listing reflects its tiny range — two localities in a single drainage — and ongoing threats from habitat alteration and invasive species. The Tocumbo population (the spring that gave the fish its name) is considered possibly extinct in the wild: the fish has not been seen there since around 2015, the bathing pool was emptied and cleaned around 2016, and a 2018 survey found masses of invasive goodeids and poeciliids but no polka-dot splitfins. The San Juanico population persists as a small group in nearshore areas.
The principal threats are the development and modification of its spring habitats, water extraction and pollution in a small, heavily used catchment, and competition from introduced fishes. Notably, the species carries no risk category under Mexico's national list (NOM-059-SEMARNAT-2010), so much of the practical safeguarding falls to international conservation aquarist networks. The Goodeid Working Group and allied hobbyist breeders maintain captive lineages of both populations as a hedge against extinction in the wild — a reminder that for some Mexican goodeids the aquarium has become a genuine conservation refuge rather than merely a place of display.