Taxonomy & naming
Chapalichthys encaustus was first described by David Starr Jordan and John Otterbein Snyder in their account of fishes collected from the rivers of Mexico, published in the Bulletin of the United States Fish Commission for 1899. They originally placed it in the genus Characodon, as Characodon encaustus, working from an adult female holotype that Snyder had collected on 26 December 1898 in Lago de Chapala near Ocotlán, Jalisco. The genus Chapalichthys was erected a few years later, in 1902, by Seth Eugene Meek, who recognised that the Chapala-basin form warranted separation from Characodon; the genus name simply marks its association with Lake Chapala. Eschmeyer's Catalog of Fishes treats Chapalichthys encaustus (Jordan & Snyder, 1899) as the valid name, the parentheses recording the move out of the original genus.
The specific epithet encaustus is taken from the Greek enkaustos, "burnt in" — the same root that gives the painting technique encaustic — and refers to the series of dark lateral bars on the body, which the authors saw as resembling marks branded into the flank. The fish belongs to the family Goodeidae, the Mexican splitfins, and within it to the subfamily Goodeinae, the live-bearing goodeids. This is a tightly endemic lineage of the Mexican highlands, quite distinct from the New World poeciliid livebearers despite a superficially similar viviparous habit; the two groups evolved internal fertilisation and live birth independently.
Morphology
Chapalichthys encaustus is a modest-sized splitfin. Standard length reaches about 3.5 in in large females (Miller and colleagues report a maximum of 3.5 in SL), while FishBase lists total lengths of roughly 2.5 in for males and 3 in for females; males are the smaller, more slender sex. The body is moderately deep and laterally compressed over a silvery ground colour, crossed by a series of around nine short, narrow, dark vertical bars along the midline of the flank — the feature that gives the species both its scientific name and its English name, the barred splitfin. A yellow band typically edges the rear of the caudal fin, most strongly developed in males.
Sexual dimorphism is pronounced. Males develop a notably larger, taller dorsal fin and carry the goodeid "splitfin" structure — properly an andropodium — in which the first six or seven rays of the anal fin are shortened and set off by a notch from the rest of the fin, forming the intromittent apparatus used to transfer sperm. Females are more slender-finned and plainer, often showing a yellowish belly. The mouth is small and set for grazing, backed by bifid (two-cusped) teeth and a long, coiled intestine — the internal signature of a herbivore. An albinotic strain was once maintained in France, but wild fish show the silvery, dark-barred wild pattern.
Habitat
The species is endemic to Jalisco and Michoacán in west-central Mexico, within the lower Río Lerma drainage. Its range centres on Laguna de Chapala — Mexico's largest natural lake — together with the adjacent Río Grande de Santiago downstream to the Juanacatlán falls, the Río Duero, Laguna Cajititlán and the Canal Sahuayo. Two subpopulations are recognised: one in Lake Chapala itself (the type locality) and one in the lower Lerma system. Remaining strongholds include the Los Negritos lakes near Sahuayo and the Presas Verduzco and Orandino near Jacona de Plancarte. A population in Presa La Vega in the upper Río Ameca appears to be an accidental introduction around 2005, probably carried in with tilapia stocking.
It is a fish of quiet and slow-moving water — the margins of lakes, ponds, canals and river channels rather than open current. Vegetation in its habitats ranges from sparse to absent, with green algae, water hyacinth and Scirpus where present, over substrates of silt, mud, clay, sand and rock. The fish keeps to shallow water, rarely deeper than about 4 ft and usually under a metre, and tolerates turbid, even muddy conditions; one field collector recorded catching it in water of around 61 °F on a December morning with visibility under 4 in. FishBase characterises the species as freshwater, demersal and tropical, with recorded chemistry of pH 7.0–7.8, hardness around 10–20 dH and temperatures of roughly 68–82 °F.
Feeding
Chapalichthys encaustus is essentially herbivorous, and its anatomy reflects it: bifid teeth for rasping, a long convoluted gut for fermenting plant material, and a comb of around 20–28 slender gill rakers suited to gathering fine matter. In the wild it grazes the aufwuchs — the film of algae, microorganisms and detritus that coats submerged surfaces — and feeds on filamentous green algae. Its trophic level is estimated near 2.0, at the low, plant-leaning end of the scale. The fish is not strictly vegetarian, however; it has been recorded jumping clear of the surface to snatch flying insects, taking animal prey opportunistically when the chance presents itself.
In the aquarium it does best on a diet that respects this herbivorous bias. Algae-based and vegetable-rich foods, supplemented with fibre, keep the gut working as it should, while occasional small live or frozen invertebrate foods round out the diet and bring fish into breeding condition. A purely high-protein, meat-heavy regime suits it poorly; as with many grazing fishes, steady access to algal and plant matter matters more than rich feeding.
Mating
Reproduction in Chapalichthys encaustus is viviparous and internal, following the goodeid pattern rather than the poeciliid one. Fertilisation takes place inside the female, with the male using the andropodium — the notched, modified front portion of the anal fin — to transfer sperm. This is the goodeid equivalent of the gonopodium seen in poeciliids such as guppies and mollies, but it is a distinct structure: where the poeciliid gonopodium is a fused, rod-like organ formed from the whole anal fin, the goodeid andropodium is simply the anal fin with its leading rays shortened and separated by a notch. Goodeids also lack the long-term sperm storage that lets a guppy produce many broods from a single mating.
Courtship is tied to the warming of spring. Males, conspicuous with their enlarged dorsal fins and yellow caudal banding, display to the plainer females, and breeding is seasonal rather than continuous, with the young of the year appearing in late spring. Beyond this, fine detail of the species' courtship behaviour in the wild is data sparse; the broad picture is of a seasonally breeding, internally fertilising splitfin in which the male's enlarged fins and colour serve the same display role that bright spotting does in the poeciliids.
Breeding
As a goodeid, Chapalichthys encaustus is a matrotrophic livebearer: rather than provisioning the egg with yolk and then simply incubating it, the female nourishes her developing embryos throughout gestation through trophotaeniae — ribbon-like placental structures that extend from the embryo's vent and absorb nutrients from the maternal ovarian tissue. This produces relatively large, well-developed young in modest numbers, in contrast to the many small fry of egg-yolk-dependent livebearers. Gestation runs through the spring, with young reported born in late May. Brood sizes are small: Meek recorded a 2.5 in female carrying 21 near-term embryos, each about 0.5 in long.
The newborn fry are large and fully formed, and the species is an obliging colony breeder because adults rarely eat their own young — an unusual and welcome trait that lets fry grow up alongside their parents. Keepers report best results with a seasonal rhythm that mirrors the wild cycle: a cool winter rest below about 68 °F for two or three months, after which spawning resumes at around 68–70 °F; breeding tails off again above roughly 77 °F. A well-vegetated, algae-rich tank and high water quality carry the fry through, and because they take prepared and algal foods readily, raising successive generations is straightforward for a dedicated livebearer keeper.
In the aquarium
Chapalichthys encaustus is a peaceful, undemanding splitfin well suited to a dedicated goodeid or coolwater livebearer setup, and it is widely kept within the conservation-aquarium community precisely because it breeds reliably and rarely predates its fry. A broad, comparatively shallow tank suits it best — keepers recommend something on the order of 65 US gal or more for a colony — with gentle to moderate flow and, importantly, high oxygenation; figures around 8 mg/l or above are cited as a target. Large, frequent water changes of 60–80% per week keep the well-oxygenated, clean conditions the species favours.
The single most important husbandry point is temperature management on a seasonal cycle. These are subtropical-highland fish, not tropical ones: a cool winter rest below 68 °F for two to three months conditions them to spawn, with the breeding window opening around 68–70 °F and closing as the water passes about 77 °F. Sustained warm-tropical temperatures suppress breeding and shorten lifespan. Diet should be algae- and vegetable-led with fibre-rich foods, matching the fish's herbivorous gut. Kept this way — cool, clean, oxygen-rich and grazed — the barred splitfin is a hardy and rewarding subject.
Conservation
Chapalichthys encaustus is assessed as Vulnerable on the IUCN Red List, under criteria B1ab(i,ii,iii,iv)+2ab(i,ii,iii,iv), reflecting its restricted range and a continuing decline in extent, area, habitat quality and number of locations. Mexico's national list, NOM-059-SEMARNAT-2010, does not assign it a risk category, but the wild trajectory is downward. The species has already been extirpated from the mainstems of the Lerma and Santiago rivers, where pollution rendered the water uninhabitable, and it has declined within Lake Chapala itself under pressure from introduced fishes — notably the molly Poecilia sphenops and the mosquitofish Gambusia yucatana, which compete with and prey upon the native splitfins.
The story is the familiar one for the Mexican goodeids: a narrowly endemic, highland-lake-and-river fauna squeezed by habitat degradation, water extraction and invasive species across the Lerma–Chapala–Santiago basin. Surviving strongholds such as the Los Negritos lakes and the Presas Verduzco and Orandino are correspondingly important. Captive populations maintained by hobbyist conservation networks act as a genetic reservoir against further wild losses, which is much of why this species is so prominent in the livebearer-keeping community. For the aquarist, the responsible practice is to maintain documented, locality-based stock and never to release fish into local waters.