Taxonomy & naming
Poecilia catemaconis was described by the American ichthyologist Robert Rush Miller in 1975, with Lake Catemaco in Veracruz, Mexico, as its type locality — the lake from which the species takes both its name and its entire known distribution. Eschmeyer's Catalog of Fishes treats Poecilia catemaconis Miller, 1975 as the valid name, placing it in the genus Poecilia within the family Poeciliidae, the New World livebearers. Within Poecilia it falls in the subgenus Mollienesia, the group that contains the mollies proper, and the fish is in every respect a molly in body plan, water preference and grazing habit.
The family Poeciliidae unites the guppies, mollies, platies, swordtails and their relatives — small, internally fertilising, mostly live-bearing toothcarps of the Americas. The Catemaco molly sits among the Mexican mollies of this assemblage, several of which are tied to particular drainages or lakes in the lowlands of the Gulf slope. The most substantial modern treatment of the species is Meyer and Radda's 2000 paper in the Annalen des Naturhistorischen Museums in Wien, which remains the principal reference behind the data recorded for it in FishBase.
Morphology
Poecilia catemaconis is a modest-sized molly with clear sexual size dimorphism: females are the larger sex, reaching about 4 in in total length, while males top out at roughly 3 in. As in other mollies the female is the deeper-bodied, plainer fish, described by keepers as drab with only a slight blue iridescence over the abdomen, whereas the male carries the colour — the basis of the "bicolor" common name — with yellow-orange and iridescent tones distributed through the anal, caudal and dorsal fins.
The defining male structure is the gonopodium, a slender intromittent organ formed from modified rays of the anal fin and used to transfer sperm to the female internally, the hallmark of the live-bearing poeciliids. An interesting wrinkle reported by hobbyists keeping the species is that males appear to develop along two schedules: some mature early and stay comparatively small, while others are "late-developing," retaining a female-like anal-fin shape for four to six months before the gonopodium forms and they take on full male coloration. The overall form is otherwise that of a typical benthopelagic molly — a compact, surface-and-midwater grazer rather than a deep-bodied or elongate specialist.
Habitat
The Catemaco molly is a narrow endemic, known only from Lake Catemaco in the state of Veracruz, southern Mexico — a large freshwater lake of volcanic origin set in the Los Tuxtlas uplands near the Gulf coast. FishBase records the species as freshwater, benthopelagic and non-migratory, occupying the tropical waters of the lake rather than its inflowing or outflowing streams. Its confinement to a single lake basin is the central fact of its biology and the reason its conservation status attracts attention.
The water it favours is hard and alkaline, in keeping with its molly lineage. FishBase gives a pH range of about 7.2 to 8.2 and a wide hardness tolerance of roughly 6 to 30 dH, with a tropical temperature range of about 75–82 °F. A hobbyist keeping the fish measured tank water of around pH 7.5, a carbonate hardness near 6 dKH and a very high general hardness (over 400 ppm) on an aragonite substrate, and reported the fish thriving — consistent with the species' preference for hard, alkaline, mineral-rich conditions and its reported tolerance of slightly brackish water typical of the molly group.
Feeding
Poecilia catemaconis is a low-trophic-level grazer: FishBase places it at a trophic level of about 2.5, indicating a diet leaning toward plant matter and detritus rather than animal prey. In the wild this means the typical molly regime of aufwuchs — the film of algae, diatoms, microorganisms and organic debris that coats submerged surfaces — supplemented by whatever small invertebrates it encounters while grazing. As a lake-dwelling molly it works the shallows and surfaces where this growth is richest.
In the aquarium the species is undemanding and accepts standard prepared foods, but its grazing biology means a meaningful share of vegetable and algal matter benefits it: spirulina-based flakes or pellets, blanched vegetables, and a well-lit tank that grows some algae for the fish to browse. Hobbyists raising the species have used small-fish powdered feeds for growing stock and report no special difficulty in feeding adults, which take ordinary aquarium fare readily. As with other mollies, a diet skewed too far toward protein-rich foods is less suitable than one that respects the fish's herbivore-leaning natural diet.
Mating
Reproduction in Poecilia catemaconis is internal and live-bearing, as in all poeciliids. Fertilisation takes place inside the female: the male uses the gonopodium — the modified anal fin — to transfer sperm, and there is no external spawning or egg-laying. As reported by keepers, mating behaviour follows the familiar molly pattern, with males pursuing females and engaging in dominance chasing and light fin-nipping among themselves, the larger and more developed males pressing their attentions on the females.
The two-track male development noted under morphology has a behavioural dimension as well: early-maturing males begin courting and competing while still small, whereas late-developing males spend months looking and behaving more like females before the gonopodium forms and they enter the breeding hierarchy. Detailed wild data on courtship, sperm storage and superfetation are sparse for this particular species; while sperm storage is widespread in poeciliids and superfetation occurs in some Poecilia, there is no firm published evidence specific to P. catemaconis, so it is better noted as data-sparse than asserted.
Breeding
Poecilia catemaconis is viviparous, the female carrying the developing young internally and giving birth to free-swimming fry rather than laying eggs. Hobbyist observations of the species in captivity describe a female producing broods at roughly monthly intervals, with brood size increasing as she matured — a first birth of only two fry, growing through about five to around sixteen fry in later broods. This pattern of small early broods enlarging with the female's size and age is characteristic of livebearers generally.
The newborn fry are fully formed miniature fish, able to swim and feed immediately. Keepers have raised them on newly hatched brine shrimp and finely powdered small-fish foods, and report that — unlike guppies — the adults of this species largely leave the fry alone, with low predation on the young observed in the aquarium. That relatively low fry predation makes the Catemaco molly comparatively easy to breed in numbers for a livebearer, an important consideration given its conservation profile and its scarcity in the hobby.
In the aquarium
Poecilia catemaconis is a rare fish in the hobby, circulating chiefly among specialist livebearer keepers and aquarium societies; recent captive stock in the United States traces in part to collector donations distributed to clubs in 2025–2026. It is, however, a forgiving fish to keep once obtained. Hobbyist accounts describe it as hardy and untroubled by illness in suitable water, kept successfully in a tank as small as a 20-gallon long with only infrequent (roughly 20% biweekly) water changes — a robustness in keeping with its molly heritage.
Water chemistry is the one thing to get right: like other mollies it wants hard, alkaline, mineral-rich water, comfortably around pH 7.2 to 8.2 and high in general hardness, with an aragonite or crushed-coral substrate helping to buffer and harden the water. Temperatures in the mid-20s Celsius suit it. Behaviour is typical molly — males chase and lightly nip as they establish a pecking order and pursue females — so a group with a female-skewed sex ratio reduces harassment of any single fish. For a keeper interested in conserving an endemic species in captivity, its willingness to breed and its low fry predation make it a rewarding and manageable project.
Conservation
Poecilia catemaconis has not been formally assessed by the IUCN Red List: FishBase (Version 2025-2, Global) records it as Not Evaluated, and no published IUCN assessment or species assessment ID could be located. The absence of a Red List entry should not be read as an absence of concern. The species is a single-lake endemic, confined entirely to Lake Catemaco, and a USGS/American Fisheries Society compilation cited through FishBase lists it as vulnerable (criteria 1, 2 and 5) — a designation that reflects exactly that narrow, one-basin distribution rather than a formal IUCN category.
Any single-lake endemic is inherently exposed: a fish with no second population has no refuge from lake-wide threats such as pollution, water-level change, habitat alteration, or the introduction of non-native species into its only home. Lake Catemaco is a populated, economically used lake, and the molly is recorded by FishBase as having both commercial fisheries and aquarium-trade interest. Given that profile, deliberate maintenance of healthy captive populations among livebearer specialists has real conservation value as a hedge, and the standard aquarist caution applies in reverse to the usual molly concern: stock should be kept and bred responsibly, never released, since the species' true scarcity is in the wild, not the tank.