Taxonomy & naming
Poecilia teresae was described by Donald W. Greenfield in 1990, in Copeia, from material collected in the Mountain Pine Ridge of Belize (Greenfield, D.W. 1990. Poecilia teresae, a new species of poeciliid fish from Belize, Central America. Copeia 1990: 449–454). The specific epithet is an eponym: the species is named for Teresa "Terry" Greenfield, the senior author's wife, which is also the origin of the alternative common name "Terry's molly." Eschmeyer's Catalog of Fishes treats Poecilia teresae Greenfield, 1990 as the valid name, placing it in the genus Poecilia within the family Poeciliidae, the New World livebearers.
The genus name Poecilia derives from the Greek poikilos, "with many colours" or "varied," a reference to the variegated patterning typical of these fishes. Within Poecilia, teresae belongs to the short-finned mollies — the subgenus Mollienesia as revised by Palacios and colleagues in 2016 — and phylogenetic work has placed it within the Poecilia sphenops species complex (Bagley et al. 2015). Earlier relationships within this group were examined by Rodríguez (1997, Copeia), which remains a principal reference for the species' systematic placement.
The species is geographically restricted, and its records have been refined since the original description. Poeser (2003) revisited the type material, and additional collections have come from Mount Pleasant Creek near Belmopan (Ake 2005). Records reported from the Calakmul Biosphere Reserve in Mexico (Vega-Cendejas et al. 2013) are regarded as questionable and in need of verification, so the confirmed range remains southern Belize.
Morphology
Poecilia teresae is a small short-finned molly. Reported maximum body size is about 2 in standard length (Lucinda 2003; FishBase Ref. 30499), placing it among the smaller members of its genus. As with mollies generally, the body is moderately deep and laterally compressed, with a dorsally flattened head and an upturned mouth suited to grazing at and near the surface.
Detailed published descriptions of live coloration are sparse — the fish is rarely kept or photographed, and FishBase carries no image on file — so claims about its appearance should be made cautiously. The diagnostic characters separating teresae from other Belizean mollies are set out in Greenfield's 1990 original description and rest on meristic and morphometric features (fin-ray and scale counts, body proportions and gonopodial structure) rather than on conspicuous colour. As in all poeciliids, the most reliable external sex character is the male's anal fin, modified into a gonopodium.
Beyond the original description, the morphological literature on this species is thin. Where precise counts or measurements are needed, the Greenfield (1990) paper and Poeser's (2003) re-examination of the type series are the appropriate primary sources rather than secondary summaries.
Habitat
Poecilia teresae is endemic to the Macal River drainage in the Maya Mountains of southern Belize, in the Cayo District around the Mountain Pine Ridge. The type locality is a small stream in the Mountain Pine Ridge Forest Reserve (Greenfield 1990; Poeser 2003), and the species has also been recorded at Mount Pleasant Creek south of Belmopan (Ake 2005). Its extent of occurrence is small — IUCN figures range from roughly 55–497 mi² — and it is treated as effectively a single-location species, occupying just one to three locations.
The characteristic habitat is small upland streams and isolated pools in tropical pine forest: clear, cool, fast-flowing water cutting through granitic rock and often terminating in high waterfalls. Greenfield (1990) noted that these waterfalls block the upstream movement of other fishes, leaving the headwater reaches above them isolated — a setting that helps explain how a narrowly endemic molly persists in this rugged terrain. The IUCN classifies the habitat as permanent inland rivers, streams and creeks, including waterfalls, within a freshwater system.
The species appears genuinely rare within this restricted range. A drainage-wide survey of the Monkey Tail branch of the Macal River yielded only a single individual (Esselman & Allan 2010), and the IUCN records the population trend as unknown. Near Belmopan, populations occupy small streams and dry-season pools that are more exposed to human pressures than the remote reserve interior.
Feeding
No detailed dietary study of Poecilia teresae has been published, and specifics for this species are data-sparse. FishBase lists an estimated trophic level of about 2.6, consistent with a largely herbivorous-to-omnivorous diet typical of short-finned mollies, which graze heavily on algae (aufwuchs) and detritus and supplement this with small invertebrates.
In the clear, fast upland streams it inhabits, the most plausible food base is the periphyton — algae and biofilm growing on rock and submerged surfaces — together with small aquatic invertebrates and drifting organic matter. This grazing habit fits the molly body plan, with its upturned mouth and flattened head adapted to scraping surfaces near the water's edge.
Because the species is essentially absent from aquarium culture, there are no established captive-feeding notes for it specifically. By analogy with its congeners, a diet emphasising algal and vegetable matter alongside small live and prepared foods would be expected to suit it, but this is inference from the genus rather than documented practice for teresae.
Mating
Like all poeciliids, Poecilia teresae has internal fertilisation. The male transfers sperm to the female using the gonopodium, an intromittent organ formed from modified rays of the anal fin — the defining reproductive structure of the New World livebearers. No courtship behaviour has been described specifically for this species, but short-finned mollies of the subgenus Mollienesia typically combine male display with persistent mating attempts rather than building nests or defending breeding territories.
As in other mollies, females can be expected to store viable sperm internally, allowing a single mating to fertilise more than one successive brood. Superfetation — the simultaneous carrying of broods at different developmental stages — is not documented in the Mollienesia mollies, including teresae; among poeciliids it is instead a feature of other lineages such as Poeciliopsis and Heterandria. For teresae specifically, the published record on reproductive behaviour is essentially empty, so these statements are drawn from the well-characterised biology of the subgenus to which it belongs.
What can be said with confidence is the broad mode: internal fertilisation via a gonopodium, female sperm storage, and viviparity. Anything finer — display sequences, mate preferences, or population-specific mating tactics — has not been studied in this rarely encountered fish.
Breeding
Poecilia teresae is viviparous, giving birth to free-swimming young rather than laying eggs, as is universal among the live-bearing poeciliids. Mollies of the subgenus Mollienesia are lecithotrophic — the developing embryos are nourished chiefly by yolk laid down before fertilisation, with little or no maternal provisioning during gestation (low matrotrophy) — rather than placental. No species-specific brood data have been published for teresae itself.
Drawing on the general biology of the genus, gestation in tropical mollies runs on the order of three to six weeks, with the exact period depending strongly on temperature, and females produce successive broods using stored sperm. Brood size in mollies scales with female size and condition, but no counts are available for this particular species, so any figure would be a guess; the honest position is that the reproductive output of teresae is undocumented.
The newborn fry of mollies are fully formed, capable of swimming and feeding from birth, and in the wild rely on cover and microhabitat to escape predation. For teresae, the isolated upland pools and waterfall-bounded stream reaches it occupies presumably serve that role, but no field study of recruitment or fry ecology in this species has been carried out.
In the aquarium
Poecilia teresae is not a fish of the aquarium hobby. It is rarely kept, is not utilised in trade, and there is no established body of husbandry experience for it; the IUCN explicitly notes the absence of any trade use. As an Endangered, narrowly endemic species, it is in any case a poor candidate for casual collection, and aquarists are far more likely to encounter its widespread relatives such as the common molly or the sailfin mollies.
The species' natural conditions — clear, cool, fast-flowing, well-oxygenated upland streams over granite — differ from the warm, still water in which mollies are usually maintained. Any responsible attempt to hold teresae would therefore need to reproduce that flowing, oxygen-rich, comparatively cool stream environment rather than the standard molly setup, and would belong to a conservation or research programme rather than to general fishkeeping.
Because documented experience is effectively nil, no care rating beyond "specialist / not in the hobby" can be given honestly. The practical takeaway for aquarists is simply that this is a wild endemic best left in situ, of interest mainly as an example of how isolated mountain drainages generate restricted-range livebearers.
Conservation
Poecilia teresae is assessed as Endangered on the IUCN Red List under criterion B1ab(iii) (Lyons, T.J. 2020. Poecilia teresae. The IUCN Red List of Threatened Species 2020: e.T191754A2002441; assessed 10 October 2019, published 2020). The listing reflects a very small extent of occurrence, occupancy of effectively a single location, and a continuing decline in habitat quality. The population trend is recorded as unknown, and the species appears genuinely rare: a drainage-wide survey of the Monkey Tail branch of the Macal River yielded only one individual (Esselman & Allan 2010).
The principal threats are tied to development in the Mountain Pine Ridge and Chiquibul region: hydropower and dam construction on the Macal drainage, logging, and mining and quarrying. Near Belmopan, the streams and dry-season pools the species occupies face domestic pollution, urban and agricultural expansion, and cattle grazing, with military exercises reported to cause solid-waste accumulation in seasonal pools. Because the species is confined to so few sites, any one of these pressures can affect a large fraction of the global population at once.
The species does occur within protected areas, including the Mountain Pine Ridge Forest Reserve and likely the Chiquibul National Park, which offers some buffer against habitat loss. Even so, the combination of a tiny range, apparent rarity and active development pressure underlies its Endangered status. Conservation here is fundamentally about safeguarding the integrity of a handful of Maya Mountain streams rather than managing harvest, since the fish is not exploited.