Taxonomy & naming
Gambusia regani was described by the American ichthyologist Carl Leavitt Hubbs in 1926, and Eschmeyer's Catalog of Fishes treats Gambusia regani Hubbs, 1926 as the valid name. It is placed in the genus Gambusia within the subfamily Poeciliinae of the family Poeciliidae, the New World live-bearing toothcarps, in the order Cyprinodontiformes. The species epithet honours Charles Tate Regan (1878–1943), the British ichthyologist and longtime worker at the British Museum (Natural History), whose work on fish classification was foundational to the field.
Gambusia is a large and taxonomically complex genus of small livebearers — the mosquitofishes — ranging across the southern United States, Mexico, Central America and the Caribbean. Many of its species are narrow-range endemics restricted to single drainages, springs or river systems, and the genus has been repeatedly revised as new forms have been recognised and old ones split. Gambusia regani belongs to this assemblage of regionally restricted Mexican species. The modern checklist treatment of the South and Central American Poeciliidae by Lucinda (2003) serves as a principal reference for the species' standing within the family.
Morphology
Gambusia regani is a small fish with the modest, unadorned appearance typical of the mosquitofishes. Reported maximum sizes are about 1.5 in total length for males and around 2 in for females, the larger female size reflecting the pronounced sexual size dimorphism that runs throughout the genus. The body is the compact, slightly elongate, dorsally flattened-headed form characteristic of Gambusia, built for life in shallow surface waters.
As in all poeciliids, the sexes are readily told apart by the anal fin: in mature males the front rays are modified into a slender, rod-like gonopodium used to inseminate females, while females retain a plain, fan-shaped anal fin and a deeper, fuller body, particularly when gravid. Detailed colour and meristic notes for this species are sparse in the accessible literature; like most mosquitofishes it is essentially plain — silvery to olive-grey, without the bright ornamentation of the more familiar aquarium livebearers — so identification within the genus relies on fin structure, body proportions and locality rather than on conspicuous colour.
Habitat
Gambusia regani is a freshwater, benthopelagic fish endemic to Tamaulipas in north-eastern Mexico, where it occupies a narrow geographic range. It inhabits small streams and small rivers and, especially, the adjacent shallow marshes and pools that fringe them — the warm, still, weedy edge habitats favoured by mosquitofishes generally.
The waters it lives in are tropical and warm, reported in the range of roughly 72–82 °F, with pH from about 6.5 to 7.8 and hardness up to around 25 dH — neutral to moderately hard, near-neutral conditions. This preference for shallow marginal waters, marshes and pools is typical of Gambusia, fishes that thrive in the warm, oxygen-poor, vegetation-choked shallows that many other fishes avoid. Because its entire known range is confined to a small part of one Mexican state, the species is inherently vulnerable to local habitat change even though it is hardy in the habitats it does occupy.
Feeding
Specific dietary studies for Gambusia regani are not available, but its feeding ecology can be inferred from its body form, habitat and the well-documented habits of its many congeners. With an estimated trophic level of about 3.2, it is a small carnivore-leaning omnivore — a micro-predator of the shallows.
Like mosquitofishes generally, it almost certainly feeds on small aquatic invertebrates, insect larvae (including mosquito larvae, the basis of the group's common name and reputation), zooplankton and other tiny drifting and surface prey, supplemented by algae and organic detritus taken opportunistically from the marsh and pool margins it inhabits. Its upturned mouth and surface-oriented body plan suit feeding at and just below the water surface, where mosquito larvae and fallen insects concentrate. In the absence of species-specific data, the diet is best described as that of a generalist micro-predator of warm, shallow, weedy waters.
Mating
Gambusia regani is a viviparous livebearer, and reproduction is internal — the defining trait of the family Poeciliidae. Fertilisation takes place inside the female after the male transfers sperm using the gonopodium, the modified anal fin that gives male poeciliids their characteristic rod-like organ. In Gambusia, males are typically much smaller than females and reproduction is driven largely by persistent male pursuit and frequent sneak insemination attempts rather than by elaborate courtship, a mating system shared across the mosquitofishes.
Detailed observations of courtship behaviour specific to G. regani are not recorded in the accessible literature, and on this point the data are sparse. What is documented is the reproductive output: sexual maturity is reached after roughly three months, allowing the species to turn over generations quickly. As in other Gambusia, females are likely able to store sperm internally, so that a single insemination can fertilise more than one successive brood — a capacity that, combined with rapid maturation, underlies the high reproductive resilience reported for the species.
Breeding
After internal fertilisation the female Gambusia regani carries the developing embryos and gives birth to free-swimming young rather than laying eggs. Gestation lasts about 24 days, after which the female produces a brood of roughly 10 to 20 fully formed fry. With sexual maturity reached at around three months, populations can grow quickly, and FishBase reports a high resilience for the species, with a minimum population doubling time of under fifteen months.
The newborn fry are miniature, independent fish able to swim and feed immediately. As is usual among Gambusia and other poeciliids, there is no parental care, and adults — including the mother — will readily prey on their own young, so in the wild the fry depend on dense marginal vegetation for cover. Superfetation, the simultaneous carrying of broods at different developmental stages, occurs in some poeciliids but has not been specifically documented for this species; the available reproductive data describe a straightforward single-brood gestation of roughly three to four weeks producing a modest brood of live young.
In the aquarium
Gambusia regani is not a commonly kept aquarium fish — it is a plain, narrow-range Mexican endemic of scientific and conservation interest rather than an ornamental livebearer — and dedicated hobby husbandry accounts are scarce. Where it is maintained, it is a specialist's or conservation aquarist's fish kept by livebearer enthusiasts rather than a general community species.
Its requirements can be read straight from its native habitat: warm water in the low-to-high 20s Celsius (about 72–82 °F), near-neutral to moderately hard water (pH roughly 6.5–7.8, hardness to about 25 dH), and the planted, shallow, gently moving set-up that suits small mosquitofishes. Like other Gambusia it would be expected to be hardy and easy to breed, but also potentially nippy and best kept among robust tankmates or in a single-species tank. As with all Gambusia, one firm caution applies: members of this genus are notorious invaders when released outside their range, so any captive stock must never be allowed to enter local waters.
Conservation
Gambusia regani is assessed as Data Deficient (DD) on the IUCN Red List, with the most recent assessment dated 28 March 2018. The Data Deficient category means there is not enough information on the species' distribution, population status and trends to assign it to a threatened or non-threatened category — it is a statement of uncertainty rather than of safety. CITES and CMS list the species as Not Evaluated.
That uncertainty is itself a concern, because G. regani is a narrow-range endemic confined to a small part of Tamaulipas, Mexico. Small-range freshwater fishes restricted to streams, marshes and pools in a single region are intrinsically exposed to localised threats — water extraction, habitat modification, pollution and the introduction of non-native fishes — and a Data Deficient listing flags exactly the kind of species that may be at risk but lacks the survey data to demonstrate it. For an aquarist, the responsible posture toward such a fish is twofold: support the documentation and captive maintenance of poorly known endemics, and never release Gambusia of any kind into wild waters, where the genus's hardiness and live-bearing fecundity make it a damaging invader.