Taxonomy & naming
Gambusia sexradiata was described by the American ichthyologist Carl Leavitt Hubbs in 1936. Eschmeyer's Catalog of Fishes treats Gambusia sexradiata Hubbs, 1936 as the valid name, placing the species in the genus Gambusia within the family Poeciliidae and the subfamily Poeciliinae — the New World livebearers. The order is Cyprinodontiformes, the toothcarps, which unites the killifishes and the live-bearing poeciliids.
Gambusia is a large and taxonomically demanding genus of small, sexually dimorphic livebearers centred on Mexico, the southern United States, the Greater Antilles and Central America. Several congeners overlap or border the range of G. sexradiata — among them the widespread Gambusia yucatana of the Yucatán and Belize — and field identification within the genus relies on fine details of the male gonopodium and fin-ray counts rather than colour, since the fishes are superficially alike. The species epithet itself, sexradiata, points to such a meristic feature.
The English name in circulation for the species is "teardrop mosquito"; in Spanish it is known as guayacón del sureste, the "southeastern guayacón," guayacón being a regional Mexican name applied broadly to small poeciliids. As with much of the genus, the bulk of the documented natural history comes from regional faunal work rather than from a dedicated species monograph; Greenfield and Thomerson's account of the fishes of the continental waters of Belize is a principal reference for the species as it occurs in the southern part of its range.
Morphology
Gambusia sexradiata is a small fish. FishBase records a maximum of about 1.5 in standard length in males or unsexed specimens and roughly 1.5 in standard length in females — figures that place it among the smaller members of an already small-bodied genus. Standard length here is measured to the base of the tail rather than the tip, so total length runs somewhat greater. The body is the typical Gambusia shape: compact, slightly compressed, with a flattened dorsal profile and an upturned mouth suited to feeding at and just below the water surface.
Colouration is muted and functional rather than ornamental — the fish are predominantly grey to olive-silver, fitting the cryptic, predator-exposed life of a small surface-dwelling poeciliid in open shallows. As in other mosquitofishes, the sexes differ: males are smaller and more slender than the deeper-bodied females, and gravid females develop the dark abdominal patch common to pregnant livebearers. Detailed pigment patterning specific to this species is sparsely documented; the most diagnostic structure is the male gonopodium, the modified anal fin used in mating, whose form distinguishes G. sexradiata from its congeners.
Habitat
The species is a benthopelagic inhabitant of warm lowland waters along the Atlantic (Gulf and Caribbean) slope of southern Mexico and northern Central America. Its range runs from the Río Nautla in northern Veracruz southward, taking in the Atlantic drainages of inland Mexico in Oaxaca and Chiapas, presumably across the base of the Yucatán Peninsula to Quintana Roo, and south to southern Belize in the Moho River drainage; it also occurs in the Río Usumacinta drainage and in isolated waters of the Petén region of Guatemala.
Gambusia sexradiata is a fish of freshwater and brackish water alike, found in both standing and flowing habitats — ponds and the margins of rivers — and typically near shore where aquatic vegetation is present. Like other Gambusia of the region it keeps to shallow, sunlit edges, the same kind of shoreline cover that gives small surface-feeders both food and refuge. FishBase characterises its environmental tolerances as broadly tropical, with a temperature range of about 72–79 °F, a pH of roughly 6.5–7.8 and hardness up to around 18 dH. This blend of warmth-loving, low-flow, vegetation-edge habitat and tolerance of brackish conditions is consistent across the mosquitofishes and underlies their success in disturbed and coastal waters.
Feeding
Gambusia sexradiata feeds mainly on benthic, vegetation-associated invertebrates together with terrestrial organisms taken from the shoreline and water surface, and FishBase places it at a trophic level of about 3.5 — squarely in the small-carnivore range. The upturned mouth and surface orientation typical of the genus suit a fish that skims small drifting and floating prey as well as picking invertebrates from submerged plants.
As with mosquitofishes generally, the diet centres on small aquatic invertebrates, and members of the genus are well known for consuming mosquito larvae and other insect larvae at the surface — the behaviour behind the common name and behind the historical use of Gambusia in mosquito control. Detailed quantitative diet studies specific to G. sexradiata are sparse, but its feeding ecology fits the familiar poeciliid pattern of an opportunistic micro-predator working the vegetated shallows.
Mating
Reproduction in Gambusia sexradiata is internal and live-bearing, as in all poeciliids. Fertilisation occurs inside the female, and the male's key adaptation is the gonopodium — a slender intromittent organ formed from modified rays of the anal fin, used to transfer sperm. In Gambusia the gonopodium is comparatively long and tipped with small hooks and spines, and its precise structure is one of the chief characters by which species in the genus are told apart.
Mosquitofishes are not courting fishes in the elaborate sense of some poeciliids; males of Gambusia typically pursue females and attempt rapid copulations rather than performing prolonged display, and a substantial part of male mating effort takes the form of these forced or sneaked matings. Species-specific behavioural detail for G. sexradiata is not well documented, but it is expected to follow this genus-wide pattern of small, persistent males and the internal-fertilisation system that defines the family. As in other poeciliids, females are able to store sperm internally, so a single mating can fertilise more than one successive brood.
Breeding
Gambusia sexradiata is viviparous, giving birth to live, free-swimming young rather than laying eggs. FishBase, drawing on regional reproductive data, reports a gestation of roughly 28 days, after which the female produces a brood of about 10 to 35 young; sexual maturity is reached at around four months of age. These figures sit comfortably within the range expected for a small Gambusia and reflect the genus's strategy of frequent, modest broods rather than large single spawnings.
Newborn poeciliids are fully formed miniatures, able to swim and feed from birth, and in Gambusia they immediately need the cover of vegetated shallows to avoid predation — including cannibalism by adults of their own kind, a common feature of the genus. The combination of live birth, early maturity and short generation time gives the species a high reproductive resilience: FishBase estimates a minimum population doubling time of under fifteen months. It is precisely this capacity for rapid increase that makes mosquitofishes effective biological agents where they are native and aggressive colonisers where they are not.
In the aquarium
Gambusia sexradiata is not a common aquarium fish, and it is best understood as a hardy, plain regional livebearer rather than an ornamental one. The husbandry that suits it follows directly from its habitat: warm water in the low-to-mid 20s Celsius, near-neutral pH around 6.5–7.8, moderate hardness, and — like several coastal Gambusia — a tolerance of slightly brackish conditions. Densely planted, vegetated margins replicate the shoreline cover the species uses in the wild and give females and fry somewhere to retreat.
As with mosquitofishes generally, the practical cautions are behavioural rather than chemical. Gambusia can be nippy and persistent, males harass females, and adults will eat fry, so an aquarist breeding the species should provide heavy plant cover or separate the young. The most important consideration, however, lies outside the tank: Gambusia are among the world's most damaging introduced fishes when released outside their range, and a small, prolific, brackish-tolerant species like this one should never be discarded into local waters. Responsible keeping means never releasing it.
Conservation
Gambusia sexradiata is assessed as Least Concern on the IUCN Red List, in an assessment dated 28 March 2018. The species is widespread across the Atlantic-slope lowlands of southern Mexico, Belize and Guatemala, tolerant of a broad range of conditions including brackish water, and reproductively resilient — none of which suggests significant range-wide threat.
As with its relatives, the conservation note for Gambusia sexradiata runs in two directions. Within its native range it is an unremarkable, abundant component of the lowland fish fauna and is not of conservation concern. But the genus as a whole is a cautionary one: Gambusia species moved beyond their natural ranges for mosquito control have become serious invasives elsewhere in the world, displacing native fishes and amphibians. For an atlas reader the takeaway is the familiar inversion — the species needs no protection at home, but its hardiness and fecundity mean it must never be released where it does not belong.