Taxonomy & naming
Gambusia zarskei was described in 2010 by Manfred Meyer, Susanne Schories and Manfred Schartl in the journal Vertebrate Zoology (60(1): 11–18), from material collected in the upper Rio Conchos system of Chihuahua, Mexico. The specific epithet honours Dr Axel Zarske, an ichthyologist at the Senckenberg Natural History Collections in Dresden, recognised for his work on the conservation biology of endangered fishes. The genus name Gambusia derives from the Cuban Spanish word "gambusino", used in the phrase "pescar gambusinos" — to fish for nothing, or to come back empty-handed — a wry reference to how little a netful of these small fishes amounts to. Eschmeyer's Catalog of Fishes treats Gambusia zarskei Meyer, Schories & Schartl, 2010 as the valid name.
Within the family Poeciliidae, the species sits in the subfamily Poeciliinae and the genus Gambusia, where it is placed in the subgenus Arthrophallus and assigned to the senilis species group. That group includes a cluster of restricted-range northern-Mexican and Texan relatives — among them G. senilis, G. hurtadoi, G. alvarezi, G. gaigei and the extinct G. amistadensis. G. zarskei is separated from these close kin chiefly by details of the male gonopodium (the arrangement of rays and the position of the terminal elbow and hooks), together with a count of eight dorsal-fin rays and the presence of a distinct black anal spot. Like much of Gambusia, the group is morphologically conservative, so diagnosis rests heavily on the fine structure of the male copulatory organ rather than on outward colour or shape.
The genus Gambusia is one of the largest in the Poeciliidae and has a complicated nomenclatural history, with many narrow endemics of the desert springs and rivers of Mexico and the southwestern United States described only in recent decades. G. zarskei is a product of that ongoing work — a population long lumped with or overlooked among its senilis-group neighbours until the 2010 study formally separated it.
Morphology
Gambusia zarskei is a very small fish, in keeping with the diminutive scale typical of the genus. Reported maximum sizes are about 1 in standard length for males and unsexed individuals and roughly 1 in standard length for females; as in most Gambusia, males generally stay under about 1 in and females under about 1.5 in. The body is the unremarkable, slightly stocky form of a mosquitofish — pale and lightly pigmented, without the bright coloration of the better-known hobby livebearers — and the species shows the modest sexual size dimorphism usual for the family, with females the larger and deeper-bodied sex.
Diagnostic features recorded in the original description and in FishBase include eight dorsal-fin rays, 30 to 31 vertebrae, and a conspicuous black anal spot. The single most important character for identification, however, is the male gonopodium: the species is distinguished from its senilis-group relatives largely by the configuration of the gonopodial rays and the form of the terminal hooks and elbow, the standard currency of Gambusia systematics. Beyond these traits the published morphological detail is sparse, reflecting how recently and from how limited a range the species was described.
Habitat
Gambusia zarskei is a freshwater, pelagic fish of subtropical streams, and is known only from the upper Rio Conchos system in the Sierra San Diego area, east of San Diego de Alcalá, in the state of Chihuahua, Mexico — roughly between 28° and 29° N and 105° and 106° W. The Rio Conchos is the largest Mexican tributary of the Rio Grande, and its upper reaches drain the high country of the Sierra Madre Occidental and the adjoining Chihuahuan Desert, a region whose isolated springs and headwater streams harbour a remarkable concentration of narrow-range endemic fishes.
The water it occupies is warm and hard: recorded temperatures fall in the range of about 75–79 °F, with reported pH around 7.9 or higher, consistent with the alkaline, mineral-rich waters of the limestone desert drainages. As a small, surface-oriented livebearer it favours the shallow, sheltered margins and slower flows where such fishes feed and shelter. Beyond these few figures the ecological data are sparse — the species is known from a very small number of collections within a single restricted area, which is precisely what underlies its conservation concern.
Feeding
No detailed diet study has been published for Gambusia zarskei, but its feeding can be inferred with confidence from its small size, surface-oriented habits and its place in the genus. FishBase places it at a trophic level of about 3.1, the value of a small omnivorous micro-predator. Like other mosquitofishes it can be expected to take small aquatic invertebrates, insect larvae — including mosquito larvae, the feeding habit that gives the genus its English name — zooplankton, and incidental algae and detritus from the margins and surface film of its stream habitat.
Members of Gambusia are opportunistic feeders that adjust their intake to whatever small prey is locally abundant, and there is no reason to think this narrow-range endemic differs. Because the species is essentially unknown in aquarium culture, there are no established captive feeding notes; the genus as a whole accepts small flake, micro-pellets and finely divided live and frozen foods readily, so any husbandry would follow that pattern. Where specifics are concerned, the honest statement is that data are sparse.
Mating
Like all poeciliids, Gambusia zarskei reproduces by internal fertilisation and is a livebearer — the original description notes no reproductive specifics, but its placement in the family settles the basic biology. The male transfers sperm to the female using the gonopodium, a slender intromittent organ formed from modified rays of the anal fin; in Gambusia this structure is elaborately built, ending in a set of hooks and an elbow whose exact geometry is species-specific and, as noted above, central to telling G. zarskei from its relatives.
Mosquitofish mating tends to be coercive rather than courtship-based: instead of the displays seen in guppies, Gambusia males typically pursue females and attempt rapid, surreptitious copulations, thrusting the gonopodium forward to make contact. Females can store viable sperm internally for extended periods, allowing a single insemination to fertilise more than one successive brood. No mating observations specific to G. zarskei have been published, so this account reflects the well-established pattern of the genus rather than direct study of the species.
Breeding
Gambusia zarskei is viviparous: after internal fertilisation the female carries the developing embryos and gives birth to free-swimming young rather than laying eggs, the defining reproductive mode of the family Poeciliidae. The published description does not report brood sizes, gestation length or developmental detail for this species, so the particulars here are drawn from the consistent biology of Gambusia rather than from direct observation.
In the genus generally, gestation lasts on the order of a few weeks and varies with temperature, broods are modest in number and scale with female size, and stored sperm lets a female produce several broods in succession from one mating. FishBase rates the species' resilience as high, with an estimated population-doubling time of under fifteen months — the rapid reproductive turnover characteristic of small livebearers, and the reason mosquitofishes recover quickly where conditions allow. Whether G. zarskei displays superfetation — the overlapping of broods at different developmental stages seen in some poeciliids — has not been documented, and given the sparse data the safe statement is that it is unknown for this species.
In the aquarium
Gambusia zarskei is essentially not an aquarium fish. It is a narrow-range endemic known only from a small area of the upper Rio Conchos, with no established presence in the hobby and no published captive-husbandry record. It is mentioned here for completeness rather than as a candidate for the home aquarium, and a Vulnerable, restricted-range species of conservation interest is best left to managed conservation programmes rather than the ornamental trade.
Were it ever maintained — for example in an ex-situ conservation context — its requirements would follow those of its desert-stream relatives: warm water around 75–79 °F, hard and alkaline chemistry with a pH near or above 7.9, and the clean, well-oxygenated conditions of a flowing-water habitat. As with other Gambusia, the genus carries a deserved reputation for fin-nipping and aggression toward tankmates, so it would not suit a mixed community. These are general inferences from the genus and the species' recorded water parameters; specific captive-care data for G. zarskei are not available.
Conservation
Gambusia zarskei is assessed as Vulnerable on the IUCN Red List, under criterion D2, with the assessment dated 4 March 2019 (species ID 140314366). Criterion D2 applies to species with a very restricted range — a small number of locations or a tiny area of occupancy — that are consequently vulnerable to the effects of plausible future threats capable of driving the species to Critically Endangered or Extinct in a very short time. The category reflects not a documented decline so much as the inherent fragility of a fish confined to a single small part of one river system.
That confinement is the heart of the concern: G. zarskei is known only from the upper Rio Conchos in the Sierra San Diego area of Chihuahua, a region of the Chihuahuan Desert where spring and headwater habitats are under pressure from water extraction, agriculture, drought and the introduction of non-native fishes. The desert drainages of northern Mexico have already lost endemic livebearers — the closely related G. amistadensis among them — so the precariousness of a single-locality species like this one is far from theoretical.
Because the published biology is so limited, the most useful conservation contribution for this species is straightforward: protect the water it lives in. Maintaining the flow, quality and integrity of the upper Rio Conchos habitat, and keeping introduced species out of it, are the measures that matter for a narrow-range endemic whose entire known world is a few kilometres of desert stream.