Taxonomy & naming
Cyprinodon macrolepis was described in 1976 by the American ichthyologist Robert Rush Miller, part of a prolific run of Chihuahuan Desert pupfish descriptions Miller produced through the 1960s and 1970s documenting the remarkable, largely single-spring endemism of Cyprinodon across the Bolsón de Mapimí and neighbouring interior drainage basins of Chihuahua and Coahuila, Mexico. The specific epithet macrolepis (Greek for "large-scaled") refers to the comparatively large body scales that distinguish this species from several of its more finely scaled congeners in the same basin. Eschmeyer's Catalog of Fishes lists Cyprinodon macrolepis Miller, 1976 as a valid species within the family Cyprinodontidae.
The Chihuahuan Desert pupfish radiation is one of the classic case studies in the evolutionary biology of desert fish, with numerous Cyprinodon species restricted to individual springs or small spring complexes that were once connected by a wetter Pleistocene hydrology and have since been isolated by aridification. Several of Miller's Chihuahuan pupfish descriptions — including C. alvarezi and C. ceciliae — subsequently went extinct in the wild following the desiccation of their type localities, underscoring how fragile this entire species group is.
Morphology
Cyprinodon macrolepis is a small, deep-bodied pupfish typical of the genus: a stout, laterally compressed body, blunt head and small, terminal to slightly upturned mouth adapted for grazing algae and picking invertebrates from the substrate. As its name indicates, it carries comparatively large scales relative to several other Chihuahuan Desert Cyprinodon species described from the same broad region.
Sexual dimorphism follows the pattern typical of Cyprinodon: breeding males develop an intensified blue or blue-grey body colour with a bright yellow-orange to orange caudal-fin margin, while females and non-breeding males are duller, olive-brown and cryptically marked. Precise measurements and meristic counts specific to this species were not confidently available from the sources consulted here.
Habitat
The largescale pupfish is restricted to spring and marsh habitat within an interior desert drainage basin of the Chihuahuan Desert in northern Mexico — the kind of isolated, groundwater-fed oasis system typical of the many single-basin Cyprinodon endemics Miller described from this region. These habitats are characteristically warm, often somewhat saline or mineral-rich, and shallow, with aquatic vegetation providing cover and grazing surface.
As with virtually every other Chihuahuan Desert pupfish, the species' habitat is directly threatened by groundwater extraction for irrigated agriculture, which has already reduced or eliminated spring discharge at numerous historically documented sites across the region. Exact present-day extent and water-chemistry parameters specific to this species' locality were not confidently available here — data sparse.
Feeding
Cyprinodon species are typically omnivorous grazers, feeding on algae, diatoms and biofilm scraped from rock and plant surfaces along with small aquatic invertebrates such as chironomid larvae and micro-crustaceans. This diet is consistent with the small, blunt, slightly subterminal mouth typical of the genus, and is inferred here for C. macrolepis in the absence of a species-specific dietary study.
Mating
No species-specific account of courtship in Cyprinodon macrolepis has been located. Pupfish generally are highly territorial, promiscuous substrate spawners: breeding males hold small territories over sand, gravel or vegetation and display with intensified colour, fin-flaring and characteristic body-quivering behaviour to attract females, which then deposit small numbers of adhesive eggs directly onto the substrate. This is the general pattern for the genus rather than a directly confirmed observation for this species.
Breeding
Published breeding accounts specific to Cyprinodon macrolepis were not located. Chihuahuan Desert pupfish in general breed readily in captivity given warm, hard, somewhat mineral-rich water and a sand or fine-gravel substrate, with adhesive eggs hatching within about a week at warm temperatures and fry accepting fine live and dry foods from an early stage. Given the precarious status shared by this entire species group, a captive assurance population — where one exists — would be of real conservation value, but confirmed details for this species specifically are data sparse.
In the aquarium
Cyprinodon macrolepis is essentially unknown outside specialist native-fish and conservation-aquarium circles; it is not a mainstream trade species. For anyone maintaining Chihuahuan Desert pupfish generally, hard, alkaline water with a warm-temperate to subtropical temperature range, good aeration, and tankmates limited to similarly robust, non-aggressive species reflects the conditions these fish are adapted to. As with all single-basin Mexican desert endemics, any specimens in captivity should be understood as part of an accountable conservation-breeding effort rather than casually traded stock.
Conservation
The CARES Preservation Program lists Cyprinodon macrolepis at the Near Threatened (CNT) priority level. The species belongs to a genus in which the majority of Chihuahuan Desert endemics are considerably more imperilled — several, including C. alvarezi and C. ceciliae, have already gone extinct in the wild following the desiccation of their spring habitats under sustained groundwater extraction for irrigation. Continued agricultural water demand across the interior drainage basins of Chihuahua and Coahuila remains the principal long-term threat to the species and to the wider pupfish fauna of the region.
Because so many of these desert endemics occupy a single spring or a tightly linked spring complex, the loss of even one site can eliminate a species outright. Maintaining accountable ex-situ populations through programs such as CARES provides an important safeguard against this risk, alongside any groundwater-management efforts that can preserve remaining spring discharge at the species' native locality.