Taxonomy & naming
Megupsilon aporus was described by Robert Rush Miller and Mary Walters in 1972, based on specimens collected at the El Potosí spring in Nuevo León, Mexico. The species was placed in the monotypic genus Megupsilon, which Miller and Walters erected to accommodate it — making it the sole member of its genus from the day of description. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) records the valid combination as Megupsilon aporus Miller & Walters, 1972.
Megupsilon belongs to the family Cyprinodontidae — the pupfishes — which comprises a large radiation of small, often highly localised killifishes concentrated in the arid Americas, the Mediterranean basin, and parts of Africa. Within the New World pupfishes, Megupsilon represents an isolated lineage; the genus was monotypic and, with the extinction of M. aporus, is now entirely lost. The American Killifish Association classified it as a Class 1 New World Non-Annual, reflecting its breeding biology as a plant-spawner rather than a seasonal soil-spawner.
Morphology
The Catarina pupfish was a small, deep-bodied fish reaching a maximum of approximately 1.5 in total length. Like other pupfishes, it had a robust, somewhat compressed body, large cycloid scales, a terminal or slightly upturned mouth suited to feeding at or near the water surface, and relatively large eyes. Sexual dimorphism was modest but present: males in breeding condition typically developed more vivid colouration compared to the plainer females.
One of the diagnostic characters that defined the genus Megupsilon was the absence of pores in the sensory cephalic lateral-line canal system — the feature encoded directly in the specific epithet aporus. This reduction or loss of cephalic sensory pores distinguishes the genus from related cyprinodontids and was among the characters that prompted Miller and Walters to erect a separate genus rather than place the species within an existing one.
Habitat
The entire known range of Megupsilon aporus was the spring system at El Potosí, Galeana municipality, Nuevo León, in the Sierra Madre Oriental of northeastern Mexico. This spring fed a small pool and associated channel within the La Hediondilla basin, an endorheic (internally draining) basin with no outlet to the sea. The water was freshwater, alkaline, and moderately hard — FishBase records a pH of approximately 7.4 and hardness around 10.5 °dH, with temperatures in the range of 64–72 °F consistent with the relatively high-altitude (roughly 1,2297 ft above sea level) spring environment.
The spring complex was shared with at least two other endemic taxa: the Potosí pupfish Cyprinodon alvarezi and an endemic crayfish Cambarellus alvarezi, highlighting the extraordinary concentration of endemism that characterises isolated desert-spring systems in Mexico. Such spring systems are biodiversity hotspots in miniature: geologically stable, thermally buffered, and long-isolated enough to generate unique species, yet catastrophically vulnerable to any perturbation of the groundwater regime that sustains them.
Feeding
As a pupfish in a spring environment, Megupsilon aporus was almost certainly an opportunistic carnivore-omnivore, as is typical across the Cyprinodontidae. FishBase characterises the species as carnivorous and egg-laying, and the broader ecological context of a small, isolated spring suggests the diet would have included invertebrates, algae, detritus, and whatever organic material the spring system produced.
Pupfishes in general are well adapted to exploiting the limited and variable food resources of arid-zone spring systems, and their broad dietary tolerance is part of what allows single populations to persist in habitats too small and fluctuating to support more specialised feeders. No detailed dietary study of M. aporus is documented in the available literature, reflecting the species' very restricted range and the limited research attention it received before its extinction.
Mating
Megupsilon aporus was a non-annual killifish — a plant or substrate spawner living in what was, at least historically, a permanent spring rather than a seasonal pool subject to annual desiccation. As a non-annual, it did not produce diapausing eggs buried in substrate peat or mud, but instead spawned adhesive eggs onto hard surfaces or fine vegetation near the bottom or among substrate rocks, with eggs developing in water without a diapause phase.
FishBase records the eggs as adhesive on rocky substrate near vegetation, consistent with the spawning behaviour of other pupfish in permanent spring habitats. The mating system of pupfishes typically involves territorial males that defend spawning sites and court passing females, with brief spawning chases and embraces that deposit small clutches of a few to several tens of eggs. There is no parental care beyond the brief spawning act itself.
Breeding
As a non-annual plant or substrate spawner, the Catarina pupfish would have laid adhesive eggs that adhered to rocky surfaces or fine vegetation within the spring pool and incubated in water over a period of days, hatching into free-swimming larvae without any dormancy phase. This mode contrasts sharply with the annual killifishes of seasonal pools, whose eggs must survive months of drought buried in the substrate.
In aquarium captivity, the species was maintained and bred by killifish hobbyists — the 14 records in the Journal of the American Killifish Association (JAKA) from 1976 to 2024 attest to its modest presence in specialist collections. However, captive populations ultimately did not survive, and no living individuals are known to remain anywhere in the world. The loss of the captive population alongside the wild population sealed the species' extinction.
In the aquarium
The Catarina pupfish was kept by a small number of specialist killifish hobbyists in Europe and North America, primarily through the exchange networks of the American Killifish Association and affiliated national societies. As a Class 1 New World Non-Annual, it was bred on spawning mops or fine substrate rather than in the peat moss used for annual species. The AKA's species records reference 14 JAKA articles spanning nearly five decades (1976–2024), indicating a persistent but very limited hobbyist presence.
Despite these captive efforts, the species was never maintained in sufficiently large, genetically diverse, or institutionally secure captive populations to survive its wild extinction. The lessons of the Catarina pupfish — alongside the better-known case of the Devil's Hole pupfish Cyprinodon diabolis — have since informed conservation protocols that now recommend formal ex situ breeding programs under zoological or aquarium institution oversight for critically endangered spring endemics, rather than reliance on the distributed hobbyist network alone.
Conservation
Megupsilon aporus is Extinct. The IUCN Red List assessment of September 2018 (Valdes Gonzales 2019) formally assigns the species the EX category, and González et al. (2020) in the journal Oryx document the extinction in detail. The sole known population occupied the El Potosí spring complex in the La Hediondilla endorheic basin of Nuevo León, Mexico. The spring pond and its associated channel desiccated completely as a result of groundwater extraction for agriculture and human use in the surrounding basin, eliminating the habitat in its entirety. Extinction in the wild is considered to have occurred by approximately 1994.
The González et al. Oryx paper frames the extinction in its broader conservation context: Megupsilon aporus was the most recently confirmed extinction of an endemic freshwater fish in Mexico, a country with exceptionally high freshwater endemism and severe pressure on its desert-spring systems from groundwater depletion, invasive species introductions (particularly Gambusia affinis), and habitat modification. The co-occurring Potosí pupfish Cyprinodon alvarezi suffered the same fate from the same spring's desiccation.
The case exemplifies the extreme vulnerability of single-spring endemics: a species whose entire global distribution fits within a small spring pool can be driven to extinction by a single well or diversion. The failure to establish a secure captive population before the wild habitat collapsed — despite the species' presence in the killifish hobby for two decades — underlines the importance of proactive ex situ programs for spring-endemic fishes wherever groundwater systems face extraction pressure.