Taxonomy & naming
Matilebias duraznensis was originally described as Cynolebias duraznensis by García, Scvortzoff & Hernández in 1995, with the type locality recorded as Durazno, Uruguay (approximately −33.42°, −56.43°), a provincial capital on the Río Negro in the centre of the country. The species was subsequently transferred to Austrolebias, the genus erected to accommodate the annual killies of the temperate southern cone of South America, and it appears in Eschmeyer's Catalog of Fishes under that combination. The authority is presented in parentheses because the species was described under a different generic placement.
FishBase additionally notes a current assignment to the segregate genus Matilebias, reflecting ongoing taxonomic revision within the Austrolebias complex. Until a broadly accepted revision formally elevates Matilebias, the name Matilebias duraznensis remains the working combination used by most practitioners. The species was confirmed as valid with a genomic voucher in a molecular study of the genus by Ponce de León et al. examining burst speciation and genome expansion in South American annual killies, which placed duraznensis among the recognised species of the clade.
This fish was long listed as Austrolebias duraznensis; Eschmeyer's Catalog of Fishes places the species in Matilebias, and the current combination Matilebias duraznensis is followed here.
Morphology
Matilebias duraznensis is one of the smallest representatives of its genus. FishBase records a maximum standard length of 1 in for males and 1 in for females; the killi.co.uk species profile gives slightly larger practical sizes of approximately 1.5 in for males and 1 in for females, likely reflecting total length rather than standard length in well-fed aquarium specimens.
The body form is typical of the smaller Austrolebias: relatively deep-bodied, laterally compressed, with a rounded head and a superior-to-terminal mouth suited for surface-oriented predation on small invertebrates. Males are more intensely coloured than females, displaying the iridescent lateral markings and fin patterning characteristic of the genus — colouration that develops with remarkable speed, appearing as early as three weeks post-hatching. Females are cryptically coloured, olive to brownish, allowing them to remain inconspicuous in the substrate-rich pools the species inhabits. Sexual dimorphism in size and colour is pronounced, as is typical across Austrolebias.
Habitat
The species is native to the lower Río Negro basin of southwestern Uruguay, with the type locality at Durazno. The Río Negro drains a substantial portion of Uruguay before joining the Río Uruguay, and its lower basin encompasses a mosaic of temporary and semi-permanent grassland pools, shallow seasonal wetlands, and flooded depressions on the Uruguayan Pampas.
These pools are characterised by warm summer temperatures, a soft muddy or silty substrate rich in organic detritus, and highly variable water chemistry tracking seasonal rainfall and evaporation. Killi.co.uk records aquarium-maintenance temperatures of 61–73 °F and approximately neutral pH for this species, reflecting the temperate, seasonally variable conditions of the Uruguayan interior rather than the warm, acidic blackwaters of tropical South American killies. The pools fill with winter and spring rains, reach maximum depth and productivity through summer, then dry completely — stranding and killing the adults — as the austral summer heat intensifies. The eggs buried in the substrate are the sole vehicle of continuity between wet seasons.
Feeding
Matilebias duraznensis is a carnivore, feeding on small invertebrates available in its shallow seasonal pool habitat: aquatic insect larvae, small crustaceans, worms, and any other animal matter small enough to be seized by its relatively broad mouth. The annual killi feeding strategy is opportunistic and intense — with a life measured in weeks to a few months, adults must feed heavily and grow rapidly to reach reproductive size before the pool dries.
In the aquarium, the species accepts a variety of live and frozen invertebrate foods: Daphnia, Moina, Artemia nauplii and adults, bloodworm, and tubifex. Live foods are strongly preferred and produce the best condition and colouration in males. Dry foods may be accepted by some individuals but should not form the core of the diet. Feeding frequency should be high given the short lifespan and rapid growth trajectory.
Mating
As an annual killifish, Matilebias duraznensis reproduces through a soil-spawning system tightly coupled to the life cycle of its ephemeral pool habitat. Courtship is initiated by the male, who displays his iridescent colouration to the female in characteristic lateral presentations. Given that males colour up and begin breeding behaviour as early as three to four weeks post-hatching — an extraordinarily short time to reproductive maturity — competition for females begins almost immediately after the cohort emerges from the soil.
When a female is receptive, the pair dive together into the soft substrate — peat, mud, or silty detritus on the pool floor — and the female is pressed or driven into the sediment by the male as eggs and sperm are released simultaneously and buried within it. Spawning bouts may be repeated multiple times across a session. Males can be intolerant of one another in the confined space of a seasonal pool or aquarium, and keeping more than one male risks injury or suppression of subordinate individuals.
Breeding
The buried eggs of Matilebias duraznensis enter a state of arrested development — diapause — that protects them through the dry season. The eggs are enclosed in a tough chorion and can withstand months of desiccation while the pool is completely dry and temperatures at the soil surface are well above the lethal limit for adult fish. Development resumes when the pool refills with rain, and hatching occurs weeks to a few months after re-wetting depending on temperature and the completeness of diapause.
In the aquarium, the standard method mirrors the natural cycle: spawning adults are maintained on a peat or coconut-coir substrate, which is removed after a spawning period of several weeks, gently squeezed to remove excess water, sealed in a plastic bag or container, and stored at room temperature for a diapause period of two to three months or longer. The peat is then re-wetted with soft, slightly acidic water to trigger hatching. Fry are minuscule on emergence and require appropriately small first foods — infusoria, micro-worms, or freshly hatched Artemia nauplii — before graduating to larger live foods. Growth is rapid; males begin colouring at around three weeks and can spawn at four weeks, making generational turnover fast even by killifish standards.
In the aquarium
Matilebias duraznensis is a manageable annual killifish suited to experienced hobbyists willing to commit to the peat-spawning and egg-storage workflow that all annual killies require. Its small size — males under 1.5 in — makes it a species that can be maintained in modest aquaria of 5.5–10 US gal per pair or trio, provided the tank has a soft substrate layer and a tight-fitting cover.
Water conditions should reflect the temperate Uruguayan Pampas: temperatures in the range of 61–73 °F, pH near neutral (around 7.0), and moderate to low hardness. The cooler end of this range slows metabolism and may extend the captive lifespan somewhat; warmer conditions accelerate breeding. Males can be kept in groups only if ample space and broken lines of sight are provided; a single male with one or two females is the safest arrangement. Because the wild lifespan is one season, captive fish should be expected to decline and die within roughly six to twelve months of hatching, making timely collection and storage of eggs essential for continuation of the line. The species is maintained in specialist killifish clubs and periodic availability through those networks is the most reliable source.
Conservation
Matilebias duraznensis has not been evaluated by the IUCN Red List and carries a status of Not Evaluated (NE). This is a notable gap: the species is a single-basin endemic confined to the lower Río Negro drainage of Uruguay, a country where seasonal grassland pools have experienced considerable pressure from agricultural intensification — drainage for pasture improvement, pesticide run-off, and the introduction of exotic species, including mosquitofish (Gambusia affinis), which have proven destructive to annual killi populations throughout South America.
Uruguay's native Austrolebias fauna is among the most diverse and threatened on the continent, and several congeners from adjacent basins have been assessed as Critically Endangered or Endangered by the IUCN. The Río Negro basin has been affected by the construction of large reservoirs (including the Rincón del Bonete and Baygorria dams) that alter hydrology downstream and may have inundated or hydrologically disconnected historical pool habitats. Formal evaluation of duraznensis is warranted; given the pattern among its close relatives, the species may qualify for a threatened category. Hobbyist maintenance of captive lines serves a secondary conservation function for these small, fragile, and habitat-specific fish.