Taxonomy & naming
Austrolebias univentripinnis was described by Wilson J. E. M. Costa and Matheus Cheffe in 2005, in Zootaxa volume 1052 (pages 41–48), on the basis of specimens collected from temporary pools on the Jaguarão River floodplains within the Mirim Lagoon basin, Rio Grande do Sul, southern Brazil. The Catalog of Fishes (Fricke, Eschmeyer & Fong) recognises the valid name as Austrolebias univentripinnis Costa & Cheffe, 2005.
Within Austrolebias, the species falls into a clade that Costa & Cheffe identified alongside A. bellottii, A. melanoorus, A. arachan, A. adloffii, and A. charrua — a grouping of Pampa-region annuals linked by shared derived characters in the unpaired fin skeleton and body proportions. The genus Austrolebias belongs to the neotropical subfamily Cynolebiatinae, the South American annual killifish, within the family Rivulidae.
The specific epithet univentripinnis is Latin, combining uni- (one, single), ventri- (belly, ventral), and -pinnis (fin) — a reference to a distinctive morphological character of females: the long, triangular anal fin that, together with fin origin position and body-depth ratios, helps diagnose the species within the genus.
Morphology
Austrolebias univentripinnis is a small species, reaching a maximum of approximately 1.5 in standard length (SL) in males according to FishBase. The original description by Costa & Cheffe notes several diagnostic characters: females possess a notably long, triangular anal fin — unusually elongated relative to related species — while males have the dorsal fin origin placed more anteriorly on the body (closer to the head) compared with some congeners. Males also have a shorter anal-fin base, a lesser body depth, and a wider basihyal (the main element of the tongue skeleton) relative to body size.
Colouration in life follows the pattern typical of many Austrolebias: males are more vividly marked, with the intensity and patterning serving as species-recognition signals in the dark, tannin-stained temporary pools they inhabit. Females are cryptically coloured, in the typical brownish tones that characterise female rivulids. The body is moderately compressed, the head blunt, and the mouth upturned — the characteristic profile of a fish that snaps prey from the water column or surface in shallow pools.
Habitat
The species is associated with the Mirim Lagoon basin in the far south of Brazil, a flat, seasonally flooded lowland region where the Jaguarão River forms the border between the Brazilian state of Rio Grande do Sul and Uruguay. The It Rains Fishes database and the original description document its occurrence in temporary pools on floodplain grasslands — the typical habitat of Pampa-region Austrolebias, which form and disappear with the Southern Hemisphere winter rains. The species has also been recorded in Uruguay and possibly Paraguay and Argentina, reflecting the broad, interconnected drainage of the Río de la Plata system.
Water chemistry at type-locality sites is moderate by killifish standards: FishBase and the original description record pH around 7.0, temperature ranging from approximately 64 °F in cooler winter conditions to 75–77 °F as pools warm in late spring, and hardness around 5 dGH — soft but not acidic blackwater. The pools are typically shallow, often turbid with suspended material, and richly stocked with invertebrates during their brief existence.
Feeding
Like all Austrolebias, A. univentripinnis is an insectivore in nature, targeting the invertebrates — aquatic larvae, small crustaceans, and surface-fallen insects — that proliferate in newly flooded seasonal pools. FishBase records the diet as insectivorous. The upturned mouth is well suited to snatching prey at or near the water surface, and the species, in common with other annual killies, must feed intensively and grow quickly to reach breeding condition before the pool dries.
In aquarium care, the species accepts live and frozen foods readily: small Artemia nauplii and adults, Daphnia, Cyclops, bloodworm (chironomid larvae), and other suitably sized invertebrate foods. Live or frozen foods elicit the strongest feeding response and best condition. A varied, invertebrate-rich diet closely mirrors the natural regime and is especially important during the conditioning of breeding pairs.
Mating
Austrolebias univentripinnis is an annual killifish, and its mating behaviour is shaped entirely by the reality of its vanishing habitat: the breeding window between pool formation and pool disappearance is measured in weeks. Males display to females by spreading and fanning the elongated fins, using colour and fin carriage as signals in the shallow, often opaque water. Rival males contest females with lateral displays and chasing; females select among them and signal receptivity by approaching and orienting toward the substrate.
Spawning is a substrate-diving act. The pair swim alongside each other and drive together into the soft mud, peat, or detritus at the pool bottom, where the female deposits one to several eggs per dive and the male fertilises them simultaneously. The eggs are left buried in the substrate, receiving no further parental attention. In the aquarium, a peat moss or coir fibre spawning medium placed on the tank floor precisely replicates this substrate, and pairs spawn into it readily once conditioned on live foods.
Breeding
After the pair have spawned into the substrate, the buried eggs enter diapause — a state of arrested embryonic development, analogous to dormancy in seeds — that allows them to survive the complete desiccation of the pool through the dry season. In wild populations of Pampa-region Austrolebias, this dry period spans several months. Development resumes when the pool refills with the next rains, and hatching occurs quickly once the eggs sense rehydration.
In aquarium breeding, the peat or coir medium is removed from the tank after a month or more of spawning and stored barely moist in a sealed bag or container at around 68–72 °F. The It Rains Fishes database records an incubation period of approximately six weeks at 77 °F for this species, with longer development at lower storage temperatures. To trigger hatching, the keeper re-wets the medium with soft, slightly acidic water; fry emerge within hours to a day. First foods are Artemia nauplii, which fry accept from the moment they are free-swimming. Growth is rapid — annual killies invest heavily in fast maturation, and young can reach breeding size in a matter of weeks under good conditions.
In the aquarium
Austrolebias univentripinnis is a specialist fish for the killi hobbyist rather than the general community aquarium. Its small size (under 1.5 in SL) means it can be maintained in modest setups — a 10–15 US gal tank is adequate for a breeding pair or small group — but its specific requirements and short natural lifespan make it a more demanding commitment than a community fish. The tank should be tightly covered, as killies are capable jumpers.
Water parameters should reflect the species' natural range: temperature 64–75 °F (cooler than many tropical tanks; the species is a temperate-edge annual), pH around 7.0, and soft to moderately soft water (around 5 dGH). A dark substrate layer of peat or coconut coir is essential for breeding and should be made available at all times. Tanks should be densely planted or provided with cover; males can be aggressive toward one another and toward unreceptive females, and visual barriers reduce stress. The American Killifish Association maintains husbandry records for this species and is the best starting point for obtaining eggs or fry, as the species circulates mainly within specialist killi clubs rather than the mainstream ornamental trade.
Conservation
The IUCN Red List assessed Austrolebias univentripinnis as Endangered (EN) under criterion B1ab(iii) in December 2020, with Gabriel Brejão as assessor. The threatened status reflects the species' narrow range within the Mirim Lagoon basin and the ongoing degradation of its floodplain habitat. Two threats are identified as primary drivers: conversion of lowland grassland and floodplain to rice paddy agriculture, which drains the temporary pools annual killies depend on, and urban and residential development along the Jaguarão valley. The population trend is listed as unknown, a consequence of limited monitoring data for this microhabitat specialist.
Conservation actions recommended in the IUCN assessment include formal legal protection of remaining habitat patches within conservation units, active habitat recovery and long-term population monitoring, and targeted research into the species' ecology and population structure. The killifish community plays a meaningful ancillary role: the American Killifish Association and hobbyist networks maintain captive-breeding populations that serve as an insurance against local extinction, though they are no substitute for protecting the seasonal pools of the Jaguarão floodplains where wild fish still occur.