Taxonomy & naming
Pterolebias longipinnis was described by the American ichthyologist Samuel Walton Garman in 1895 in the Memoirs of the Museum of Comparative Zoology at Harvard (vol. 19(1): 142, pl. 3, fig. 14), based on material collected near Santarém, on the lower Amazon in Pará state, Brazil, during the Nathaniel Thayer expedition of August 1865. Eschmeyer's Catalog of Fishes (CAS) treats Garman, 1895 as the valid authority for the species.
The genus Pterolebias was revised by Wilson J. E. M. Costa in a 2005 paper that reassessed the phylogenetic position of the group within the South American annual killifish lineage. Pterolebias belongs to the order Cyprinodontiformes, family Rivulidae (or Aplocheilidae sensu some authors). The genus is characterised by extended dorsal and anal fins in males — the source of both the genus name and the specific epithet — and by the annual life cycle tied to seasonally drying floodplain pools.
Morphology
Males reach up to 4.5 in standard length (SL) and are among the larger annual killifishes; females are somewhat smaller and much less ornamented. The most striking feature of the male is the extreme elongation of the unpaired fins: dorsal, anal, and caudal rays are produced into long, often filamentous extensions that give the fish an almost sail-like silhouette. Meristic counts from It Rains Fishes average D 9.2 rays, A 18.8 rays, and 29.8 lateral scales.
Body colouration in males is iridescent blue-green to teal with red to orange spotting or reticulation on the flanks and vivid colour in the extended fins; exact patterning varies with locality and condition. Females are cryptically brownish with a rounded, shorter-finned profile. The body is robust and slightly laterally compressed, consistent with a bottom-dwelling annual killi that dives into soft substrate to spawn.
Habitat
The species occupies a broad arc from the lower Amazon basin (Pará state, Brazil) south through the Pantanal wetland complex of Mato Grosso do Sul (Brazil) into adjacent parts of Argentina. It is found in field-measured habitats as small as roughly 1.8 × 6 ft pools with a maximum depth of around 12 in — tiny, sunlit depressions in seasonally flooded savanna or floodplain grassland with mud or soft-detritus bottoms.
Field measurements from American Killifish Association collectors record water temperature around 90 °F in midday sun, pH 6.8, and hardness of approximately 4.5 dH. FishBase gives a broader aquarium-parameter range of pH 6.5–7.0, temperature 63–72 °F (cooler husbandry conditions reflecting the range's southern populations near the Pantanal). These pools receive full sun, contain little to no aquatic vegetation, and dry out completely during the region's annual dry season, leaving only buried eggs to carry the population through.
Feeding
In the wild, Pterolebias longipinnis feeds on small aquatic invertebrates — worms, crustaceans, and insects — typical of the invertebrate fauna of shallow, organically rich seasonal pools. Its large size relative to other annual killies allows it to take larger prey items than smaller Rivulidae.
In captivity it readily accepts live and frozen foods: bloodworms (Chironomus larvae), tubifex, brine shrimp, Daphnia, and similar invertebrate-based diets. Variety in the diet supports full development of the brilliant male colouration. Dry pellets may be accepted after conditioning but live and frozen foods should form the core of the diet for fish being prepared for breeding.
Mating
Pterolebias longipinnis is a classic annual killifish mating diver. When a male courts a receptive female, the pair come together side-by-side or in a brief embrace at or just above the pool bottom. Both fish then plunge simultaneously into the soft substrate — peat, mud, or detritus — burying themselves partially or fully as the eggs are fertilised and deposited deep in the sediment.
Males are conspicuous and actively display, extending their elongated fins and intensifying their body colours to attract females and exclude rival males. In the compact pools these fish naturally inhabit, competition between males can be fierce; the large fins and vivid markings serve as honest signals of male quality. A single wild pool may support only a handful of breeding adults, making each spawning event consequential for the local gene pool.
Breeding
After the pair dives and buries the fertilised eggs, the adults' role in reproduction is complete. The eggs remain buried in the sediment as the dry season advances and the pool evaporates; the embryos enter diapause — a state of arrested development in which metabolic activity is drastically reduced — and can persist for three to six months or longer while the substrate is dry.
In the hobby, breeding is managed on a peat-spawning protocol. A pair is conditioned on live foods, then introduced to a tank with a shallow layer of damp peat or coir as substrate. After several sessions of spawning dives the peat is removed, lightly pressed to remove excess moisture, and stored in a sealed plastic bag or container at 75–82 °F. It Rains Fishes notes that egg incubation duration depends on humidity, temperature, and the peat medium; growers typically water-test a pinch of peat at three months and re-store if the eggs are not yet developed. When the eggs are eyed-up, the peat is flooded with soft, slightly acidic water and the fry begin hatching within hours. Young fish grow quickly and can be fed Artemia nauplii and microworms from the start.
In the aquarium
Pterolebias longipinnis is kept by specialist killifish keepers rather than general community hobbyists; its annual life cycle, specific spawning requirements, and need for cool-to-moderate temperatures set it apart from mainstream fishkeeping. A pair or small group can be housed in a modestly sized aquarium — 15 US gal or more — with a tight-fitting lid (killies are accomplished jumpers), subdued lighting, and a bottom of soft substrate or a removable peat tray for spawning.
Water should be soft and slightly acidic (pH 6.5–7.0, hardness around 4–6 dH) and temperature kept in the 68–77 °F range for general maintenance; some keepers report that the cooler end of this range improves longevity in this species. The AKA notes that the fish can be shy and breeding results can be inconsistent — patience and consistency of conditions matter more than elaborate manipulation. Males should not be kept together unless the tank is large enough for clear territorial separation, as the extended fins make rivals conspicuous targets. Wild-caught or farm-bred stock should be quarantined carefully; like other annual killies they are susceptible to velvet (Piscinoodinium) and should be watched for it.
Conservation
The IUCN Red List assessed Pterolebias longipinnis as Least Concern in December 2020 (Assessment ID 160296336, species ID 49830679), noting a wide distribution across the Amazon and Paraguay (Pantanal) basins with no identified major widespread threats at the time of assessment.
Nonetheless, the ecology of the species makes it inherently vulnerable at the local scale. Its entire reproductive output for a year rests in buried eggs in tiny, isolated seasonal pools; a single drainage project, cattle-pond excavation, or conversion of savanna to cropland can eliminate a local population permanently with no possibility of recolonisation unless nearby pools remain. Seasonal pools across the Amazon–Pantanal interface face pressure from agricultural expansion (soy and beef production), channelisation of natural drainage, and climate variability altering the timing and reliability of wet-season flooding. Because Pterolebias longipinnis has a relatively broad range for an annual killifish, the species-level LC rating is probably appropriate — but monitoring of specific localities, particularly in the southern Pantanal where range contraction would occur first under intensifying land use, remains important.