Taxonomy & naming
Plesiolebias glaucopterus was described by Wilson J. E. M. Costa and Margareth T. Lacerda in 1989. The authority is presented in parentheses following standard ichthyological convention, indicating the species was subsequently placed in a different genus from the one in which it was originally described. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) records the valid combination as Plesiolebias glaucopterus (Costa & Lacerda, 1989).
The genus Plesiolebias belongs to the family Rivulidae (South American killifishes) and contains a small assemblage of annual species distributed across the central lowlands of South America, particularly in the Paraguay and upper Paraná basins. The genus name is from the Greek plēsios ('near' or 'close to') combined with Lebias, an older killifish genus name, reflecting the group's close affinities within the rivulid radiation.
The specific epithet glaucopterus derives from the Greek glaukos ('blue-grey' or 'sea-green', often referring to a pale bluish colour) and pteron ('fin' or 'wing'), alluding to the distinctive colouration of the fins that characterises the species.
Morphology
Plesiolebias glaucopterus is a miniature fish, reaching a maximum recorded total length of 1 in (FishBase). As with most annual killifishes, there is marked sexual dimorphism in colour. Males display the vivid pigmentation that typifies the genus: the fins that give the species its name (glaucopterus, 'blue-grey fin') likely bear the pale blue-grey tones visible in live specimens, set against body colouration patterned in ways typical of Plesiolebias — rich browns, blues, and iridescent scales.
Females are considerably plainer, as is usual across the South American annual killifish clade. They are typically brownish or cryptically coloured, smaller-bodied than males at equivalent ages, and lack the bright fin pigmentation. This pronounced sexual dichromatism facilitates pair identification in the aquarium.
The body plan conforms to the generalised rivulid shape: moderately robust and cylindrical, with a slightly flattened dorsal profile, a rounded caudal fin, and dorsal and anal fins set far back on the body — an arrangement shared across the South American annual killifish families and associated with their benthic, substrate-spawning lifestyle.
Habitat
The type locality of Plesiolebias glaucopterus is a lagoon near the Rio Paraguay in the vicinity of Cáceres, Mato Grosso state, Brazil. This places the species within the upper Paraguay River basin, a region characterised by the vast seasonal floodplain of the Pantanal — one of the world's largest tropical wetland systems. On its margins and in the surrounding lowland savannas (cerrado), seasonal pools and lagoons fill during the wet season rains and dry out completely during the austral dry season.
FishBase records water parameters of pH 6.5, temperature 72 °F, and hardness of approximately 10 dH for this species, indicating moderately soft, slightly acidic conditions — consistent with the weathered sediments and organic-rich waters typical of Pantanal floodplain pools. The pools inhabited by annual killifishes in this region tend to be shallow, warm, and heavily vegetated with aquatic macrophytes and emergent grasses during the wet season.
The annual cycle drives everything about the biology of this species. Rising wet-season rains fill the pools from roughly October to March; the fish hatch, grow, mature, spawn, and die within this wet-season window. By the dry season the pool has evaporated and all adults are dead; survival of the population depends entirely on the eggs buried in the substrate.
Feeding
FishBase classifies Plesiolebias glaucopterus as omnivorous. In the wild, small annual killifishes in the Paraguay basin feed opportunistically on whatever invertebrates, zooplankton, and small arthropods are available in the seasonal pools they inhabit. Given their small adult size — just 1 in — prey items are correspondingly minute: copepods, water fleas (Daphnia and related cladocerans), small insect larvae, and similar micro-invertebrates would constitute the bulk of the diet.
In the aquarium, fry require very fine first foods after hatching: infusoria and paramecium (single-celled ciliates) are the standard starting point, as the newly hatched fish are tiny and cannot take even newly hatched brine shrimp (Artemia nauplii) at first. As they grow, Artemia nauplii and micro-worms become appropriate, and adults can take small live and frozen foods such as Daphnia, Cyclops, and fine frozen bloodworm. The species is not known to accept dry prepared foods readily, and a diet centred on live or frozen invertebrate prey is recommended to maintain condition.
Mating
Plesiolebias glaucopterus is an annual killifish that spawns in the substrate — the defining reproductive mode of the South American annual killifish clade. Males are brightly coloured and compete for access to females; courtship involves lateral displays in which the male presents his coloured flanks and fins to the female. When a female is receptive, the pair moves to the substrate together.
The spawning act involves the male and female pressing side by side and pushing together into soft substrate material — peat, mud, or detritus — burying one or more eggs per dive. The eggs are left embedded in the substrate with no parental attendance. A single spawning session may involve many such dives and result in multiple eggs deposited over the substrate surface and into deeper layers. Males reportedly show a tendency to dominate and may harass females in a confined space, so provision of cover and careful monitoring of female condition is important.
Breeding
The eggs of Plesiolebias glaucopterus undergo diapause — a period of arrested embryonic development that allows them to survive the complete desiccation of the pool during the dry season. Incubation at Cáceres-area latitudes in nature spans the full dry season; in the aquarium, eggs require six to nine months of storage in damp (but not wet) peat or coir before being re-wetted to trigger hatching. This is among the longer incubation periods recorded for South American annual killifishes and is a significant practical challenge for hobbyists.
The standard aquarium method involves spawning the fish onto a layer of soft peat or coconut coir on the tank floor, then removing the medium after several weeks of spawning activity, gently squeezing out excess water until the medium is just damp, and storing it in a sealed bag or container at room temperature for the required incubation period. After six to nine months, the medium is placed in a shallow dish and fresh, soft, slightly acidic water is added to trigger hatching. Fry from this species are very small at hatching and require infusoria as a first food before graduating to Artemia nauplii.
A notable feature reported for the species is a high proportion of males among the fry — experienced keepers should be prepared for skewed sex ratios, which can create management challenges when males outnumber females substantially. The source of the sex-ratio skew is not well understood. Adults are short-lived, typically surviving only the duration of the wet season in nature; in captivity, well-maintained individuals may survive somewhat longer but the species is not long-lived.
In the aquarium
Plesiolebias glaucopterus is regarded as a species for experienced killifish keepers, not a beginner's fish. The primary demands are the management of the long diapause period — six to nine months of careful peat storage — and the provision of very fine first foods for the tiny hatchlings. Adult fish are small and peaceful toward one another by killifish standards, though males may pursue females persistently in small spaces, so a species tank with multiple females per male and dense surface cover of floating plants is advisable.
A small aquarium (2.5–5.5 US gal) suffices for a breeding group. Water conditions should replicate the soft, slightly acidic conditions of the Pantanal floodplain periphery: temperature around 72 °F, pH near 6.5, and low hardness (around 10 dH or softer). A dark substrate of fine peat or sand with a dedicated spawning layer of commercial peat or coir allows natural substrate-diving behaviour. Dim lighting and dense planting reduce stress. Males are best kept at a ratio of one male to two or more females to limit female harassment.
The species is not widely available in the trade; it circulates mainly among killifish specialist societies and dedicated keepers. Acquiring stock generally means contacting killifish associations such as the American Killifish Association, where members trade eggs and fish among themselves. Its small size, delicate husbandry requirements, and limited commercial availability mean it will remain a niche species valued primarily by enthusiasts interested in South American annual killies as a group.
Conservation
Plesiolebias glaucopterus was assessed as Least Concern (LC) by the IUCN in 2018. The assessment reflects the species' occurrence within the Paraguay River basin — a large hydrological system — and the absence of evidence of major population decline at the time of assessment. The Pantanal and its surrounding cerrado seasonally flooded landscapes support a diverse annual killifish fauna, and the species is not known to be restricted to a single, uniquely threatened locality.
Nonetheless, annual killifishes as a group are inherently vulnerable: they exist only as eggs during the dry season, and the destruction or permanent alteration of their seasonal pool habitat eliminates populations with no possibility of recolonisation from buried eggs. The cerrado biome surrounding the upper Paraguay basin is one of the most heavily converted biomes in South America, with large-scale soy agriculture and cattle ranching draining and filling seasonal pools across the region. Climate variability that shortens or disrupts the wet season can also cause egg mortality if pools dry before the annual cohort completes spawning.
The introduction of non-native fishes — including mosquitofish (Gambusia spp.) and other invasive species — into seasonal pools is a further recognised threat to annual killifishes across their South American range. The long-term security of Plesiolebias glaucopterus depends on the conservation of natural Pantanal and cerrado seasonal wetlands, habitats that are currently under substantial and increasing pressure from agricultural expansion in Mato Grosso state.