Taxonomy & naming
Plesiolebias lacerdai was described by Wilson J.E.M. Costa in 1989, with a type locality in the floodplains of the Rio das Mortes, a major tributary of the Araguaia River in Mato Grosso and Tocantins states, central Brazil. The species is placed in the family Rivulidae, the New World annual and non-annual killifishes, and within that family in the genus Plesiolebias — a group of very small, seasonally annual fishes endemic to the central Brazilian plateau and surrounding drainages. The Catalog of Fishes (Fricke, Eschmeyer & Fong) records the valid name as Plesiolebias lacerdai Costa, 1989.
The genus name Plesiolebias combines the Greek plesios (near, close) with lebias, an ancient name for a small fish — reflecting the genus's close relationship to Leptolebias and other miniature rivulid genera. The species epithet lacerdai honours Marco Túlio Lacerda, the Brazilian ichthyologist who discovered the species. One distinctive meristic feature of P. lacerdai is the position of the dorsal fin: it originates above the first to third anal fin rays, further forward than in many congeners, a character useful for identification. Meristic counts recorded from the type series and subsequent specimens include approximately 11 dorsal fin rays, 15 anal fin rays, and around 23 lateral scales. The name Hypsolebias lacerdai is treated as a synonym of this species in Eschmeyer's Catalog of Fishes.
Morphology
Plesiolebias lacerdai is a miniature fish, reaching a maximum of 1 in total length — placing it among the smallest annual killifishes and among the smallest vertebrates to complete a terrestrial dry season as a buried egg. The body form follows the typical rivulid pattern: moderately elongate and compressed, with a rounded head, superior to slightly terminal mouth, and fins set well back. The dorsal fin's anterior position, originating over the first few anal fin rays, is a distinguishing character within the genus.
As in other Plesiolebias species, males are more vividly coloured than females, with patterning across the flanks and unpaired fins that serves in courtship and species recognition in the typically tannic, turbid waters of flooded grasslands. Females are comparatively plain and deeper-bellied when gravid. Given the 1 in maximum size, all measurements and fin counts must be taken with care; the species is best identified in combination with locality data and meristic counts rather than colour pattern alone.
Habitat
The species is known from the seasonal floodplains of the Rio das Mortes drainage, a right-bank tributary of the Araguaia River in the states of Mato Grosso and Tocantins, central Brazil. The Araguaia basin lies within the Cerrado biome — the Brazilian savanna — where the climate is sharply seasonal: a pronounced wet season floods shallow grassland pools and oxbow fringes, followed by a long dry season during which these pools evaporate completely.
Plesiolebias lacerdai inhabits these temporary pools during the wet season. The water is typically soft, warm, and slightly acidic, coloured by leaf litter and organic matter but far less darkly tannic than Amazonian rainforest streams. Water temperatures in Cerrado seasonal pools follow the ambient air temperature and can be high in the dry-season tail, but during the wet-season breeding period they are moderated by rainfall and are typically in the mid-20s °C. The pools are shallow, with soft muddy or peaty substrate into which the fish bury their eggs — the defining requirement for an annual killi habitat.
FishBase records the distribution as confined to the Araguaia basin in Mato Grosso and Tocantins. No populations are documented outside this core drainage.
Feeding
Like other small annual killifishes, Plesiolebias lacerdai is a micro-predator of invertebrates. In the temporary pools it inhabits, the prey base during the wet season is dominated by zooplankton, small aquatic insects, mosquito larvae, ostracods, and other microcrustaceans. The superior to terminal mouth position is well suited to taking prey at or near the water surface as well as in the water column.
In the aquarium, the species accepts live and frozen micro-foods readily: Daphnia, Cyclops, Artemia nauplii, tubifex, and bloodworm are all suitable. Live foods are preferred and elicit the most natural foraging behaviour. Given the 1 in adult size, food particle size must be calibrated carefully; even adults accept relatively small items, and juveniles require infusoria or paramecium cultures for the first one to two weeks before they can manage freshly hatched Artemia nauplii.
Mating
Plesiolebias lacerdai is a substrate-diving annual killifish. Courtship occurs within the water column above the spawning substrate — soft mud or peat — with the male displaying his colour pattern to the female. When the female signals receptivity, the pair dive together into the substrate: the male positions alongside or above the female, and eggs are deposited directly into the soft bottom material, where they are fertilised and immediately concealed. Each spawning dive yields one or a small number of eggs. The behaviour is repeated many times over the breeding period.
The eggs are equipped with an outer filamentous coat or adhesive surface that anchors them in the substrate. Once the seasonal pool dries out, the eggs enter diapause — a state of arrested embryonic development that allows survival through the dry season in the dried-out substrate. This diapause is obligate for at least some stages of development; bypassing it by constant wetness can impair hatching success. The male-biased sex ratio often observed in cultured populations (sometimes 90% or more males) is a characteristic of P. lacerdai noted by hobbyists and is not fully explained, though it is not unique to this species within the genus.
Breeding
In nature, eggs buried in the substrate pass the entire dry season — potentially many months — in diapause, hatching only when seasonal rains refill the pool. In the aquarium hobby, the standard technique mirrors this cycle: a pair or small group is kept on a deep layer of peat or coconut coir, into which they deposit eggs over several weeks. The substrate is then removed, gently pressed to remove excess water, placed in a sealed plastic bag or airtight container, and stored at a stable warm temperature (around 75–79 °F) in darkness. Storage duration for P. lacerdai is notably long compared with most annual killifishes: It Rains Fishes and hobbyist experience indicate an incubation period of six to nine months, longer than many rivulid annuals. Attempting to hatch the eggs prematurely yields poor results.
After the storage period, the peat is placed in a shallow container and soft, slightly acidic water is poured over it. Fry begin hatching within hours to a day or two. The fry are tiny and require infusoria or paramecium as first food for one to two weeks; only then are freshly hatched Artemia nauplii appropriate. Growth to maturity takes approximately two months under good conditions. Brood sizes vary, but productive pairs can yield substantial numbers of eggs over the spawning period — offset by the challenging sex ratio and fry-rearing demands. This species is rated by experienced hobbyists as one for the dedicated killifish specialist, not a beginner.
In the aquarium
Plesiolebias lacerdai is among the more demanding killifishes in cultivation, and sources consistently rate it as suitable only for experienced killifish keepers. It is sensitive to transport and handling stress, and losses during transit are not uncommon even when fish arrive in good condition. The very small adult size (1 in TL) means it is vulnerable to any tankmate large enough to swipe at it, and it is best kept as a species in a small, calm aquarium.
The recommended setup is a small, dark aquarium — 5.5–10 US gal suffices — with a deep layer of soft peat or coconut fibre substrate, dense floating plants or surface cover to diffuse light, minimal water current, and no aggressive or fast-moving companions. Water should be soft and slightly acidic, matching Cerrado floodplain conditions: temperature around 75–79 °F, pH in the 6.0–7.0 range, low hardness. The American Killifish Association recognises the species and can be a resource for locating captive-bred stock and connecting with hobbyists who have experience with the long diapause protocol.
Given the obligate diapause and the six-to-nine month wait between spawning and hatching, P. lacerdai rewards patience above all. The challenge of the long incubation period, the poor sex ratio, and the tiny fry make it a fish for the collector drawn to unusual annual biology rather than one for the general planted-tank hobbyist.
Conservation
The IUCN Red List assessed Plesiolebias lacerdai as Least Concern in 2018 (assessment dated 2018-11-07), based on a range that, while restricted to the Araguaia basin and Rio das Mortes drainage, was not considered to be severely fragmented and had no documented rapid population decline at the time of assessment.
Nonetheless, the species faces the pressures typical of Cerrado seasonal-pool annuals. Brazil's Cerrado biome is one of the world's most threatened biodiversity hotspots, heavily converted to soybean and cattle agriculture, with seasonal pools — the exclusive habitat of annual killifishes — frequently drained, filled, or rendered unsuitable by agrochemical run-off. The Rio das Mortes drainage (the name means River of Deaths) runs through a heavily farmed region of Mato Grosso and Tocantins; agricultural expansion in these states has been rapid. Because P. lacerdai depends on temporary seasonal pools that must remain undisturbed through the entire wet season for spawning, and through the entire dry season for diapause, even partial drainage or pool alteration can eliminate local populations entirely.
The Least Concern status should be read against these pressures, and against the known pattern of South American annual killies: narrow-range endemics restricted to ephemeral habitats that are rarely surveyed and can disappear between assessments. Monitoring of the Rio das Mortes populations and broader Cerrado conservation are both relevant to the long-term outlook for this species.