Taxonomy & naming
Plesiolebias aruana was described by Kenneth J. Lazara in 1991, with the type locality recorded as temporary pools near the city of Aruanã, middle Araguaia River basin, state of Goiás, Brazil (approximately 51°03′W, 14°55′S). Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) records the valid combination as Plesiolebias aruana (Lazara, 1991), within the family Rivulidae, the South American killifishes — the largest family of annual killifish in the world.
The genus Plesiolebias is a small, phylogenetically distinct lineage within Rivulidae, characterised by a combination of meristic and morphological traits that separate it from the closely related genera Maretacoara and Simpsonichthys, both of which share its seasonal pool habitat in central and northeastern Brazil. It rains Fishes notes that P. aruana co-occurs at some localities with Maretacoara lacortei, Simpsonichthys costai, Pituna compacta, and Trigonectes rubrimarginatus — a community of syntopic annuals that together illustrate the remarkable diversification of the Rivulidae in Neotropical seasonal wetlands.
The species epithet aruana refers directly to the city of Aruanã (also spelled 'Aruana' in older literature), the geographic landmark closest to the type locality. No valid synonyms are recorded in the current literature.
Morphology
Plesiolebias aruana is a small, slender killifish with a maximum recorded total length of 1 in (FishBase; It Rains Fishes), placing it among the smaller members of its genus. Meristics from the species description and reproduced in the It Rains Fishes profile include: dorsal fin rays D 9, anal fin rays A 16, dorsal-to-anal distance index D/A 7, and lateral line scale count 24 — figures consistent with the genus and reflecting the short, deep-bodied proportions typical of Plesiolebias.
As in most annual rivulids, sexual dichromatism is pronounced. Males display vivid lateral patterning — rows of iridescent spots or chevron marks on a contrasting base colour — while females are the typical cryptic brownish pattern common to all annual killi females, providing camouflage during the brief productive season before the pools dry. The small adult size is characteristic of highly seasonal fish that must complete an entire reproductive cycle within a few weeks of pool formation, selecting strongly for early maturity rather than growth.
Habitat
The natural range of Plesiolebias aruana is restricted to the Araguaia-Tocantins River basin of central Brazil, specifically the temporary pools and seasonal wetlands of the middle Araguaia, in the states of Goiás and Tocantins (FishBase; It Rains Fishes). These are shallow, turbid to clear, heavily vegetated pools that form on the cerrado (Brazilian tropical savanna) during the summer wet season — typically from October or November — and desiccate fully by the end of the dry season, usually between May and September.
Water conditions in these seasonal pools are typically warm (FishBase records a temperature range of 75–79 °F), soft, and slightly acidic to near-neutral, consistent with the laterite and sandy soils of the cerrado biome. Dissolved oxygen can be low as photosynthetic algae and decomposing plant matter accumulate in the shallowing pools toward the end of the wet season. The substrate is soft mud or sandy silt — the medium into which spawning pairs dive to bury their eggs, and in which those eggs must survive the months of drought.
This species does not occur in permanent rivers or streams; it is a strict temporary-water specialist. Its distribution within the Araguaia-Tocantins basin reflects the availability of suitable seasonal grassland and gallery-forest pool habitats rather than any direct connection to the perennial river channel.
Feeding
In the wild, Plesiolebias aruana and its congeners are understood to be generalist micropredators, targeting small invertebrates — aquatic insect larvae, microcrustaceans, worms, and other zooplankton — that concentrate in the short-lived seasonal pools alongside the fish. The limited duration of the habitat means feeding must be intense and opportunistic; growth from hatching to sexual maturity takes only about two months (It Rains Fishes), which demands high-calorie invertebrate prey throughout.
In the aquarium, small live and frozen foods are the ideal diet: baby brine shrimp (Artemia nauplii), daphnia, cyclops, micro-worms, and finely chopped tubifex. Dry foods may be accepted but should not dominate the diet of fish this size. Fry require infusoria or paramecia for the first one to two weeks of life before they can handle nauplii — the tiny size of the hatchlings (characteristic of P. aruana) makes this an important and sometimes logistically demanding stage of the breeding cycle.
Mating
Plesiolebias aruana is an annual killifish and a substrate-spawning species whose reproductive strategy is built entirely around the certainty of pool desiccation. Spawning takes place in pairs within the water column just above the soft substrate. After a brief courtship display in which the male's colours intensify, the pair align flank-to-flank and dive simultaneously into the soft mud or peat layer at the pool bottom, where a small number of eggs are deposited and fertilised in a single dive. The pair then resurface, recover, and repeat the dive sequence over extended periods.
Each dive delivers only a few eggs at a time, but a breeding female can produce a substantial number of eggs across a full season. The It Rains Fishes profile notes that P. aruana is actually a prolific egg-producer — the challenge in the hobby is not a low clutch size but rather a strongly skewed sex ratio in the offspring, with 90 % or more of hatching fry often developing as males. The biological basis of this skew is not fully understood but may be influenced by incubation temperature or diapause duration.
Breeding
The eggs of Plesiolebias aruana undergo diapause — a period of arrested embryonic development — while buried in the dried substrate of the pool. In the wild, this diapause spans the entire dry season, with eggs persisting in the cracked mud until the rains return and pools refill, typically six to nine months later (It Rains Fishes). Hatching is triggered by the rehydration of the substrate.
In the hobby, the peat or coir spawning medium is collected after four to six weeks, lightly rinsed to remove excess fry food residue and infertile eggs, then sealed in a labelled plastic bag at room temperature (approximately 75–79 °F) and stored in darkness for a minimum of six months — the full incubation period documented for this species. It Rains Fishes emphasises that the incubation period is non-negotiable: rewetting the medium before diapause is complete produces very low hatch rates. After storage, the medium is placed in a shallow tray and covered with soft, slightly acidic water at 75–79 °F; fry emerge over a period of days to weeks.
Hatchlings are very small — among the smallest of any Plesiolebias species — and require infusoria or paramecia as first foods for one to two weeks before graduating to newly hatched brine shrimp nauplii. Maturity is rapid: surviving fry can reach breeding condition within approximately two months, consistent with the short annual cycle of the wild population. The expected extreme male bias in offspring means that breeders are advised to maintain multiple pairs and multiple peat batches to obtain enough females for a viable breeding group.
In the aquarium
Plesiolebias aruana is considered a challenging species even by killifish standards, and is recommended only for experienced annual-killifish keepers. Its small size (1 in) means it can be housed in a modest aquarium of 10 US gal, but the tank must be species-only or limited to similarly sized, non-predatory annual killies; the fish is too small and delicate for general community conditions. A dark substrate, dim lighting, and dense floating or fine-leaved plants reduce stress and encourage natural behaviour.
Water should be soft and slightly acidic, in keeping with the cerrado pool chemistry of the native habitat — temperatures of 75–79 °F (FishBase) are appropriate, and the tank should be tightly covered to prevent escape and maintain humidity above the water surface. Filtration should be gentle; a small sponge filter is ideal. The key husbandry challenge is managing the breeding cycle: the long mandatory diapause period, the very small fry, and the male-skewed sex ratio all require planning and patience. It Rains Fishes describes the species as 'difficult to maintain' and notes it is rarely encountered in the hobby even among specialist killi keepers.
Despite these challenges, P. aruana is a rewarding species for those willing to commit to the full annual-killifish protocol. The males are attractively patterned, the biology is fascinating, and successfully raising a generation through the long diapause carries a satisfaction few other freshwater fish can match.
Conservation
The IUCN Red List assessed Plesiolebias aruana as Least Concern (LC) in 2018, with the assessment published on 7 November 2018 by the Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio), Brazil (IUCN Red List). This assessment reflects an absence of evidence for rapid population decline rather than documented abundance — annual killies living in temporary cerrado pools are inherently difficult to survey, and population size estimates are highly uncertain.
The species is a narrow endemic of the middle Araguaia basin, an area that has experienced significant and ongoing land-use change. Brazil's cerrado biome — one of the world's most biodiverse tropical savannas — has lost an estimated half of its original extent to agriculture and cattle ranching, and the conversion of seasonal grassland pools and gallery-forest wetlands directly destroys the only habitat this species can use. Drainage, soybean cultivation, and altered fire regimes eliminate the temporary pool systems on which P. aruana and its syntopic killifish community depend.
Because annual killies exist as dormant eggs between wet seasons, small habitat patches can maintain viable egg banks for some years even after surface conditions degrade — but this resilience is not unlimited. A single episode of deep tillage, pool infilling, or severe drought breaking out of normal seasonal rhythm can eradicate a local population from which no recolonisation is possible. The narrow range of P. aruana means that such local losses are also range-wide losses. Monitoring of remaining seasonal pool systems in Goiás and Tocantins, and inclusion of temporary cerrado wetlands in conservation planning for the Araguaia-Tocantins basin, are necessary to ensure the long-term survival of this and several co-occurring endemic annuals.