Taxonomy & naming
Boraras brigittae was originally described by Dieter Vogt in 1978 as Rasbora urophthalma brigittae, treating it as a subspecies of the urophthalma group within the large, historically catch-all genus Rasbora. The species was elevated to full species rank and transferred to the newly erected genus Boraras by Kottelat & Witte in 1999 when they diagnosed Boraras as a clade of very small Southeast Asian miniature cyprinids diagnosably distinct from Rasbora. The current valid name, as recorded in GBIF and consistent with Kottelat & Witte's revision, is Boraras brigittae (Vogt, 1978), with the authority in parentheses marking the transfer from the original genus.
Boraras comprises a small cluster of micro-cyprinids from the Sundaland region and Indochina, all characterised by their diminutive size (rarely exceeding 1 in) and their preference for highly acidic, organically stained blackwater. The genus is closely allied to Rasbora and Microrasbora, and species in all three genera were historically pooled under Rasbora before molecular and morphological work clarified their relationships. The species epithet honours Brigitte, the wife of the original describer, Dieter Vogt.
Morphology
The chili rasbora is a tiny, slender, laterally compressed fish reaching only about 0.5–1 in in standard length, with FishBase citing a maximum total length of 1.5 in though this likely reflects an outlier; common length in aquarium literature is consistently given as 0.5–1 in SL. The body is semi-transparent with a pale pinkish-silver base colour. A bold, blood-red to deep crimson horizontal stripe runs from the snout through the eye and along the flank to the caudal peduncle, bordered above by a thin dark line and below by a silver-white zone — in fully coloured males this stripe is vivid enough to account for the common name 'chili rasbora'.
Sexual dimorphism is subtle but present. Males are more intensely coloured and slimmer, with the red stripe typically brighter and the finnage showing more colour. Females are slightly deeper-bodied and, when gravid, visibly rounder in the belly. The fins are largely hyaline with reddish tones at their bases. The small, upturned mouth reflects a micropredator that feeds in the middle and upper water column.
Habitat
Boraras brigittae is endemic to southwestern Borneo, Indonesia, known from swampy lowland habitats in the region centred on Bandjarmasin and extending westward to the Jelai Bila river basin near Sukamara. Its habitat is the tropical peat-swamp forest — shallow, heavily shaded pools and streams underlain by deep accumulations of decomposing organic matter, the water permanently stained dark amber-to-black with humic and tannic acids leached from fallen leaf litter and peat.
The water chemistry in these habitats is extreme by most freshwater standards: pH can fall below 4.0, hardness is negligible (often 18–179 ppm as recorded by SeriouslyFish), and temperatures remain warm year-round, typically 77–82 °F (FishBase; SeriouslyFish gives a broader tolerance of 68–82 °F in aquaria). The substrate is soft, often composed of leaf litter and fine organic sediment. Light penetration is low, and dissolved oxygen can be reduced. The extent of occurrence for the species is estimated at around 87,0 mi² (IUCN, 2020), but this figure encompasses the broader southwestern Borneo swamp region rather than confirmed occupied habitat.
Feeding
In the wild, Boraras brigittae is a micropredator, subsisting on the minute invertebrates of the peat-swamp water column: small insects, mosquito larvae and other dipteran larvae, aquatic worms, tiny crustaceans and zooplankton. Its small mouth limits prey size, and it is adapted to hunting very small, slow-moving or drifting organisms in dim, highly acidic water where food resources may be patchy.
In the aquarium it responds well to appropriately sized live and frozen foods — Daphnia, Artemia nauplii, micro-worms, and Paramecium for conditioning — alongside quality micro-pellets or small crushed flakes. Because of its tiny mouth, standard tropical flakes need to be ground fine or replaced with dedicated nano-fish foods. Variety in the diet maintains best colour and breeding condition. It feeds actively in the middle and upper water column and is not competitive enough to hold its own against larger or more assertive tankmates at feeding time.
Mating
Boraras brigittae is an egg-scattering, non-guarding spawner. There is no nest, no pair bond beyond the act of spawning, and no parental care of any kind. In aquarium conditions the species spawns continuously in small batches rather than in discrete seasonal events, which reflects the relatively stable year-round conditions of its equatorial peat-swamp homeland. A conditioned male pursues a gravid female through fine-leaved vegetation, pressing alongside her as eggs and milt are released among the plants or onto the substrate.
The adhesive eggs sink and cling to plant leaves, moss, or fine substrate particles. Adults will graze on the eggs if they encounter them, so planted cover and soft substrate that helps conceal spawning sites both serve the purpose of improving survival in a heavily planted community setup. There is no change in behaviour by either parent after spawning.
Breeding
Breeding Boraras brigittae in captivity is straightforward once water chemistry is matched to the species' blackwater requirements. A dedicated breeding tank — even a small vessel of 2.5–5.5 US gal — should be set up with very soft, acidic water (pH 4.0–6.5, negligible hardness), dim lighting and a generous mat of fine-leaved plants or Java moss to catch eggs and protect fry. A thin layer of dark substrate or bare base over dark material reduces light stress on both adults and larvae. Peat filtration or the addition of Indian almond leaves helps replicate the tannin-stained blackwater chemistry.
Eggs hatch in approximately two days at 79–82 °F. The tiny larvae are free-swimming within a few days and require infusoria or commercially prepared liquid fry food for the first week or so before graduating to Paramecium and freshly hatched Artemia nauplii. Because of the microscopic size of the fry, predation and accidental removal during water changes are the main hazards. Growth is steady but slow relative to larger danionins. Adults can be left in a heavily planted tank to spawn naturally, with some fry surviving to adulthood among dense vegetation.
In the aquarium
The chili rasbora is a peaceful, shoaling nano fish that should be kept in groups of ten or more — in smaller numbers individuals are shy and pale, but a well-sized shoal in a planted blackwater tank produces a display of colour disproportionate to the species' tiny size. Its diminutive scale means it must not be kept with any fish large enough to treat it as prey; ideal companions are other micro-cyprinids, small Otocinclus, dwarf shrimp (though very small shrimp fry may be eaten) and similarly scaled peaceful species.
The aquarium should reflect the species' blackwater origins: soft, acidic water (pH 4.0–7.0), temperature 72–82 °F, gentle filtration (a sponge filter is ideal as it avoids creating strong currents), dim lighting or heavy floating plant cover, tannin-stained water from leaf litter or peat, and a dark substrate. Under these conditions the red coloration reaches its maximum intensity. The species is long-lived for its size — three or more years — and thrives without the need for extremes of temperature or aggressive feeding regimens, making it well suited to a carefully maintained nano or blackwater biotope setup.
In hard, alkaline, or brightly lit aquaria the chili rasbora fades in colour, becomes stress-prone, and is more susceptible to disease; matching water chemistry to the Bornean peat swamp is the single most important husbandry decision.
Conservation
Boraras brigittae is assessed as Data Deficient (DD) on the IUCN Red List (assessed February 2019, published 2020). The DD listing reflects insufficient data on its area of occupancy, the number of distinct populations, and population trends — not evidence of security. The species' entire known range falls within the peat-swamp forests of southwestern Borneo, one of the world's most intensively converted tropical ecosystems. Bornean lowland peat swamps have been drained, burned and replaced by oil-palm and acacia plantations at a large scale over recent decades, and the highly acidic, oligotrophic conditions these fish require are eliminated when peat-swamp hydrology is disturbed.
The extent of occurrence figure (~87,0 mi², IUCN 2020) covers a broad geographic area but does not equate to available intact habitat; the actual area of undisturbed peat swamp within that range is substantially smaller and continues to shrink. Collection for the ornamental fish trade adds a further pressure, though the species does breed readily in captivity and captive-bred stocks supply a significant proportion of the hobby market. A proper assessment of wild population status would require systematic field surveys of remaining Bornean peat-swamp forest.