Taxonomy & naming
Boraras naevus was formally described by Kevin W. Conway and Maurice Kottelat in 2011. The genus Boraras was erected by Kottelat and Vidthayanon in 1993 to accommodate a set of dwarf, sexually mature miniaturised cyprinids that had previously been scattered through the large and paraphyletic genus Rasbora. The name Boraras is an anagram of Rasbora, reflecting this origin. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) recognises Boraras naevus as the valid name for this species, with the authority Conway & Kottelat, 2011; the species has no prior synonyms under another combination.
The epithet naevus is Latin for 'a spot or mark on the skin,' a direct reference to the sexually dimorphic lateral blotch that distinguishes this species from its congeners. Six other species of Boraras are currently recognised — including the well-known B. brigittae (chilli rasbora) and B. maculatus (dwarf rasbora) — and B. naevus is placed among them on the basis of its extremely small adult size, pelvic and pectoral fin structure, and the shared blackwater peat-swamp ecology of Sundaland and adjacent Indochina. All Boraras are placed in the family Cyprinidae (subfamilially within Danioninae) by most modern classifications.
Morphology
Boraras naevus is one of the smallest cyprinids in the world, with standard lengths typically between 15 and 1 in and a maximum of roughly 1 in SL. The body is laterally compressed, deep relative to its length, and the head is small with a slightly upturned terminal mouth suited to surface and mid-water feeding. Coloration in life includes a reddish to coppery body ground colour, consistent with the genus but variable according to condition and lighting.
The defining character is the lateral blotch from which the species takes its name. Females carry a small, roughly circular dark spot on the mid-flank; males bear a conspicuously larger, elongated oval marking in the same region. This sexual dichromatism in blotch shape — not just size — distinguishes B. naevus from its closest relatives within Boraras. Beyond the blotch, males tend to be slimmer and more intensely coloured; females are deeper-bellied and appear rounder when ripe with eggs, as is typical across the genus.
Habitat
The type locality of Boraras naevus is a swamp north of Surat Thani in the lower Tapi River drainage, Peninsular Thailand, on the Gulf of Thailand slope. More broadly, the species is known from the lower Tapi drainage in southern peninsular Thailand — a region characterised by low-lying coastal and near-coastal lowland swamps, slow-moving forest streams, and seasonally flooded ground that was historically dominated by freshwater swamp forest.
As with other Boraras, B. naevus inhabits soft, acidic, heavily tannin-stained blackwater environments. Aquarium literature records water parameters of pH 5.0–7.0, temperatures of 68–82 °F, and hardness of 18–179 ppm — a wide but consistently soft and warm range. The natural substrate is likely leaf litter and fallen wood on dark, peaty sediment. These blackwater swamps are highly productive at the microbial and zooplankton level despite their apparent austerity, and they support a specialised community of miniaturised fishes including multiple Boraras species.
Feeding
Boraras naevus is a micropredator. Its small mouth and disproportionately large eyes relative to body size are adaptations for hunting very small prey items in dim, tannin-stained water. In its natural blackwater swamps it feeds on tiny invertebrates — copepods, ostracods, insect larvae, small worms and similar meiofauna — that proliferate in the leaf-litter substrate and the water column.
In the aquarium it requires correspondingly small foods. Live or frozen Daphnia, micro-worms, baby brine shrimp (Artemia nauplii), and cyclops are ideal staples that elicit natural hunting behaviour. High-quality flake food ground to a fine particle size may be accepted but should not constitute the entire diet; the species feeds best when offered varied live and frozen items that match the prey size its tiny mouth can handle. Like all Boraras it is not competitive with larger or faster fishes at feeding time.
Mating
Boraras naevus is an egg-scattering, non-guarding spawner. Spawning behaviour follows the pattern common to danionine cyprinids: a conditioned male pursues a gravid female through dense vegetation, fine-leaved moss or leaf litter, pressing alongside her and releasing milt as she expels eggs. The eggs are small and weakly adhesive, settling among plants or onto the substrate.
There is no pair bond and no parental behaviour beyond the act of spawning itself. Sexual dimorphism — most obviously in the size and shape of the lateral blotch but also in overall body proportions — allows reliable sex identification in healthy adult fish, which is useful when selecting pairs or groups for breeding attempts. The species is noted as a continuous spawner in appropriate conditions, producing small clutches at frequent intervals rather than in single large events.
Breeding
In the aquarium, Boraras naevus has been bred by dedicated hobbyists, though its small size and specific water requirements make it more demanding than many danionine cyprinids. A separate breeding vessel with soft, acidic water (pH around 5.5–6.5) at 75–79 °F, heavily planted with fine-leaved plants or java moss, provides the conditions most likely to elicit spawning. A peat-filtered or blackwater setup with very gentle filtration reduces egg mortality.
Fertilised eggs hatch in approximately two days at tropical temperatures. The larvae are extremely small at hatch and require appropriately tiny first foods — infusoria, commercially available rotifers, or paramecia — before graduating to newly hatched brine shrimp nauplii. Adults will consume both eggs and fry given the opportunity, so removing adults after spawning is strongly advised. Fry growth is slow relative to larger cyprinids, and patience is required before juveniles are large enough to accept standard small frozen foods. This species is infrequently bred in captivity, and animals available in the trade are likely either wild-caught or produced by specialist breeders.
In the aquarium
Boraras naevus is an advanced-level nano fish, best suited to an experienced keeper willing to provide the soft, acidic blackwater conditions it requires. A species-only or carefully chosen community tank of 10–15 US gal is appropriate; tankmates should be limited to similarly tiny, peaceful fishes that share its water chemistry requirements — other Boraras species, small Sundadanio, or comparably sized rasboras. The species is easily intimidated by larger or more active fish and may refuse to feed openly in busy tanks.
Water quality is critical: soft (under 100 ppm hardness), acidic (pH 5.5–7.0), warm (72–81 °F), and well-oxygenated but with very gentle flow. Tannin-stained water from dried leaves or peat filtration replicates the natural blackwater environment, reduces stress and may encourage natural colouration and breeding behaviour. Dense planting with fine-leaved plants and floating cover provides security. This is a shoaling species — a group of at least eight to ten individuals allows natural schooling behaviour and reduces the vulnerability of any single fish. When kept correctly and fed well, it is a rewarding display fish whose dimorphic blotch and rich body colour make it quietly striking in a well-planted blackwater aquascape.
Conservation
Boraras naevus is assessed as Data Deficient (DD) on the IUCN Red List (2022.2 assessment). Data Deficient does not imply security; rather, it reflects that so little is known about the species' actual distribution, population size and population trends that no threat category can yet be assigned. The species is known from only its type locality and nearby localities in the lower Tapi drainage of southern peninsular Thailand — an extremely restricted known range.
The broader conservation context is cause for concern. Peninsular Thailand's coastal lowland swamps have been heavily transformed: the natural swamp forest and peat-swamp habitats that Boraras naevus depends upon have been largely drained, cleared and converted to paddy fields, oil-palm plantations and other agricultural land. This process began decades before the species was scientifically described and continues today. Several other narrow-range Boraras species face analogous pressures across Sundaland, and the guild as a whole is vulnerable to habitat loss at a scale that threatens local extirpation before populations can even be assessed. Further field surveys of the lower Tapi drainage are needed to determine the species' actual distribution and abundance.