Taxonomy & naming
Cyanogaster noctivaga was described by Mattox, Britz, Toledo-Piza and Marinho in 2013 from material collected in the Rio Negro drainage of the Amazon basin. The original description erected both a new genus and a new species, placing the fish within the subfamily Stevardiinae on the basis of morphological characters, including a distinctive single conical tooth in the outer premaxillary tooth row — a feature absent in other stevardiines examined at the time. Eschmeyer's Catalog of Fishes records the valid name as Cyanogaster noctivaga Mattox, Britz, Toledo-Piza & Marinho, 2013.
The genus name is formed from the Greek kyanos (blue) and gaster (belly), a direct reference to the iridescent blue-violet abdominal region that is the fish's most striking visible character. The specific epithet noctivaga is Latin, combining nox (night) and vagare (to wander), reflecting the collecting circumstances: specimens were taken exclusively during nocturnal surveys between 20:00 and 24:00 over rocky plateau substrates, suggesting genuinely crepuscular or nocturnal habits rather than simply a collecting artefact.
Morphology
Cyanogaster noctivaga is one of the smallest characiforms described, with a maximum recorded standard length of 0.5 in. The body is notably translucent, with the internal organs visible through the body wall, and the overall appearance in life is of a near-invisible sliver of water with a conspicuous iridescent blue abdominal band — the combination of transparency and chromatic colour is unusual among small South American characids.
Fin counts recorded for the species include two unbranched plus eight branched dorsal-fin rays (D ii+8) and 19–22 anal-fin rays. Both the dorsal and anal fins carry lines of black chromatophores along their margins, providing modest but distinct patterning against the otherwise hyaline body. The single conical outer premaxillary tooth distinguishes the species within its subfamily at the time of description.
Habitat
The type locality of Cyanogaster noctivaga is the Rio Negro drainage of the Amazon basin, which drains a vast area of lowland northern Brazil and adjacent Colombia. The Rio Negro is the world's largest blackwater river, with water stained tea-brown to near-black by dissolved humic and fulvic acids leached from surrounding forest soils and leaf litter. Conductivity is extremely low, pH is acidic, and dissolved mineral content is close to nil.
Specimens were collected at approximately 5 ft depth over rocky plateau substrates during the dry season in October, entirely during nocturnal survey hours. This habitat association — shallow, fast-flowing or still acidic blackwater with rock substrates, active at night — distinguishes Cyanogaster noctivaga from many co-occurring small tetras that are diurnal and occupy open water or vegetated margins. Its range appears to include both Brazil and Colombia based on distributional records associated with the Rio Negro system.
Feeding
Field data on the natural diet of Cyanogaster noctivaga is limited. FishBase records insect larvae as a dietary component, which is consistent with the nocturnal activity pattern: many aquatic insect larvae are themselves more active or vulnerable at night, and a nocturnally foraging miniature fish in shallow rocky habitat would be well-positioned to exploit them.
In the aquarium the species has not been widely maintained, and detailed feeding records are sparse. By analogy with other miniature stevardiine and characid species of similar size and ecology, it would be expected to accept very small live and frozen invertebrates — micro-worms, baby brine shrimp, and similar micro-foods — though data remains limited for this species specifically.
Mating
No direct observations of courtship behaviour in Cyanogaster noctivaga have been published. As a stevardiine characid, the species would be expected to practise egg-scattering open-water or plant-associated spawning, with the male pursuing and displaying to the female prior to release of eggs.
The iridescent blue abdominal colouration that names the genus may plausibly function in intraspecific signalling — either mate choice or species recognition in the low-light, tannin-stained blackwater environment where visual cues are reduced. Whether the blue belly is expressed equally in both sexes, or differs between sexes as a basis for dimorphic signalling, remains undocumented in the literature available.
Breeding
Captive breeding of Cyanogaster noctivaga has not been documented in the available literature or hobby sources. Based on the broader Stevardiinae pattern, the species would be expected to scatter adhesive eggs among fine-leaved plants or over substrate, with no parental care; the adults would likely consume their own eggs if not separated.
The extremely small adult size — 0.5 in SL — means that any eggs and larvae would be correspondingly tiny, requiring the finest available live foods for the first-feeding stage. Practical breeding, if attempted, would call for very soft, warm, acidic water replicating Rio Negro conditions, a mature tank with minimal flow, and a separate rearing vessel to protect eggs and fry.
In the aquarium
Cyanogaster noctivaga is not an established aquarium fish and is essentially unknown in the hobby. Its extremely small size, specialised blackwater habitat, and nocturnal behaviour make it a challenging subject even for experienced keepers of nano and microspecies. Any attempt to maintain it would call for a mature, heavily tannin-stained aquarium with very soft, acidic, warm water, very low flow, and fine substrate or leaf litter mimicking its Rio Negro rocky-plateau habitat.
The species' translucent body and blue belly shimmer would be most visible under dim or red-spectrum lighting that mimics low-light blackwater conditions and aligns with its nocturnal activity window. It should not be mixed with anything large enough to eat it; ideal tankmates, if any, would be similarly sized or smaller microspecies tolerant of identical soft-acid water. No care rating can responsibly be assigned given the absence of established hobby experience.
Conservation
Cyanogaster noctivaga is assessed as Least Concern (LC) on the IUCN Red List, with the assessment dated 4 March 2021. The LC designation reflects that the Rio Negro drainage — despite being the world's largest blackwater river system — remains largely intact relative to many other Amazonian drainages, and the species is not known to face imminent large-scale habitat loss or targeted exploitation.
Nevertheless, the species was described only in 2013 and its precise range, population size, and ecological requirements remain poorly documented. The Rio Negro basin faces incremental pressures from deforestation, climate-driven hydrological change, and artisanal mining in parts of its catchment. As a small, presumably habitat-specific fish tied to rocky blackwater substrates, Cyanogaster noctivaga could be more range-restricted and sensitive to local disturbance than a bare LC listing suggests; further survey work would be needed to fully characterise its distribution and abundance.