Taxonomy & naming
Nematobrycon lacortei was formally described by Stanley H. Weitzman and William L. Fink in 1971 in the journal Beaufortia (volume 19, number 248, page 59), with the type locality recorded as probably the Río Calima, Colombia. The holotype (USNM 205594) and paratypes deposited in BMNH, CAS, FMNH, USNM, and ZMA anchor the species firmly in the San Juan River basin of Colombia's Pacific versant. Eschmeyer's Catalog of Fishes lists the valid name as Nematobrycon lacortei Weitzman & Fink, 1971 and places it in the subfamily Rhoadsiinae within Characidae.
The genus Nematobrycon is a small Neotropical group most familiar to aquarists through N. palmeri, the emperor tetra. N. lacortei is its closest described relative and broadly similar in body plan, but the two are separable by colour pattern — most notably in the male's intensely multicoloured iridescence — and by the fine structural details used in the original description. Because the species was described by Weitzman and Fink (not reassigned from another genus), no parenthetical brackets appear around the authority in the valid name.
Morphology
Nematobrycon lacortei is a small, slender-bodied characid reaching a maximum standard length of approximately 1.5 in. Like its congener N. palmeri it is laterally compressed with a moderately deep body, large eyes, and the trident-like extension of the caudal fin's central rays that gives the genus part of its common-name cachet — 'emperor' referencing those elongated fin filaments in dominant males.
The colour pattern is the species' defining feature and the source of its 'rainbow' vernacular: healthy males display an extraordinary combination of iridescent blues, greens, reds, and purples that shift with angle and light intensity. A bold, dark lateral band runs from behind the operculum to the caudal peduncle, underlining the iridescent body colours. Females are more subtly marked, with less intense colour and without the elongated caudal rays of mature males — a reliable guide to sexing adult fish in an aquarium setting.
Habitat
In the wild, Nematobrycon lacortei occupies small, isolated forest pools in the headwaters of the Río Calima, part of the San Juan River drainage system on the Pacific slope of Colombia. These are plant-rich, often shaded environments — the classic Neotropical blackwater or clearwater forest pool, with soft, acidic water tinted by tannins leaching from leaf litter and woody debris. Reported physico-chemical parameters from Seriously Fish place the natural range at roughly 68–82 °F, pH 5.0–7.2, and hardness 18–179 ppm, reflecting the wide variation possible across pool microhabitats within the same catchment.
The Río Calima drains the Western Cordillera of the Colombian Andes before entering the San Juan, a Pacific-slope river with relatively poor connectivity to the main Andean headwater systems. This geographic isolation — small forest pools in a restricted catchment — means the species has a naturally narrow range even within its Colombian homeland. Surveys of the precise distribution remain limited, contributing to its Data Deficient status with the IUCN.
Feeding
Nematobrycon lacortei is an omnivore. In its natural forest-pool habitat it likely takes small invertebrates — chironomid larvae, microcrustaceans, small worms, and terrestrial insects fallen to the water surface — alongside any fine organic detritus and algae available in plant-rich, slow-moving water.
In the aquarium the species adapts readily to a varied diet. Seriously Fish recommends a mix of small live and frozen foods (chironomid larvae, Daphnia, Artemia/brine shrimp) together with a quality dried staple. Keeping foods small is important given the fish's compact size: oversized particles are ignored or difficult to swallow. Frequent small meals tend to improve condition and colour in this species more reliably than single daily rations.
Mating
Nematobrycon lacortei is an egg-scattering species with no parental care. Males court females with displays in which they intensify their iridescent body colours and extend the elongated central caudal-fin rays — an advertisement of fitness that parallels the behaviour seen in N. palmeri. Rival males display against one another as well, and groups of males kept together are noticeably more vivid than solitary fish.
Spawning takes place among fine-leaved aquatic plants or over synthetic spawning mops, with the pair releasing small, adhesive eggs into the vegetation. Soft, acidic water and dim lighting appear to encourage spawning activity. As with most egg-scattering characids, the breeding adults will consume eggs given the opportunity, so separation or dense spawning cover is needed to protect the spawn.
Breeding
Breeding Nematobrycon lacortei follows the same pattern as other small characids from soft, acidic South American waters. A dedicated breeding setup with fine-leaved plants or spawning mops, very soft and acidic water (pH 5.0–7.0, gH 1–8), and a temperature of approximately 75–81 °F (75–80 °F) provides optimal conditions. Low light levels reduce stress and encourage the pair to move into the spawning medium.
Eggs are small and adhesive and hatch within 24–36 hours at breeding temperatures; the fry become free-swimming approximately four to five days after hatching. At this stage they are tiny and require infusoria or equivalent micro-foods as a first diet before graduating to newly hatched brine shrimp nauplii and finely powdered flake. Adults should be removed promptly after spawning to prevent egg predation. Tank-bred stock is available in the specialist hobby and is strongly preferred over wild-caught fish given the species' restricted range.
In the aquarium
Nematobrycon lacortei is a peaceful schooling species best kept in groups of eight or more. A minimum aquarium of roughly 120 × 30 × 12 in (~30 US gal) is recommended by Seriously Fish to give the school adequate space, though many aquarists house smaller groups in slightly more modest tanks with success. The tank should be planted, with areas of shade created by floating vegetation or overhanging leaves, driftwood, and perhaps a peat substrate or tannin-stained water to replicate the species' darkwater origins. A dark substrate and subdued lighting bring out the colours to their full extent.
Water chemistry should be soft and acidic — pH 5.0–7.2, hardness in the low range — and temperature stable in the 68–82 °F window. The species is not demanding provided extremes are avoided, but it is not as tolerant of hard or alkaline water as many common tetras. Good tankmates are other small, peaceful Neotropical species from similar water conditions: small corydoras, dwarf cichlids such as Apistogramma, and other soft-water characids. The rainbow tetra is not commonly available in the trade, but specialist breeders and aquarium societies occasionally offer stock.
Conservation
Nematobrycon lacortei is assessed as Data Deficient (DD) on the IUCN Red List (2022.2 assessment). Data Deficient does not mean the species is secure — it means that insufficient information exists to assign a threat category. Given its apparently small, naturally fragmented range in isolated forest pools of the Río Calima headwaters, and the general pressure on Pacific-slope Colombian forests from agriculture and infrastructure development, the species' actual situation in the wild may be more precarious than the DD listing suggests.
The rarity of this fish in scientific collections and in the trade reflects both the remoteness of its type locality and the challenges of surveying small forest pools in a poorly accessed region. Hobbyist breeding programmes represent the most reliable safeguard currently in place: captive-bred populations maintained by dedicated breeders ensure that the species remains available without placing collecting pressure on wild fish. Aquarists keeping or breeding this species contribute directly to that insurance population.