Taxonomy & naming
Hemibrycon carrilloi was described by the Danish-Colombian ichthyologist Guillermo Dahl in 1960 in his paper on new freshwater fishes from western Colombia, published in the journal Caldasia (volume 8, number 39). The type locality is Quebrada La Noche, a small stream tributary to the upper Atrato River, in the municipality of Riosucio, Antioquia Department, at an elevation of approximately 1804 ft above sea level. Eschmeyer's Catalog of Fishes maintains Hemibrycon carrilloi Dahl, 1960 as a valid name within the genus Hemibrycon.
Hemibrycon is a medium-sized genus of characids distributed across northern and western South America, with centres of diversity in the Andes-flanking drainages of Colombia, Ecuador, and Peru. The genus belongs to the subfamily Stevardiinae (Characidae), a diverse clade of small to medium-sized South American characins. Several species of Hemibrycon occupy fast, clear Andean and sub-Andean streams, and H. carrilloi fits this general guild.
The specific epithet carrilloi is a patronym honouring Dr Jorge Carrillo, who at the time of description held the position of Director of the Colombian Fisheries Department. The genus name Hemibrycon derives from the Greek roots hemi (half) and bryko (to bite), a reference shared with related genera and alluding to the jaw structure typical of the group.
Morphology
FishBase records a maximum standard length of 2.5 in for Hemibrycon carrilloi, placing it among the smaller members of its genus. Like other Hemibrycon, it is a slender, fusiform fish adapted to life in flowing water, with a moderately compressed body, a forked tail, and the two dorsal fins characteristic of the order Characiformes — a short main dorsal and a small, fleshy adipose fin behind it.
Detailed morphological data for this species are sparse in the accessible literature, and a thorough meristic and morphometric description is available only in Dahl's original 1960 account. The benthopelagic habit attributed to the species by FishBase suggests it occupies the lower and middle portions of the water column, using both the substrate zone and open water, as is typical of Hemibrycon in hill-stream habitats.
Data on coloration in life are not available in the sources consulted for this account. In preserved specimens, members of Hemibrycon typically show a lateral stripe running from behind the operculum to the base of the caudal fin, a humeral spot, and a dark blotch at the caudal peduncle, but the exact pattern in H. carrilloi requires reference to the original description and freshly collected material.
Habitat
Hemibrycon carrilloi is recorded from the upper Atrato River basin and the Baudó River drainage of western Colombia — both systems draining the Pacific-facing slopes and foothills of western Colombia. The Atrato is a large blackwater-to-clearwater river that flows northward through the Chocó department to the Gulf of Urabá on the Caribbean, while the Baudó is a separate, shorter river draining the Serranía del Baudó directly to the Pacific. The type locality, Quebrada La Noche near Riosucio, is a small creek at about 1804 ft elevation, typical of the sub-Andean foothill streams where species of Hemibrycon commonly occur.
The IUCN account categorises the species as inhabiting small creeks and permanent rivers and streams in freshwater. Sub-Andean streams in the Chocó biogeographic region are typically clear to lightly stained, fast-flowing, well-oxygenated, and cool to warm depending on elevation; they are underlain by rocky or sandy substrates with patches of leaf litter. This western Colombian lowland-to-foothill zone is among the wettest and most biodiverse regions on Earth, yet many of its small-stream fishes remain poorly documented.
Beyond the type locality and the Baudó River, precise locality data for H. carrilloi are limited. The IUCN assessment notes the species' occurrence in both the upper and lower Atrato basin as well as the Baudó, but the population is described as rare and no systematic surveys of its distribution have been published in the available literature.
Feeding
No specific dietary data for Hemibrycon carrilloi appear in the sources consulted for this account. Members of the genus Hemibrycon are generally regarded as opportunistic omnivores or invertivores in fast-flowing Andean streams, feeding on aquatic invertebrates — insect larvae, small crustaceans, and other benthic organisms — as well as terrestrial invertebrates that fall onto the water surface and, in some species, algal material.
Given the species' benthopelagic habitat association and its occurrence in small, clear creeks, it likely forages in the mid-water and near the substrate, taking whatever small food items the current delivers. The food habits of H. carrilloi have not been formally studied and data sparse notes should be applied to any specific dietary claims until field studies are available.
Mating
No published observations of mating behaviour in Hemibrycon carrilloi are available. Based on reproductive biology reported for other members of Hemibrycon and the broader family Characidae, the species is presumed to be an egg-scatterer, releasing non-adhesive to slightly adhesive eggs in open water or among substrate materials without building a nest. Males likely pursue and court females during brief spawning events, with no pair bond forming.
In many small Andean characids, spawning is triggered by seasonal changes in water level and temperature associated with the wet season, which drives fish into flooded margins and smaller tributaries where eggs can develop with reduced predation pressure. Whether H. carrilloi follows such a pattern is unknown.
Breeding
Hemibrycon carrilloi has not been reported in the aquarium hobby and no captive-breeding accounts have been located. The general pattern for egg-scattering characids applies by default: spawning over fine substrate or vegetation, adhesive to semi-adhesive eggs that receive no parental care, and fry that are small and require appropriately sized first foods.
Without captive observations of this species, incubation times, fecundity, and larval development are unknown. The species' rarity and restricted distribution mean that collection for the ornamental trade is not documented, and any captive specimens would likely derive from research collection rather than the hobby supply chain.
In the aquarium
Hemibrycon carrilloi is not known in the aquarium hobby and is not commercially available. As a small, slender characid from fast, clear Andean foothill streams, it would in principle require well-oxygenated, clean water with good current, a slightly acidic to neutral pH, and temperatures in the low-to-mid 20s Celsius consistent with its sub-Andean habitat at 1804 ft elevation.
With a maximum size of 2.5 in SL, it would be manageable in a moderately sized aquarium if specimens were ever obtained through research or conservation programs. Its habitat in clear, swift creeks suggests sensitivity to warm, stagnant, or polluted water. This account is provided for completeness; practical husbandry details await any future hobbyist or researcher who works directly with the species.
Conservation
Hemibrycon carrilloi is assessed as Data Deficient (DD) on the IUCN Red List (assessed 9 October 2014, published 2016), a status that reflects the absence of adequate information on population size, trends, and the threats acting on the species rather than a determination that it is not at risk. The assessor, F. Villa-Navarro, noted that the species is rare at the localities where it has been found, that no population size data exist, and that no occurrence in protected areas has been documented.
Interestingly, the species was previously listed as Least Concern (LC) in 2009 before the 2016 reassessment downgraded it to DD on the basis of insufficient data — a reminder that a DD listing can represent deteriorating knowledge rather than a stable situation. The Chocó region of western Colombia, which encompasses the Atrato and Baudó drainages, faces significant pressure from gold and platinum mining, agricultural expansion, deforestation, and infrastructure development, all of which degrade small streams. Until systematic surveys establish the actual extent of the species' range, population trends, and tolerance of habitat change, its true conservation situation remains unknown.