Taxonomy & naming
Brycon guatemalensis was described by the British ichthyologist Charles Tate Regan in 1908. It belongs to the family Bryconidae — the machacas and their relatives, formerly lumped into the pacu-and-piranha family Serrasalmidae in older literature but now recognised by the Catalog of Fishes (Eschmeyer, California Academy of Sciences) as a distinct family within Characiformes. The genus Brycon is large and taxonomically difficult, with many nominal species historically confused on the basis of overlapping colouration and body proportions; Brycon guatemalensis is the widely accepted valid name for the Central American machaca across its Atlantic-slope range.
The common name "machaca" is used throughout Central America for this species and is also applied loosely to other regional Brycon and Bramocharax species, so common-name identification alone is unreliable; the species is also traded and referenced under "macabi tetra" in some English-language sources. GBIF lists the species (taxon ID 2353514) as a valid, accepted name within the Neotropical characin fauna of Central America.
Morphology
Brycon guatemalensis has the classic Brycon build: a moderately elongated, laterally compressed, silvery body built for sustained swimming rather than ambush, with a forked tail and a small adipose fin typical of characins. FishBase records a maximum published length of 23 in total length for males and unsexed individuals, with sexual maturity reached around 10.5 in — figures that place this species well above the size of the small shoaling tetras most aquarists associate with the characin order.
The jaw carries strong, close-set teeth suited to its mixed diet of fruit, seeds, insects and other prey, and the body is countershaded silver, consistent with a fish that spends much of its time in open water and near the surface, where it forages under the forest canopy along riverbanks.
Habitat
The species occupies lakes, rivers and streams across the Atlantic slope of Central America, from the Grijalva River system in Mexico through Guatemala, Honduras and into eastern Panama, with a separate Pacific-slope population confined to the Choluteca River basin in Honduras. It is a freshwater, benthopelagic fish found from sea level up to about 1969 ft elevation, and it tolerates a broad tropical temperature range, with FishBase citing roughly 70–93 °F across its range.
Riparian vegetation is an integral part of its habitat rather than incidental cover: machaca actively forage beneath overhanging trees for fallen and low-hanging fruit and seeds, so intact streamside forest canopy is closely tied to the species' feeding ecology and, by extension, to seed dispersal for the plants it feeds on.
Feeding
Diet shifts markedly with age. Juvenile Brycon guatemalensis take insects, leaves, fruits and seeds in a fairly general omnivorous pattern, while adults become primarily frugivorous, specialising on fruits and seeds that fall — or are plucked directly — from riparian trees. The species is noted for its ability to leap clear of the water surface to take fruit and seeds from low branches, a foraging behaviour that makes it a locally important agent of seed dispersal along the rivers it inhabits.
Adults also continue to take plant material alongside invertebrates and small fish, so the diet is best described as omnivorous with a strong frugivorous emphasis in maturity, rather than strictly herbivorous or strictly predatory.
Mating
Data on courtship behaviour specific to Brycon guatemalensis is sparse in the sources available here. What is documented is the reproductive biology surrounding spawning: females reach 50% sexual maturity at approximately 10.5 in, and the species has a group-synchronous ovary organisation, meaning a batch of eggs matures and is released together rather than being laid continuously in small increments. Potential annual fecundity has been estimated at roughly 701 eggs, increasing allometrically with female body size — larger females produce disproportionately more eggs.
Breeding
Brycon guatemalensis has an extended spawning season of roughly eight months beginning in July, coinciding with the wet season across its Central American range. Reproduction has been documented as egg deposition in nests on sandy stream bottoms, rather than open-water broadcast spawning or any form of nest-guarding parental care; the eggs are effectively left in the substrate rather than tended by the adults.
This is a wild, river-spawning pattern typical of larger migratory or semi-migratory characins, and there is no evidence in the sources here of the species being bred in home aquaria — it is not a hobbyist-bred fish, and its size and habitat requirements make captive breeding impractical outside of research or aquaculture settings.
In the aquarium
Brycon guatemalensis is not a realistic aquarium fish for the vast majority of keepers. At a documented maximum of 23 in total length, it is a large, active river fish that needs correspondingly large volumes of water and strong, sustained water flow to thrive — conditions far beyond a standard home aquarium. Where it is kept at all, it is typically in large public aquaria, research facilities or very large pond systems in tropical climates, not community tanks.
As a fast-growing, omnivorous species with a strong frugivorous streak in adulthood, any keeper considering this fish should be prepared to offer fruit and plant matter alongside protein and to plan for an adult that will outgrow almost any home setup. It should never be released into waters outside its native range if a keeper can no longer accommodate it.
Conservation
The IUCN Red List assessed Brycon guatemalensis as Least Concern in 2018, reflecting its wide Atlantic-slope distribution across Mexico, Guatemala, Honduras and Panama and a population with medium resilience (a minimum population doubling time of 1.4–4.4 years). The species is not currently considered at elevated risk of extinction.
Machaca is, however, a food fish across parts of its Central American range — it appears in regional cuisine and has been recognised by the Slow Food Foundation's Ark of Taste as a traditional food species — so local fishing pressure and the health of riparian forest along its rivers (which supplies much of its adult diet) are relevant long-term factors for regional populations, even though the species as a whole is not currently threatened.