Taxonomy & naming
Hyphessobrycon myrmex was described by Murilo N. L. Pastana, Fernando Dagosta, and Ana Lúcia A. Esguícero in 2017 in the Journal of Fish Biology (volume 91, issue 5, pages 1301–1318, DOI: 10.1111/jfb.13449). The type locality is the Rio Formiga, a tributary of the upper Rio Juruena, which forms part of the upper Rio Tapajós drainage in Mato Grosso, Brazil. The species is placed in Hyphessobrycon, the large and taxonomically challenging genus of small to medium-sized characids that spans much of tropical and subtropical South America.
The species name myrmex derives from the Greek word for ant. The name carries a double meaning: it alludes both to the fish's remarkably small adult size and to the name of the type-locality river — Rio Formiga, formiga being Portuguese for ant. Eschmeyer's Catalog of Fishes records the valid name as Hyphessobrycon myrmex Pastana, Dagosta & Esguícero, 2017. Because it was described in Hyphessobrycon, no parenthetical authority is used.
Morphology
Hyphessobrycon myrmex is a dwarf tetra, reaching approximately 1 in standard length (roughly 1 in total length), making it one of the smaller members of the genus and a miniature fish by any measure. The body is typical of the genus: moderately compressed laterally, with a relatively deep profile and a distinct Hyphessobrycon build.
A key distinguishing character noted in the original description is a broad, diffuse dark midlateral stripe formed by dense dark chromatophores on the lower half of the body, concentrated above the anal fin. The dorsal fin carries dense dark chromatophores on both the unbranched rays and the distal portions of the branched rays, giving it a darkened, contrasted appearance. Sexual dichromatism is pronounced: alpha males are intensely red to orange on the body, while females and immature individuals are pale yellow to greenish and lack the red coloration. The intensity of male coloration varies with dominance and condition.
Habitat
Hyphessobrycon myrmex is known from the Rio Formiga and the upper Rio Juruena basin in Mato Grosso, Brazil, within the broader Rio Tapajós drainage system. Collection data indicate that the species inhabits transparent, clear water over substrates of stone and sand, with moderate accumulations of vegetal debris. The habitat is characterized by shallow rapids, placing this species in lotic rather than still-water environments.
A notable ecological observation from FishBase is that approximately 50 percent of individuals in the type-locality population were found to be parasitized by isopods of the family Cymothoidae — an unusually high infestation rate that may reflect particular microhabitat or population density conditions. The overall water conditions of the upper Juruena basin are those of a typical Amazonian tributary: warm, soft, and relatively acidic, consistent with the broader distributional patterns of Hyphessobrycon in central Brazil.
Feeding
Gut-content analysis reported in FishBase characterizes Hyphessobrycon myrmex as an omnivore with a strong invertivore tendency. The stomach contents include Ephemeroptera (mayfly) nymphs at around 64 percent of the diet by proportion, making aquatic insect larvae the dominant prey item. Chironomidae (midge) larvae and microcrustaceans each contribute roughly 10 percent, while filamentous algae and vascular plant tissue make up approximately 10 and 6 percent respectively.
In the aquarium, the species can be expected to accept a wide variety of small prepared and live or frozen foods. Given the wild diet's heavy emphasis on small invertebrates, offering live or frozen Daphnia, Artemia nauplii, and small chironomid larvae alongside quality micropellets or finely crushed flake will best replicate natural feeding habits and support good coloration in males.
Mating
Hyphessobrycon myrmex is an egg-scattering characid with no parental care, consistent with the general reproductive strategy of the genus and family. Males display strong sexual dichromatism, with dominant or alpha males exhibiting the most intense red-orange coloration during courtship and competitive encounters. Female coloration remains muted regardless of reproductive state, and immature individuals of both sexes resemble females.
As in other small Hyphessobrycon, spawning is expected to occur among fine-leaved aquatic vegetation or similar spawning substrate, with the pair releasing small adhesive eggs that settle among plant matter. The pronounced dichromatism suggests that visual mate choice plays a role, with males competing for female attention through color and fin display.
Breeding
Hyphessobrycon myrmex has been bred successfully in captivity; Aquarium Glaser noted captive-bred specimens entering the trade, reducing dependence on wild-caught imports and making the species more accessible to hobbyists. This follows the pattern typical of small, colorful Hyphessobrycon, which often adapt well to aquarium breeding once conditions are met.
Spawning in captivity is expected to follow the general Hyphessobrycon protocol: well-conditioned adults introduced into a tank furnished with fine-leaved plants or synthetic spawning mops, with slightly soft and warm water. Eggs are adhesive and should be separated from the parents, which will eat them, as quickly as possible. Fry are very small given the diminutive adult size, and initial foods should be appropriately fine — infusoria or commercial liquid fry foods before advancing to newly hatched brine shrimp nauplii.
In the aquarium
Hyphessobrycon myrmex is well suited to nano and miniature biotope aquaria given its maximum size of around 1 in total length. A small group showing multiple males will encourage the striking red-orange display coloration; a single male kept alone or with only larger fish may never develop full intensity. A planted tank with subdued lighting and a dark substrate will show off the contrasting dark lateral markings and male coloration to best effect.
Water conditions replicating the soft, warm, slightly acidic Amazonian tributary habitat are recommended: temperatures in the mid-to-upper 20s Celsius, slightly soft and acidic to neutral water. Tankmates should be similarly small and peaceful; nano tetras, small rasboras, pygmy corydoras, and small livebearers all make compatible companions. The species should be kept away from aggressive or much larger fish. Given its tiny size, filtration should provide gentle flow without creating currents strong enough to stress the fish.
Conservation
Hyphessobrycon myrmex has not been formally assessed by the IUCN Red List; FishBase and other sources confirm a status of Not Evaluated, reflecting the fact that the species was only described in 2017 and formal conservation assessments of recently described Amazonian small-bodied fishes often lag several years behind their taxonomic description.
The species is currently known only from the Rio Formiga and the upper Rio Juruena basin in Mato Grosso, Brazil. This restricted known range means that habitat quality in the Cerrado-Amazonian transition zone — subject to agricultural expansion, deforestation, and water extraction — is likely the principal conservation concern, even without a formal IUCN assessment. The existence of captive-bred populations in the aquarium trade provides some buffer but should not substitute for monitoring of the wild population.