Taxonomy & naming
Hemigrammus guyanensis was described by the French-Swiss ichthyologist Jacques Géry in 1959, in a paper published in the Bulletin Mensuel de la Société Linnéenne de Lyon that revised several new characoid species from French Guiana. The type locality is Sable Creek (Crique Sable) in the upper Mana River drainage, French Guiana. The valid name as maintained in Eschmeyer's Catalog of Fishes is Hemigrammus guyanensis Géry, 1959.
A complicating element in the species' taxonomy is the possible synonymy with Hemigrammus falsus Meinken: Lima, Correa and Ota (2016) discussed the relationship between these names in a broader treatment of Hemigrammus diversity, but a formal synonymy has not yet been published, and the two are currently treated as distinct pending further revision. The genus Hemigrammus Gill, 1858 belongs to the family Characidae and encompasses a large number of small schooling tetras distributed broadly across lowland South America.
Morphology
Hemigrammus guyanensis is a small, slender tetra of typical Hemigrammus build: a moderately compressed, fusiform body with a rounded snout, large eyes, and an adipose fin. Published size data are limited, but the species falls within the range typical of the genus, generally not exceeding 1.5–2 in standard length in adults.
The most diagnostic character separating H. guyanensis from its closest relative H. ocellifer is a conspicuous dark midlateral stripe running along the flank, which is more strongly developed and continuous in guyanensis than in ocellifer. Body coloration is otherwise the pale silvery-translucent ground typical of many small Hemigrammus species, with the stripe lending a clean linear contrast. The caudal peduncle region may carry a dark spot or intensification of pigment consistent with the genus, though detailed meristic data are sparse in the available literature.
Habitat
The species inhabits stream pools and backwaters at middle and upper river reaches, favoring slower or still sections of water rather than swift main-channel currents. It has been recorded from the Maroni, Mana, Approuague, and Oyapock river systems in French Guiana, with additional records from Suriname, and from Pará, Maranhão, and Ceará states in northeastern Brazil.
This distribution spans both the Guiana Shield rivers draining northward into the Atlantic and some of the northeastern Brazilian drainages, pointing to a wide but not continuous range across lowland tropical forest and transition zones. Like many small characids, it likely occupies densely vegetated margins and shaded areas with leaf litter and submerged woody debris, sheltering from current and predators in calmer microhabitats within otherwise moderate-gradient stream systems.
Feeding
Hemigrammus guyanensis, like most small Hemigrammus tetras, is a generalist micropredator and opportunistic omnivore in the wild. The natural diet is presumed to consist primarily of small invertebrates — aquatic insect larvae, microcrustaceans, and zooplankton — supplemented by fallen terrestrial invertebrates and, to a lesser extent, plant material and algae.
In the aquarium it readily accepts a varied diet of small dried foods such as quality flake and micro-pellets, and benefits from regular supplementation with frozen or live foods including Daphnia, Artemia nauplii, Moina, and bloodworm. A varied diet supports good coloration and reproductive condition; the midlateral stripe shows best in well-fed, unstressed fish under appropriate lighting.
Mating
Hemigrammus guyanensis is an egg-scattering characid with no parental care, following the reproductive pattern general to the genus. Males are typically slimmer than females and may develop more intense stripe coloration when in breeding condition. Courtship involves active pursuit and display behavior, with males chasing females through fine-leaved vegetation.
Spawning is triggered by suitable environmental cues — a slight drop in water temperature, increased water changes, or the onset of rainfall-simulating conditions. Eggs are broadcast among fine-leaved aquatic plants or spawning mops, to which they may adhere briefly before settling. Neither parent guards eggs or fry.
Breeding
Breeding Hemigrammus guyanensis in captivity follows the standard approach for small egg-scattering tetras. A well-conditioned pair or small group can be set up in a dedicated breeding tank with dense fine-leaved planting such as Java moss or synthetic spawning mops, dim lighting, and soft, slightly acidic water consistent with the Guiana Shield blackwater environment where the species is most abundant.
Adults readily consume their own eggs, so removing them to a separate rearing vessel after spawning, or using a mesh barrier to protect eggs, is advisable. Eggs hatch within 24–48 hours at tropical temperatures, and the tiny free-swimming fry require infusoria or commercial liquid fry foods initially, progressing to newly hatched Artemia nauplii as they grow. The species has not been extensively documented in captive-breeding literature, so hobbyists should treat the above parameters as broadly applicable Hemigrammus guidance rather than species-specific data.
In the aquarium
Hemigrammus guyanensis is a peaceful, schooling tetra suitable for community aquaria with other small, non-aggressive species. It shows best in groups of eight or more, where its schooling behavior and midlateral striping create visual interest. A planted tank with subdued lighting and a dark substrate or background will bring out the stripe contrast most effectively.
Water conditions that reflect the Guiana Shield origin — soft, slightly acidic to neutral water, temperatures in the range of 73–82 °F — suit the species well, though it likely tolerates a somewhat broader range given its distribution extends into more variable northeastern Brazilian waters. The IUCN notes this species enters the international aquarium trade; wild-sourced stock from French Guiana or Suriname may occasionally be available, while captive-bred stock is preferred from a conservation perspective. It is not a common hobbyist tetra and may require effort to source.
Conservation
Hemigrammus guyanensis is assessed as Least Concern (LC) on the IUCN Red List, based on an assessment published in 2023 reflecting a 2021 evaluation. The population is considered stable, and the species is described as locally relatively common within its range. Its relatively broad distribution across French Guiana, Suriname, and parts of northeastern Brazil provides some buffer against localized threats.
The primary documented threats are gold mining activities, particularly in Suriname, which cause deforestation and siltation of stream habitats — pressures that affect many small characids in the Guiana Shield. The species also enters the international aquarium trade, though this is not considered a significant driver of population decline at current levels. Continued monitoring of gold-mining impacts on Guiana Shield stream systems is the most relevant conservation action for this and many co-occurring small fish species.