Taxonomy & naming
Hemigrammus silimoni was described by Heraldo A. Britski and Flávio C.T. Lima in 2008 as a new species from the upper Rio Tapajós basin in Brazil. The valid name, as registered in Eschmeyer's Catalog of Fishes, is Hemigrammus silimoni Britski & Lima 2008. The Catalog places the species in the family Characidae and notes that its distribution spans both the Juruena River drainage (a tributary of the Tapajós) and the Aripuanã River drainage, which belongs to the Madeira basin — two separate major river systems within Mato Grosso State.
Hemigrammus is one of the largest genera of small tetras and belongs within the Characidae, the family that dominates small characiform diversity across tropical South America. The 2008 description distinguished H. silimoni from congeners on morphological grounds; the original paper, published in a specialist ichthyological journal, provides the formal diagnosis. No subspecies are recognised.
Morphology
Hemigrammus silimoni is a small, slender-bodied tetra reaching a maximum recorded standard length of approximately 1.5 in. Like other Hemigrammus, it has a fully scaled caudal fin — the character that historically separated Hemigrammus from the closely related Hyphessobrycon — and the generally streamlined, moderately compressed body plan typical of open-water tetras from fast-flowing habitats.
Detailed colour pattern descriptions from the literature are sparse for this species. The habitat data suggest a fish adapted to clear, swift water over sand and rock substrates, which in many small tetras correlates with a relatively plain or silvery colouration that provides camouflage against bright substrates. The specific morphological characters distinguishing it from congeners are documented in the original description by Britski and Lima.
Habitat
The type locality of Hemigrammus silimoni is a section of the Rio Juruena, a major left-bank tributary of the upper Rio Tapajós in Mato Grosso State, Brazil. Field notes recorded at collection cite a river roughly 50 metres wide and 1.7 to 2.0 metres deep, with swift-running, crystal-clear water over a sand and rock bottom. These are classic conditions for the clean-water, high-gradient streams of the Brazilian Shield: low turbidity, well-oxygenated, and relatively stable in chemistry.
The species' range extends beyond the Juruena drainage into the Aripuanã River system, a major tributary of the Madeira, indicating that it tolerates some variation in the hydrogeographic context while remaining tied to clear-water, swift-current environments. FishBase characterises it as a tropical freshwater, benthopelagic species, suggesting it occupies both the mid-water and substrate-adjacent zones of its river habitat.
Feeding
Gut-content analysis of Hemigrammus silimoni, as reported via FishBase, recorded algae and vegetal matter, indicating an omnivorous diet with a notable plant-material component. This places the species among the many small characids that supplement invertebrate prey with benthic algae and organic detritus, particularly in swift, well-lit, rocky-bottomed streams where periphyton is productive.
In captivity H. silimoni would be expected to accept the standard range of prepared and live foods offered to small tetras: quality micro-pellets or fine flake, supplemented with small live or frozen invertebrates such as Daphnia, Artemia nauplii, and copepods. The plant-matter component of the wild diet suggests some vegetable enrichment — spirulina-based foods or blanched greens — may be beneficial for long-term maintenance.
Mating
No published observations of mating behaviour specific to Hemigrammus silimoni are available in the sources examined. As a member of Hemigrammus, it would be expected to follow the broad characid pattern: egg-scattering open-water spawning with no pair bond and no parental care. Males in spawning condition typically pursue females actively, with courtship involving close following and lateral display.
Fine-leaved aquatic vegetation and root tangles along riverbanks are the typical spawning substrate for small tetras in similar swift-water habitats, where adhesive or semi-adhesive eggs may lodge among vegetation or settle into crevices in rocky substrates. Data from the wild for this species are absent from the available literature.
Breeding
No specific captive breeding records for Hemigrammus silimoni appear in the available literature, and the species is not established in the ornamental trade. Based on the biology of comparable Hemigrammus species, breeding would be expected to follow the standard egg-scattering model: a small group or conditioned pair placed in a densely planted spawning tank releases adhesive eggs among fine-leaved plants or spawning mops, after which the adults should be removed to prevent egg predation.
Incubation and fry development times are unreported for this species. At tropical temperatures typical of Mato Grosso rivers — roughly 75–82 °F — eggs of small characids generally hatch within 24 to 48 hours, with fry becoming free-swimming a few days later and requiring very fine first foods such as infusoria or commercial liquid fry food before graduating to newly hatched brine shrimp. These are general characid expectations rather than confirmed data for H. silimoni.
In the aquarium
Hemigrammus silimoni is not an established aquarium species and is unlikely to be found in the ornamental trade. Its type locality conditions — swift, crystal-clear, well-oxygenated water over sand and rock, in a moderately wide river — suggest that it would require good water movement and high dissolved oxygen in captivity, parameters similar to those favoured by other clear-water Brazilian stream tetras.
Should specimens become available, a setup mimicking a clean, fast-water Cerrado or Amazon-shield stream environment would be appropriate: efficient filtration with strong flow, a sand and smooth-pebble substrate, and subdued planting consistent with a high-current habitat. The small adult size of approximately 1.5 in SL suggests keeping in groups, as is standard for schooling tetras, with peaceful tankmates of comparable size. Water temperature in the range typical of Mato Grosso lowland rivers — roughly 75–82 °F — and near-neutral pH would be a reasonable starting point.
Conservation
Hemigrammus silimoni is assessed as Least Concern on the IUCN Red List, with the most recent assessment carried out under the auspices of the Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio) and accepted in the 2025-2 version of the Red List (assessment year 2022). The Least Concern designation reflects the absence of evidence for rapid population decline or severe range restriction meeting threatened thresholds.
The species' occurrence in the Rio Juruena and Aripuanã drainages places it within the broader Tapajós and Madeira basins of Mato Grosso — regions that have experienced significant deforestation, agricultural conversion, and hydroelectric development pressure in recent decades. While the current assessment finds no imminent threat, the clear-water, swift-current stream habitats the species appears to prefer are sensitive to catchment disturbance, sedimentation, and altered flow regimes. Continued habitat monitoring in the upper Tapajós and Madeira basins is relevant to the long-term status of this and many other endemic stream fishes of the Brazilian Shield.