Taxonomy & naming
Hemigrammus eigenmanni was originally described as Moenkhausia eigenmanni by the French ichthyologist Jacques Géry in 1964. The type locality is the Río Manacacias at its confluence with the upper Río Meta near Restrepo, in the Colombian department of Meta. The holotype is deposited at the National Museum of Natural History, Washington (USNM 198640). Eschmeyer's Catalog of Fishes recognises the current valid combination as Hemigrammus eigenmanni (Géry, 1964), with the parenthetical authority reflecting the generic transfer away from Moenkhausia.
Within the family Characidae the species is placed in the subfamily Pristellinae according to current higher-level classification recognised by the Catalog of Fishes. The specific epithet honours Carl H. Eigenmann (1863–1927), the American ichthyologist whose foundational monographs on South American characins laid the groundwork for all subsequent tetra taxonomy. Eigenmann's name has been applied to multiple fish species across different genera, reflecting his outsized influence on Neotropical ichthyology.
The genus Hemigrammus is one of the larger and more challenging genera in the family, containing numerous small-bodied tetras that are diagnosed, in part, by the presence of scales on the lower lobe of the caudal fin — a character distinguishing them from the otherwise similar genus Moenkhausia, where the original description placed this species.
Morphology
Hemigrammus eigenmanni is a slender, fusiform tetra reaching a maximum recorded standard length of 2 in. Like most members of the genus it has a moderately compressed, elongated body, large eyes relative to head length, and the two-part (adipose + rayed) dorsal fin arrangement characteristic of characids. The caudal fin is forked, and the lower lobe bears scales, consistent with Hemigrammus.
Detailed published colour notes for this species are sparse in the available literature; the general body is presumed silvery with the lateral stripe patterning typical of the genus. As with many Hemigrammus, sexual differences likely involve female fullness in the abdomen during the breeding season, but specific dimorphism data for H. eigenmanni are not well-documented in accessible sources.
Habitat
The species is known from the upper Río Meta system in the Colombian Llanos, with the type locality at the Río Manacacias near Restrepo. The Meta is a major Orinoco tributary draining the eastern Andean piedmont and the vast grassland plains of the Colombian Llanos, a region characterised by strongly seasonal rainfall that drives dramatic wet-season inundation and dry-season contraction.
FishBase records water parameters for this species as 73–81 °F, pH 6.0–7.2, and hardness up to 15 dH — conditions reflecting the moderately soft, warm, slightly acidic to neutral waters typical of Andean-piedmont and lowland Llanos streams. The fish is classified as freshwater, benthopelagic, and tropical in FishBase, consistent with life in the mid-water column of warm Orinoco-drainage streams. The upper Río Meta corridor supports high fish diversity, and H. eigenmanni shares its range with numerous other small characids.
Feeding
Specific gut-content studies for Hemigrammus eigenmanni are not available in the literature reviewed. Based on the feeding ecology of the genus and its Llanos habitat, the species is expected to be an omnivorous opportunist targeting small aquatic invertebrates, insect larvae, zooplankton, and minor amounts of plant or detrital material — a feeding strategy typical of small characids in South American rivers.
In the aquarium, Hemigrammus tetras generally accept high-quality dried foods including flake and small pellets, and condition best on supplemental live and frozen foods such as Daphnia, brine shrimp, and bloodworm. A varied diet is advisable to support good health and any breeding attempts.
Mating
No published courtship descriptions specific to Hemigrammus eigenmanni are recorded in the sources available. The species is expected, by analogy with other Hemigrammus and the broader Characidae, to follow the egg-scattering spawning mode typical of the group: males pursuing females through fine-leaved vegetation or open water, releasing sperm in concert with the female's egg release, with no pair bond and no parental care following spawning.
For other Hemigrammus, males often display lateral flank-to-flank postures during competition for access to females, and spawning is typically initiated by changes in photoperiod, temperature, or water chemistry. The adhesive or semi-adhesive eggs fall among fine-leaved plants or substrate after fertilisation.
Breeding
Captive breeding records for Hemigrammus eigenmanni are not documented in widely accessible hobbyist literature, likely reflecting the species' rarity in the trade. Based on the genus, successful breeding would be expected to follow the standard Hemigrammus approach: a conditioning period with live or frozen foods, a spawning tank with fine-leaved plants or spawning mops, slightly soft and acidic to neutral water in the range documented for the species (pH 6.0–7.2, 75–79 °F), and separation of adults from eggs immediately after spawning to prevent predation.
Eggs are typically adhesive, attaching to plant material or settling on the substrate, and hatch within 24–48 hours at tropical temperatures. Fry are small and require infusoria or commercial micro-fry foods as initial nutrition before progressing to newly hatched brine shrimp. No parental care is provided; adults will consume both eggs and fry given the opportunity.
In the aquarium
Hemigrammus eigenmanni is rarely encountered in the ornamental trade and is primarily of interest to hobbyists seeking Colombian Orinoco-drainage species or less-common Hemigrammus. When available, it would be suited to a well-planted community aquarium with soft to moderately hard water at 73–81 °F and a neutral to slightly acidic pH, matching its natural range parameters.
As a small schooling characid it should be kept in groups of six or more, providing a sense of security and allowing natural social behaviour. Tankmates should be similarly sized, peaceful fish that share compatible water requirements — other small tetras, dwarf cichlids from the Orinoco basin, and small Corydoras would be appropriate companions. The species has high resilience and low vulnerability scores on FishBase, suggesting reasonable adaptability to aquarium conditions.
The aquarium should be well-filtered with moderate flow, providing clear, clean water with some surface movement. Subdued lighting with floating plants or overhanging vegetation can help mimic the shaded conditions of piedmont streams and reduce stress. Because captive-bred stock is essentially unavailable, any specimens offered for sale are likely wild-caught; ethical sourcing considerations therefore apply.
Conservation
Hemigrammus eigenmanni is assessed as Least Concern (LC) on the IUCN Red List, with the most recent assessment published on 16 November 2020. The Least Concern designation reflects the species' occurrence in the relatively large and still largely intact upper Río Meta watershed and the absence of documented rapid population declines or severe habitat loss across its range at the time of assessment.
The upper Río Meta and the Colombian Llanos face ongoing pressures from agricultural expansion, hydrocarbon extraction, and seasonal burning, and long-term water quality and habitat integrity in the region warrant monitoring. However, as currently assessed, the species does not approach threatened thresholds and is not considered at immediate risk of extinction at the global level.