Taxonomy & naming
Phenacogaster beni was described by Carl H. Eigenmann in 1911, with a type locality at Villa Bella on the Río Beni in Bolivia. The genus Phenacogaster Eigenmann, 1907 belongs to the family Characidae and currently encompasses several dozen small South American tetras distributed across the Amazon and Orinoco basins. Eschmeyer's Catalog of Fishes is the authority for the valid name; the species remains in Phenacogaster, with the authority in parentheses not required as placement in the original genus has been retained.
The species epithet beni refers directly to the Río Beni, the major Bolivian tributary of the Madeira system where the type was collected. The genus name Phenacogaster combines the Greek phenacos (deceiver) with gaster (belly), an allusion shared by other Eigenmann genera of the period to morphological features that confounded early classification. Within the genus, P. beni is distinguished from congeners by a combination of scale counts, body proportions, and pigment details; definitive species-level identification often requires examination of preserved material.
Morphology
Phenacogaster beni is a small, elongate characid with a maximum recorded standard length of 1.5 in — typical for the genus, whose members cluster in the 1–2 in range. The body is slender and moderately compressed, with the relatively large eye and terminal mouth characteristic of small micropredatory tetras. Like all Phenacogaster it lacks an adipose fin, a feature that distinguishes the genus from many other small South American characids at a glance.
Pigmentation is subdued: the flanks carry a dark lateral stripe or midlateral blotch series running from behind the opercle toward the caudal peduncle, typically terminating in a caudal spot. The overall body coloration is silvery to pale yellowish. Sexual differences, where known in congeners, involve female fullness at spawning condition; males of related species may show minor fin-ray extensions but these are not well documented for P. beni specifically.
Habitat
The species is recorded from the Guaporé and Beni River basins, spanning roughly 1°S to 15°S in Bolivia and adjacent western Brazil. These are major western Amazon tributaries that drain the flanks of the Andes, carrying waters that range from clear and moderately soft in headwater reaches to more turbid lowland channels. The type locality, Villa Bella (today known as Guayaramerín), sits at the confluence of the Río Beni and Río Mamoré.
Based on habitat data for closely related congeners, P. beni is expected to favour clear or slightly tannin-stained shallow water with dense submerged or marginal vegetation and moderate current. Temperatures across its range span roughly 68–82 °F, and pH values typical of Amazonian clear-water systems range from mildly acidic to neutral. Specific microhabitat data for this species are sparse in the published literature.
Feeding
Phenacogaster beni is a micropredator in the wild, feeding on small invertebrates, zooplankton, and insect larvae — the dietary mode documented across the genus and confirmed for congeners such as P. tegatus. The relatively large eye and small terminal mouth are consistent with hunting tiny, mobile prey items in clear water.
In the aquarium, close relatives accept a wide range of small live and frozen foods readily: Artemia nauplii, Moina, small Daphnia, and grindal worms are all taken. Fine-grained prepared foods — micro-pellets and powdered or small-particle flake — supplement the diet well. Feeding variety is worthwhile in captivity both for nutritional balance and to keep the fish in breeding condition.
Mating
No specific courtship observations for Phenacogaster beni have been published, but the genus follows the characid norm of egg-scattering with no parental care. Males of related species pursue females with brief lateral displays; spawning takes place among fine-leaved vegetation or submerged plant material, where adhesive eggs are deposited and left.
The species is presumed seasonal in nature, responding to temperature and water-condition cues associated with the Amazonian wet and dry seasons. In captivity, conditioning on live foods and a gradual drop in water temperature followed by a water change can stimulate spawning, following the approach that succeeds with other small South American characids.
Breeding
Breeding in Phenacogaster beni has not been reported in detail in the hobbyist literature, but the reproductive biology of the genus aligns with standard egg-scattering characid practice. A small, well-planted breeding tank or one equipped with fine-leaved synthetic spawning mops is appropriate; the adults should be removed after spawning as they will consume the eggs.
Eggs are adhesive and are expected to hatch within 24–48 hours at tropical temperatures, depending on the precise temperature maintained. The fry are very small at hatching, initially requiring infusoria or commercial fry foods before graduating to newly hatched Artemia nauplii. Growing the fry to juvenile size calls for clean water, gentle filtration, and frequent small feedings. No brood is very large given the fish's tiny size.
In the aquarium
Phenacogaster beni is a peaceful, schooling fish suited to a well-planted nano or community aquarium. A group of eight to ten individuals will school more cohesively and display more natural behaviour than a smaller number; the species becomes shy when kept in insufficient numbers. Given its maximum size of under 1.5 in SL, a tank of 23.5–31.5 in is adequate for a group, though it will use more space if available.
Water conditions should replicate the warm, soft-to-moderately-hard, slightly acidic to neutral Amazonian clear-water environment: temperatures around 72–81 °F and a pH of 6.0–7.5 are a reasonable target based on congener requirements. The tank benefits from subdued lighting, dark substrate, and some surface cover from floating plants to reduce skittishness. Tankmates should be similarly sized and gentle — other small tetras, dwarf cichlids, and small Corydoras are all suitable. The species is rarely traded by name and is more likely to appear as a by-catch in assortments of small wild-caught Bolivian or Brazilian tetras than as a deliberate import.
Conservation
Phenacogaster beni is assessed as Least Concern (LC) on the IUCN Red List. The assessment reflects a distribution that, while not vast, spans two major river basins in Bolivia and Brazil and does not face the range restriction or acute localised threats that would elevate concern. The Guaporé–Beni drainage system retains large areas of intact Amazonian habitat, and no targeted fishing pressure on this small and commercially minor species has been reported.
As with many small, modestly traded Amazonian tetras, the principal background threat is the progressive degradation of Amazonian river systems through deforestation, agricultural runoff, and hydrological alteration — pressures that affect the entire fish community rather than singling out P. beni. The LC assessment is appropriate given current knowledge, but the scarcity of published research means population trends are poorly quantified.