Taxonomy & naming
Pyrrhulina beni was described by Norman Ethered Pearson in 1924, with a type locality of the lower Río Beni, a major left-bank tributary of the Amazon in Bolivia. The Catalog of Fishes (Eschmeyer, CAS) records the valid name as Pyrrhulina beni Pearson, 1924; the species is placed in the subfamily Pyrrhulininae within the family Lebiasinidae. Lebiasinidae is a small Neotropical family of elongate characiforms — the pencilfish-and-relatives assemblage — quite distinct from the large, diverse family Characidae that most hobbyists think of as "tetras."
The genus Pyrrhulina itself contains a number of species ranging across the Amazon, Orinoco, and Guiana shield drainages. Members are united by their elongate body plan, surface-dwelling habits, and characteristic spawning biology in which eggs are deposited on hard substrates and guarded by the male. Pyrrhulina beni's range falls within the western Amazon arc: GBIF occurrence data places it in Bolivia, Brazil, and Peru, consistent with the southwestern Amazon basin that includes the Río Beni drainage and adjacent systems.
Morphology
Pyrrhulina beni is a slender, elongate fish reaching a maximum recorded standard length of 2 in. The body plan is characteristic of the genus: a long, torpedo-shaped profile, a mouth angled upward toward the surface, and a relatively small, compressed form suited to life in still or slow-flowing shallow water. The dorsal fin is positioned well back on the body, an arrangement shared by pencilfish relatives, giving the fish a distinctive rakish outline.
Coloration in Pyrrhulina species typically involves a dark lateral stripe or series of markings along the flank against a pale to golden background, often with red or orange accents on the fins in breeding males. Specific pigmentation details for P. beni are sparsely documented in the hobby literature, but the general Pyrrhulina pattern — lateral stripe, upturned posture, and the suggestion of warm fin coloration in mature males — applies to the genus as a whole. Females tend to be plainer and slightly deeper-bodied.
Habitat
Pyrrhulina beni originates from the southwestern Amazon basin, with occurrence records spanning Bolivia, Brazil, and Peru. The type locality is the lower Río Beni in Bolivia, a large white-water river draining the eastern Andes into the main Amazon system. Like other Pyrrhulina species, P. beni is expected to occupy shallow, slow or still freshwater habitats — flooded forest margins, oxbow lakes, quiet backwaters, and stream edges — where surface-associated feeding and leaf-spawning are practical.
The species is a tropical freshwater fish, consistent with the lowland Amazon environment. FishBase classifies it as benthopelagic in freshwater, and it is regarded as a low-vulnerability species in its native range. Water conditions in the Río Beni basin vary seasonally with Andean runoff; the fish likely experiences moderate temperatures through the year in a river-fed lowland setting.
Feeding
Pyrrhulina beni is a micropredator. Members of the genus feed opportunistically on small invertebrates and zooplankton near the water surface, picking off tiny insects, crustaceans, and other invertebrates at or just below the film. The upturned mouth is an adaptation for this surface and near-surface foraging mode.
In the aquarium, Pyrrhulina species accept a range of small live and frozen foods — Artemia nauplii, Moina, Daphnia, and grindal worms are all suitable — as well as fine-grade dried foods. Live and frozen foods also serve as spawning stimulants, bringing fish into breeding condition more reliably than prepared diets alone. Small, frequent feedings that mimic surface-insect falls suit the species well.
Mating
Pyrrhulina species reproduce by depositing adhesive eggs on solid surfaces such as broad leaves, flat stones, or aquarium glass, in a spawning style sometimes described as leaf-spawning. The male selects and cleans a spawning site and courts the female, leading her to the site where a clutch of 150–200 eggs is deposited. After spawning, the male guards the eggs, fanning and tending them until hatching.
Spawning activity in the genus is broadly continuous throughout the year rather than strictly seasonal, though it intensifies with improved feeding and water conditions. Males in display show heightened coloration and actively defend the nest area. The courtship stimulus most often cited in practice is the introduction of live or frozen invertebrate foods — Artemia nauplii, Moina, and grindal worms are particularly effective.
Breeding
Breeding Pyrrhulina beni follows the leaf-spawning pattern typical of the genus. A conditioned pair will deposit eggs on a flat, smooth surface — a broad aquatic or terrestrial leaf placed in the tank, a piece of slate, or clean aquarium glass — in a tightly packed, adhesive cluster. Clutch size in Pyrrhulina is in the range of 150–200 eggs per spawning event. The male guards the clutch, fanning the eggs to oxygenate them and removing any that develop fungus.
At tropical water temperatures the eggs hatch within a few days. The male continues to attend the fry briefly after hatching. Because the adults may eat eggs or fry once guarding behavior lapses, breeders often transfer the spawning surface to a separate rearing container once eggs are detected, or remove the adults after hatching. First foods for the tiny fry should be appropriately small — infusoria or commercial liquid fry foods — before moving on to newly hatched Artemia nauplii as the fry grow.
In the aquarium
Pyrrhulina beni is a small, peaceful characid suited to a quiet community aquarium of appropriate scale. Its compact size (2 in) and surface-oriented habits mean it does best in a tank with a well-fitted lid — the upturned mouth and active disposition make Pyrrhulina capable jumpers — and with calm, slow-moving water that does not push the fish away from the surface zone where it feeds and displays.
A planted aquarium with floating or broad-leaved plants provides both cover and potential spawning sites. The species is shy when kept singly; a small group of six or more individuals reduces stress and encourages more natural behavior. Tankmates should be small and non-aggressive — other small characids, dwarf cichlids, or similarly sized peaceful fish work well. Water chemistry should reflect its southwestern Amazon origins: soft to moderately soft, slightly acidic to neutral conditions and warm tropical temperatures are appropriate. Availability in the hobby is modest; P. beni is not a common species in trade compared to more familiar Pyrrhulina species.
Conservation
Pyrrhulina beni is assessed as Least Concern (LC) on the IUCN Red List, with the assessment last updated in 2020. The species has a reasonably broad distribution across the southwestern Amazon basin in Bolivia, Brazil, and Peru, and faces no known acute threats that would qualify it for a higher category of concern at the time of assessment.
As a small, apparently widespread lowland Amazon fish, P. beni is likely affected by the same broad pressures acting on Amazonian ichthyofauna generally — habitat modification, deforestation of riparian zones, and water-quality changes in parts of its range — but these do not currently appear to meet the IUCN threshold for uplisting. The species is not known to be a significant target of the ornamental-fish trade. Its low vulnerability index on FishBase reflects its small size and reproductive strategy; continuous or near-continuous spawning with moderate clutch sizes gives it resilience to modest population disturbance.