Taxonomy & naming
Hyphessobrycon agulha was described by Henry Weed Fowler in 1913 from material originating in the Madeira River, Brazil. Eschmeyer's Catalog of Fishes recognises the valid name as Hyphessobrycon agulha Fowler, 1913, placing it within the family Characidae, order Characiformes. Because the species was described directly in Hyphessobrycon and has never been transferred to another genus, the authority is written without parentheses.
The species belongs to the H. agulha species group as defined by Géry (1977), a cluster of small Hyphessobrycon characterised by a dark lower body half and a humeral spot. The genus Hyphessobrycon itself is the most species-rich tetra genus, encompassing well over a hundred described species ranging across South America east of the Andes. Several group members are similarly obscure in the hobby, and exact boundaries within the genus remain an ongoing subject of ichthyological revision.
Morphology
H. agulha is a small tetra reaching around 1.5 in standard length, with females reportedly slightly larger and notably deeper-bodied than males. The body is elongated and somewhat laterally compressed, consistent with the typical small-tetra bauplan. A humeral spot is present behind the gill cover, and the lower half of the body is distinctly darker than the upper half — the pattern that defines the agulha species group.
A distinctive anatomical feature is the presence of bony hooks on all fins, including the caudal fin — an unusual extension of a trait seen in various characids. These hooks are most developed in males and can cause fin-tangling when rival males spar or when fish are netted. The common name 'red-tailed' alludes to the colouration of the caudal region, though the overall colour scheme is relatively subdued compared with showier congeners.
Habitat
The species is native to the Madeira River basin in Brazil, with the type locality recorded from the Madeira River itself. Biotope data from the region around Porto Velho and associated tributaries describe clear-water environments with sandy substrates, driftwood, and accumulated leaf litter — the kind of blackwater or clearwater Amazonian streams typical for small characids in this drainage.
The waters of these tributaries tend toward soft, slightly acidic to neutral chemistry, reflecting the lowland Amazonian geology of the drainage. Current may be moderate to swift in the main channel, but smaller tributaries often offer calmer margins where small tetras congregate. H. agulha is described as benthopelagic — occupying the water column from near the bottom upward — consistent with foraging among leaf litter and over sandy substrate in shallow, structure-rich zones.
Feeding
In the wild H. agulha is an omnivore, taking small invertebrates and crustaceans from the substrate and water column. Its natural diet likely includes aquatic insect larvae, tiny crustaceans, microinvertebrates encountered among leaf litter, and opportunistic organic matter.
In the aquarium the species accepts a broad range of small foods without difficulty. Seriously Fish recommends bloodworm, mosquito larvae, Daphnia, Moina, and dry prepared foods. As with most small tetras, a mixed diet that includes live or frozen invertebrate foods alongside quality dry flake or micro-pellets promotes optimal condition and colour. Feeding should be sized to the fish's small mouth — foods should be correspondingly fine or small.
Mating
Like all Hyphessobrycon, H. agulha is an open, egg-scattering spawner with no parental care. In the wild, spawning presumably takes place among fine-leaved aquatic vegetation or over substrate in slower marginal waters, where adhesive eggs can anchor briefly before the adults move on. Males, which carry bony fin hooks on all fins, may use these during spawning contact or rival interactions — the function of fin hooks in Hyphessobrycon is thought to relate to close-contact male–male competition as well as possibly to spawning clasping.
Sexual dimorphism is observable: females are deeper-bodied and slightly larger, a reliable guide when selecting pairs or conditioning groups. As with the majority of small tetras, the species is not a seasonal spawner in the aquarium and will attempt to spawn under consistently good conditions.
Breeding
Captive breeding of H. agulha has not been formally reported in the literature as of available sources, and the species is noted as rare in the hobby. By analogy with related Hyphessobrycon, successful breeding would involve a dedicated spawning tank with soft, slightly acidic water, fine-leaved plants or spawning mops, and dim lighting to replicate the tannin-stained clearwater habitat of the Madeira drainage.
Eggs in egg-scattering characids are typically small and adhesive, deposited among vegetation and abandoned immediately. Adults will consume eggs and fry, so removal of the adults or transfer of the spawning medium is standard practice after spawning is observed. Fry would be expected to require infusoria or similar first foods before advancing to newly hatched brine shrimp. Breeders willing to work with this uncommon species would contribute meaningfully to establishing a captive population given its rarity in trade.
In the aquarium
H. agulha is a peaceful schooling tetra best kept in groups of eight or more, as individuals kept in small numbers may be shy or skittish. The species is rare in the trade and is primarily of interest to hobbyists seeking biotope accuracy for the Madeira River system or collectors of unusual Hyphessobrycon.
Water parameters should approximate the soft, warm conditions of its native clearwater tributaries: temperature in the range of 68–82 °F (68–82 °F), pH 5.5–7.5, and low to moderate hardness (roughly 18–215 ppm TDS as reported). A well-planted tank with dark substrate, leaf litter, driftwood, and gentle filtration creates an appropriate environment and brings out natural behaviour. The species is entirely peaceful and suitable with other small, non-aggressive fish from similar water chemistries. The bony fin hooks are worth bearing in mind when netting the fish — a fine-mesh net reduces the risk of fin damage or entanglement.
Conservation
Hyphessobrycon agulha is assessed as Least Concern (LC) on the IUCN Red List, with the assessment recorded in 2021 and the population trend considered stable. The Madeira River basin is one of the largest tributaries of the Amazon and supports extensive suitable habitat; there is no evidence of range-wide population decline sufficient to approach threatened thresholds.
The species faces no specific conservation actions identified in the current assessment. Continued monitoring of the Madeira basin is relevant given ongoing pressures on Amazonian river systems from deforestation, hydroelectric development, and gold mining — pressures that affect water quality and habitat structure across the basin. For now H. agulha's wide river-basin range and stable trend support its Least Concern status.