Taxonomy & naming
Aphyocharax agassizii was described by the Austrian ichthyologist Franz Steindachner in 1882, making it one of the earlier members of the genus to be formally named. The parenthetical authority indicates that the species was not originally placed in Aphyocharax but was subsequently transferred into it; Eschmeyer's Catalog of Fishes (the governing authority for valid names) records the current combination as Aphyocharax agassizii (Steindachner, 1882).
The genus Aphyocharax sits within the large and diverse family Characidae, the characins of the Americas and Africa. The genus is characterised by its small size, schooling habits, and generally slender, fusiform body plan. The specific epithet agassizii honours Jean Louis Rodolphe Agassiz (1807–1873), the Swiss-American naturalist who led the Thayer Expedition to Brazil (1865–1866) and whose collections furnished material for many Steindachner descriptions of Amazonian fishes.
Morphology
Aphyocharax agassizii is a small, slender characid reaching approximately 1.5 in standard length — among the smaller members of its genus. Like other Aphyocharax it has the typical streamlined, slightly compressed body of a fast-water schooling tetra, with a moderately forked tail and a small adipose fin behind the dorsal.
Specific colour pattern details for this species are not well documented in the available literature. Members of Aphyocharax tend to be semi-transparent silvery fish, sometimes with a reddish flush to the fins or a darker lateral stripe, but without detailed morphological data on record for A. agassizii, finer colour and meristic characters cannot be reliably stated here.
Habitat
Aphyocharax agassizii is recorded from the Amazon River basin in Brazil. FishBase classifies it as a benthopelagic freshwater species, meaning it occupies the water column but may also associate with bottom zones — a habitat description consistent with many small characids that forage through a range of depths in lowland Amazon waterways.
Beyond the broad basin-level distribution record, specific habitat details — substrate type, water-body category (river channel, floodplain lake, forest stream), vegetation associations, and depth preferences — are not captured in the available sources. The Amazon basin encompasses an enormous range of microhabitats, and the precise ecology of this species within that system remains undocumented.
Feeding
No specific dietary data for Aphyocharax agassizii has been recorded in the scientific literature or aquarium databases. The closely related Aphyocharax anisitsi (Bloodfin Tetra) is a documented omnivore that feeds on small insects, worms, zooplankton, and prepared aquarium foods, and it is reasonable to expect a broadly similar feeding strategy in A. agassizii given the shared genus and body form.
Small Amazonian characids of this body type generally feed in the mid-to-upper water column, picking at zooplankton, small invertebrates, and surface-fallen insects. In the aquarium, Aphyocharax species are typically undemanding, accepting quality flake foods and small frozen or live items such as Daphnia and brine shrimp.
Mating
No specific mating or reproductive behaviour data has been recorded for Aphyocharax agassizii. Based on the biology of congeners — particularly the well-studied A. anisitsi — spawning in Aphyocharax follows the egg-scattering pattern typical of small characids: males court females with chasing displays and the pair deposit small adhesive eggs among fine-leaved plants or in open water, after which neither parent guards the eggs or fry.
Spawning in related species is triggered by soft, slightly acidic water and is most active in the morning hours. Whether A. agassizii shares the same cues and preferences is unstudied, but its Amazon basin provenance — soft, warm, mildly acidic blackwater or clearwater — aligns with the conditions that favour spawning in Aphyocharax generally.
Breeding
Breeding records for Aphyocharax agassizii in captivity appear to be absent from the hobbyist literature, and no breeding data is reported in the scientific sources. The species is rarely if ever encountered in the aquarium trade, which limits the accumulation of captive husbandry knowledge.
By analogy with A. anisitsi and other small egg-scattering characids, a successful breeding approach would likely involve conditioning adults on varied live and frozen foods, providing fine-leaved plants or spawning mops, and using soft, slightly acidic water in the pH 6.0–6.5 range. Eggs would be expected to be small and adhesive, hatching within a few days at warm Amazonian temperatures, with the adults removed after spawning to prevent egg predation. These remain inferences from congener biology rather than observed data for this species.
In the aquarium
Aphyocharax agassizii is not a species regularly encountered in the aquarium trade, and no established captive-care literature exists for it. The sparse records available offer size (approximately 1.5 in SL) and a broad Amazon basin origin as the primary reference points.
Hobbyists who do obtain this species could reasonably apply the care framework used for other small Aphyocharax: a well-filtered aquarium with open swimming space and some planted cover, warm soft-to-moderately-soft water in the tropical range, and a mixed diet of small live or frozen foods alongside quality prepared foods. Its small adult size makes it suitable for nano or community tanks, but given the lack of documented captive experience, care parameters should be treated as provisional until more information accumulates.
Conservation
Aphyocharax agassizii is assessed as Data Deficient (DD) on the IUCN Red List, with the assessment dated 07 November 2018. A Data Deficient rating does not indicate that the species is not at risk; rather, it reflects that the information available is insufficient to make a reliable assessment of extinction risk. The lack of population data, range surveys, and ecological studies makes it impossible to determine whether the species faces threats warranting a higher threat category.
The Amazon basin, while vast, faces mounting pressure from deforestation, agricultural runoff, dam construction, and other land-use changes that affect lowland freshwater fish communities broadly. Without targeted survey work to establish the range, population size, and habitat associations of A. agassizii, its actual conservation status remains unknown. The DD designation is a call for more research rather than a reassurance of security.