Taxonomy & naming
Hasemania hanseni was described by the American ichthyologist Henry Weed Fowler in 1949. The authority is written with parentheses — (Fowler, 1949) — indicating that the species was originally placed in a different genus; Fowler assigned it to Pristicharax, a name now treated as a synonym of Hasemania. Eschmeyer's Catalog of Fishes (Catalog of Fishes, CAS) is the governing authority for the valid name, which it records as Hasemania hanseni.
Hasemania belongs to the family Characidae and is one of the most distinctive small genera in the family by virtue of a morphological peculiarity shared with few other characids: the adipose fin — the small rayless fin between the dorsal and caudal fins so characteristic of the order — is absent or reduced to a vestige in most members of the genus. This makes Hasemania immediately separable from the majority of similarly sized Neotropical tetras. The genus itself was named in honour of John D. Haseman, the American naturalist and fish collector who conducted extensive expeditions through South American river systems in the early twentieth century.
Type specimens of H. hanseni include paratype ANSP 72105 (Academy of Natural Sciences of Philadelphia) from Goiás, and material catalogued as MZUSP 35676 from Distrito Federal, confirming the species' presence across the central Brazilian highland drainages. A subsequent description of the related species H. piatan, published in a Brazilian scientific journal, made reference to these specimens in placing H. hanseni within the genus.
Morphology
Hasemania hanseni is a small, slender-bodied characid. FishBase records a maximum standard length of 1.5 in for males and unsexed individuals, placing it among the smaller members of the family. The body form is elongate and laterally compressed in the typical tetra fashion, with a short head, small mouth, and the forked caudal fin characteristic of the group.
The most notable morphological feature of the genus — and one that immediately separates Hasemania from most similarly-sized Characidae — is the near-total absence of the adipose fin. In the vast majority of characids this small, fleshy, rayless fin is a reliable feature sitting between the dorsal and the upper caudal lobe; in Hasemania it is either absent entirely or reduced to a tiny knob of tissue. This character is shared across the genus and is retained in H. hanseni.
Detailed colour notes for H. hanseni are sparse in the literature. From its placement within Hasemania and from specimen data, the species is expected to be a silver-bodied small tetra, likely with the pale to yellowish lateral stripe and perhaps a caudal spot that characterise the genus, though precise chromatophore data are not available in currently accessible sources.
Habitat
Hasemania hanseni is known from freshwater streams and tributaries in central Brazil, with confirmed records from Goiás and the Distrito Federal. This region forms part of the Cerrado biome — the vast Brazilian tropical savanna — where streams run through seasonally dry grassland and gallery forest over laterite and sandy substrates, often draining into major river systems including the upper tributaries of the Rio São Francisco basin.
The closely related H. nana, which has been studied more thoroughly, inhabits creeks and small tributaries of the Rio São Francisco basin, occurring in both white-water and blackwater environments. Given H. hanseni's confirmed distribution in the same central Brazilian highland drainage network, comparable biotopes — shallow, clear-to-tannin-stained streams with moderate flow, sandy or gravelly substrates, and riparian vegetation providing overhead shade and leaf litter — are the likely habitat for this species as well. FishBase records the ecology as pelagic and freshwater, within a subtropical range.
Feeding
No species-specific dietary data for Hasemania hanseni are available in currently accessible sources. From the biology of congeners and the feeding guild typical of small characids in the cerrado stream fauna, H. hanseni is presumed to be a generalised micropredator and omnivore, taking small aquatic invertebrates — midge larvae, microcrustaceans, and other zooplankton — along with small terrestrial insects that fall to the water's surface, and possibly filamentous algae or detritus as a dietary supplement.
In the aquarium, congeners such as H. nana adapt readily to a variety of prepared and live foods, accepting flake, micro-pellets, and frozen foods including bloodworm, Daphnia, and brine shrimp. H. hanseni would be expected to behave similarly, though specific captive dietary observations remain unpublished in sources available for this article.
Mating
No mating observations specific to Hasemania hanseni are recorded in the literature available for this account. The genus Hasemania, and small Characidae broadly, are egg-scattering open spawners; courtship typically involves males pursuing females through fine-leaved vegetation, with spawning triggered by the pair darting through plant fronds and depositing eggs as they go.
In H. nana — the best-studied congener — spawning is non-seasonal in captivity, occurring whenever fish are well-conditioned and water quality is appropriate. Males display laterally to females and may develop slightly intensified colour during courtship. H. hanseni, as a closely related small Hasemania, is presumed to follow the same general reproductive pattern, though formal documentation is lacking.
Breeding
Hasemania hanseni has not been documented in the aquarium literature as a commonly bred species, and specific breeding protocols are not established. Based on the reproductive mode of H. nana — for which the breeding biology is well described — the following general pattern applies to the genus: eggs are adhesive and deposited among fine-leaved plants or spawning mops; the adults provide no parental care and will consume eggs if not removed; fry hatch within 24–36 hours at typical tropical temperatures and become free-swimming by 3–4 days after hatching.
For H. nana, water conditions used by breeders fall in the range of approximately 73–82 °F, pH 6.0–8.0, and moderate hardness (5–20° dH). As a co-occurring central Brazilian congener, H. hanseni would likely respond to comparable conditions. First foods for fry are typically infusoria or commercial liquid fry preparations, graduating to newly hatched brine shrimp nauplii as the fry grow. Breeding H. hanseni in captivity remains a task for the specialist; data sparse.
In the aquarium
Hasemania hanseni is rarely encountered in the aquarium hobby and is not an established trade species. Its small size — around 1.5 in SL — makes it suitable for small aquaria, and its character as a centralBrazilian stream fish suggests that a biotope-inspired setup with soft, slightly acidic to neutral water, sandy substrate, some leaf litter, and fine-leaved plants would suit it well. Moderate filtration and regular water changes are appropriate; strong current is not required.
Given the habitat preferences of congeners and closely related cerrado-stream tetras, water temperatures in the 72–82 °F range and pH values from approximately 6.0 to 7.5 are a reasonable starting point. As a small, presumably schooling characid, a group of six or more individuals is preferable to a lone specimen or a pair. Tankmates should be similarly small and peaceful. The diet can follow the pattern established for H. nana: a variety of small live and frozen invertebrate foods alongside quality flake or micro-pellets. Due to its rarity in trade, most aquarists interested in the species would need to source it through specialist breeders or collectors.
Conservation
Hasemania hanseni is assessed as Least Concern (LC) on the IUCN Red List, with an assessment date of 7 November 2018 as recorded by FishBase. The Least Concern designation indicates that, at the time of assessment, the species did not meet the thresholds for a threatened category and was not considered to be declining at a rate that would warrant listing.
The species is endemic to central Brazil, with records from Goiás and the Distrito Federal — a region that lies within the Cerrado biome, one of the world's most threatened biodiversity hotspots. The Cerrado has suffered extensive habitat loss through agricultural conversion, particularly for soy and cattle production, and the stream systems that support small endemic characids like H. hanseni are vulnerable to siltation, pollution, and flow alteration from land-use change. A Least Concern listing should be read in the context of this broader landscape pressure, and localised populations may face stresses not fully captured in the global assessment.