Taxonomy & naming
Apareiodon hasemani was described by Carl H. Eigenmann in 1916. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) lists it as a currently valid species in the genus Apareiodon, family Parodontidae, order Characiformes.
Parodontidae — the "scrapetooths" or "toothed headstanders" — are a small Neotropical family of specialised algae grazers, separated from the superficially similar Hemiodontidae and Curimatidae by an inferior (underslung), suckerlike mouth armed with a single row of closely set, spatulate premaxillary teeth used to rasp periphyton from rock surfaces, along with a fusiform body and a single unbranched pectoral-fin ray. Apareiodon hasemani is one of several Apareiodon species restricted to a single Brazilian river system, in this case the São Francisco basin, which has produced a number of locally endemic parodontids.
Morphology
Apareiodon hasemani is a small, elongate, fusiform characin reaching a documented maximum of about 5 in standard length — modest even by the standards of the smaller genera covered in this group, and far removed from the large, powerful characins (Brycon, Salminus) that share the family-level neighbourhood. The body is streamlined and rounded in cross-section, an adaptation shared across Parodontidae for holding station and manoeuvring in fast current rather than for open-water cruising.
The diagnostic features of the genus are functional rather than showy: an inferior, sucker-like mouth positioned for scraping surfaces below the fish, a single row of flattened, spatulate teeth on the premaxilla adapted for rasping algae and biofilm from rock, and a single unbranched ray leading the pectoral fin. Colouration and fine-scale morphological detail for this particular species are not documented in the sources consulted here; data sparse beyond the family-level description and the confirmed maximum size.
Habitat
Apareiodon hasemani is endemic to the São Francisco River basin in Brazil, a large, entirely Brazilian river system running through the country's east-central interior to the Atlantic. Within that basin it is a benthopelagic fish of fast-flowing water, favouring rocky or stony substrates in riffles and current-swept stretches rather than still backwaters — habitat consistent with its role as a rheophilic (current-loving) algae grazer.
As with other Parodontidae, this habitat preference is tied directly to feeding ecology: fast, well-oxygenated, current-scoured rock surfaces support the growth of the algal biofilm the species depends on. Specific water chemistry data (pH, hardness, temperature range) for A. hasemani were not found in the sources consulted; data sparse on those specifics.
Feeding
Apareiodon hasemani is a daytime algae grazer, consistent with the family-wide Parodontidae feeding strategy: the underslung, spatulate-toothed mouth is used to rasp periphyton — algae and associated biofilm — from the surface of rocks in current. This places it among the omnivore/detritivore grazers of the broader "Hemiodus & minor characins" group, alongside the hemiodus, curimatas and Prochilodus, and in clear contrast to the predatory Brycon and Salminus.
No aquarium-specific feeding notes were found in the sources consulted for this species; in general, algae-grazing parodontids are best approximated in captivity with a diet built around algae wafers, blanched vegetables and biofilm-rich, well-conditioned surfaces rather than protein-heavy fare, but this is an inference from family biology rather than a documented account of A. hasemani specifically.
Mating
No courtship, pairing or spawning behaviour has been documented for Apareiodon hasemani in the sources consulted for this article. Data sparse — this is a poorly studied riverine endemic with no aquarium-breeding record, and nothing should be assumed about its mating behaviour beyond what is confirmed for the family.
Breeding
There is no confirmed breeding-mode record for Apareiodon hasemani in the sources reviewed here, and no evidence it has been bred in captivity. It should not be assumed to be a substrate nest-spawner, an egg-scatterer, or any other specific mode without direct evidence; honestly, this is simply undocumented for this species. Some related Neotropical algae-grazing characins are egg-scatterers over current-swept substrate, but that pattern is not confirmed for A. hasemani specifically and is not stated here as fact.
In the aquarium
Apareiodon hasemani has essentially no presence in the aquarium trade or hobby literature; the sources consulted for this article are limited to taxonomic and conservation databases and a general habitat/behaviour reference, with no aquarium-keeping accounts found. Its small adult size (about 5 in SL) and biology as a current-loving, rock-grazing algae feeder suggest that, were it ever kept, it would need strong water flow, oxygen-rich conditions and algae-covered hardscape rather than a typical still, planted community setup — but this is inference from its wild ecology, not a documented care record.
Given the complete absence of hobby data, prospective keepers should not expect to source this species commercially; it is best regarded as a river-specific scientific curiosity rather than an aquarium fish at this time.
Conservation
Apareiodon hasemani is assessed by the IUCN Red List as Least Concern, an assessment dated 7 November 2018. As a São Francisco basin endemic, its conservation outlook is tied to the health of that single Brazilian river system — a heavily used basin subject to damming, water abstraction and agricultural pressure across its length — even though the species itself is not currently considered threatened.
There is no indication in the sources consulted that A. hasemani is targeted by the aquarium trade or by fisheries; its Least Concern status reflects an apparently stable, if narrow, native range rather than any commercial exploitation.