Taxonomy & naming
Apareiodon machrisi was described by the Brazilian ichthyologist Lauro Travassos in 1957. It belongs to the genus Apareiodon within the family Parodontidae, a small group of South American characins commonly called "scraping characins" or "toothed headstanders" for their distinctive ventral, algae-rasping mouths. Parodontidae sits within the order Characiformes but is only distantly related to the pacus and piranhas of Serrasalmidae that dominate this atlas section — the resemblance ends at the shared order.
The family ranges from eastern Panama south to the Río de la Plata basin, and the genus Apareiodon is one of its principal genera alongside Parodon. Data on Apareiodon machrisi specifically is sparse in the accessible literature; occurrence records consistently place it in the Tocantins-Araguaia river system of Brazil, and no subsequent taxonomic revision affecting the species' valid combination has surfaced in the sources consulted here, so "Apareiodon machrisi Travassos, 1957" stands as the current valid name.
Morphology
Apareiodon machrisi is a slender, elongate, torpedo-shaped fish typical of its genus, with a rounded snout and a small, ventrally positioned mouth adapted for scraping rather than biting. FishBase records a maximum size of 3 in standard length, and a voucher specimen (MCP 15876) measured at 2 in SL confirms the species stays genuinely small — this is not a fish that grows into anything resembling the large, deep-bodied serrasalmids elsewhere in this family group.
Like other Parodontidae, its teeth are modified into a comb-like scraping edge suited to rasping algae and biofilm from rock and wood surfaces rather than cutting flesh or plant tissue, and the family overview notes most species in this group stay under 6 in — machrisi sits well within that, at the smaller end. Specific colour-pattern and fin-count details were not available in the sources consulted; the trade name "Brazilian algae eater" reflects its grazing habit and general silvery characin build rather than any documented striking pattern.
Habitat
The species is native to the Tocantins-Araguaia river basin of Brazil, a major Neotropical freshwater system that drains north into the Amazon delta region independently of the main Amazon channel. FishBase describes it as benthopelagic freshwater fish, consistent with a species that spends much of its time working over submerged rock, wood and sediment surfaces where algae and biofilm accumulate.
Water conditions reported for the species run warm and only mildly acidic to neutral: FishBase gives a temperature range of roughly 73–84 °F and a pH of 6.5–7.5. Data on specific tributary habitat structure, flow regime or seasonal behaviour within the Tocantins-Araguaia system was not found in the sources consulted here.
Feeding
Apareiodon machrisi is a grazer, not a predator or an omnivorous opportunist. FishBase records its diet as algae, vegetable matter and detritus, consistent with the ventral, scraper-toothed mouth shared across Parodontidae, which is purpose-built for rasping biofilm and algal growth from hard surfaces rather than taking flesh or large prey items.
In the aquarium this translates to a fish that does real, useful grazing work on algae-covered wood, rock and glass, and that benefits from a diet supplemented with algae wafers, blanched vegetables and a small amount of vegetable-forward prepared foods alongside whatever biofilm the tank supplies naturally. There is no indication in the sources of predatory or fin-nipping behaviour — this is a peaceful grazer, the opposite behavioural profile from the true piranhas elsewhere in this family group.
Mating
Courtship and spawning behaviour for Apareiodon machrisi specifically is not documented in the sources consulted here, and breeding of the species in captivity is described in hobby sources as not well documented. Data is sparse enough that no reliable account of pairing behaviour, spawning triggers or external sexual dimorphism can be given without speculation, and none is offered here.
Breeding
No confirmed spawning mode is documented for Apareiodon machrisi in the sources available. The wider family Parodontidae is not known in the hobby for established captive-breeding protocols the way many open-water egg-scattering characins are, and iNaturalist-sourced hobby notes explicitly flag that breeding of this species in the aquarium is not well documented. Rather than assign a breeding mode by inference from more common characin groups, this account states plainly that the species' reproductive biology in both the wild and captivity is effectively undescribed in the available literature — a genuine data gap rather than an oversight.
In the aquarium
Apareiodon machrisi is a small, active, undemanding fish well suited to a well-established, algae-tolerant community aquarium rather than a specimen tank — its whole ecological role is grazing algae and biofilm off hardscape, so a bare, sterile setup wastes what the fish actually does best. Hobby sources describe it as peaceful and adaptable to slightly acidic-to-neutral water, matching the FishBase range of roughly pH 6.5–7.5 and 73–84 °F.
As a schooling minor characin it should be kept in a group rather than singly, with driftwood and rock surfaces left to develop natural algae growth for it to work over, supplemented with algae wafers and vegetable matter. Given its small adult size (well under 4 in), it poses no space or predation risk to typical community tankmates, and unlike the pacus, wolf fish and true piranhas found elsewhere in this family group, there is no size trap, legal restriction or release warning associated with this species — the sources describe a genuinely low-maintenance, non-aggressive grazer.
Conservation
Apareiodon machrisi has not been formally assessed by the IUCN Red List and currently carries a status of Not Evaluated. This is common among smaller, single-basin Neotropical characins with a narrower distribution and limited commercial or scientific attention.
No specific threats, population trends or conservation pressures for the species were identified in the sources consulted. Its range in the Tocantins-Araguaia basin is subject to the general pressures facing Brazilian freshwater systems — damming, deforestation-linked sedimentation and water development — but no source-documented, species-specific conservation concern is asserted here; data is sparse and this account does not go beyond what is documented.