Taxonomy & naming
Hysteronotus megalostomus was described by Carl H. Eigenmann in 1911. It is the sole species in the genus Hysteronotus and therefore functions as its own type species. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) treats the valid name as Hysteronotus megalostomus Eigenmann, 1911.
FishBase places the genus within the family Stevardiidae (alternatively treated as the subfamily Stevardiinae within Characidae), a group formerly known as Glandulocaudinae. The site's family field is retained as Characidae in accordance with the broader circumscription used across this atlas. The species is monotypic — no subspecies or recognised variants are known.
The generic name draws on Greek: hysteros (later or behind) and noton (back), a reference to a posterior dorsal feature. The specific epithet megalostomus likewise derives from Greek: megas (large) and stoma (mouth), describing the notably wide gape that distinguishes this fish within its immediate group.
Morphology
Hysteronotus megalostomus is a small, slender characid. FishBase records a maximum standard length of 1.5 in, consistent with IUCN data reporting the largest known specimen at 1.5 in SL. The body form is typical of a small stream characid — moderately compressed, with a slightly elongate profile and the large mouth that gives the species its name.
Detailed external morphology data for this species is sparse in the public literature, reflecting its rarity and the paucity of collected material. Its placement in the stevardiid/glandulocaudine lineage implies the presence of the caudal gland tissue characteristic of many members of that group, though specific pigmentation and scale counts are not widely documented in accessible sources. Its benthopelagic habit suggests it occupies both bottom and mid-water zones in the small streams it inhabits.
Habitat
Hysteronotus megalostomus is endemic to the upper São Francisco River basin in central Brazil, where it occurs in the states of Minas Gerais and Bahia, as well as the Federal District (Brasília DF). Confirmed river systems within this range include the Rio das Velhas, the Paraopeba, the Verde Grande, and the Tatu — all headwater tributaries of the upper São Francisco.
This region lies within the cerrado biome, the vast Brazilian tropical savanna. The species inhabits small headwater riachos: shallow, often fast-flowing streams running through cerrado vegetation. These streams are characterised by seasonal flow variation, with high discharge in the wet season and reduced flow in the dry months. Water temperatures in the upper São Francisco region average roughly 64–72 °F annually. The cerrado stream environment is typically clear to slightly turbid, with substrates of sand, gravel, and leaf litter shaded by gallery forest.
The species is described as rare within this range. Its estimated extent of occurrence is approximately 260,0 mi², but actual occupied habitat is far more restricted, fragmented by the intensive land-use transformation that has affected over 50% of the cerrado biome.
Feeding
No specific dietary studies of Hysteronotus megalostomus appear in the accessible literature. Its benthopelagic habit and placement within the Stevardiinae suggest a diet based on small invertebrates — aquatic insect larvae, microcrustaceans, and similar prey — supplemented opportunistically by organic particulate matter and possibly algae or plant material.
Small stevardiid characins typically forage across the water column and along the substrate surface in stream environments, exploiting the invertebrate drift and bottom fauna of riachos. Given the species' rarity and limited collection history, direct gut-content data have not been widely published. In aquarium conditions it would likely accept small live and frozen invertebrate foods alongside fine-particle prepared diets, consistent with its small gape and stream-dwelling lifestyle.
Mating
Specific reproductive behaviour data for Hysteronotus megalostomus have not been documented in the available literature. As a member of the Stevardiinae — a subfamily whose members generally practice egg-scattering open-water or plant-associated spawning with no parental care — H. megalostomus is expected to spawn adhesive eggs among fine-leaved aquatic vegetation or submerged plant material in the stream environment.
Stevardiid tetras typically exhibit sexual dichromatism or dimorphism associated with breeding condition, and males in many related genera develop the caudal gland secretions thought to function in courtship signalling. Whether H. megalostomus displays equivalent courtship structures is not confirmed in accessible sources.
Breeding
No captive breeding records for Hysteronotus megalostomus appear in the hobby or scientific literature, and the species is essentially unknown in the aquarium trade. Based on its stevardiid affinities, the reproductive mode is inferred to be egg-scattering with adhesive eggs deposited among fine-leaved plants or bottom substrate, hatching after a period of several days at ambient stream temperatures, with no parental care and adults liable to consume their own eggs.
The rarity of wild specimens and the absence of captive populations mean that practically nothing is documented about clutch size, incubation duration, fry development, or any special requirements that might differ from more familiar stevardiid tetras. Any future captive-breeding attempt would need to replicate cerrado stream conditions: soft to moderately hard, slightly cool, well-oxygenated water with ample plant cover.
In the aquarium
Hysteronotus megalostomus is not established in the aquarium hobby and is essentially unavailable in the trade. No husbandry guidelines specific to this species exist. Its small size (under 2 in), benthopelagic stream habit, and stevardiid lineage suggest it would be suited to a small, well-planted aquarium with gentle current, soft to moderately hard water, and temperatures in the 64–72 °F range that approximate its cerrado stream habitat.
Given its rarity and restricted endemic range, any specimens entering the hobby would almost certainly be wild-caught, and the conservation case for preferring captive-bred stock applies strongly. The species is best regarded as a scientific and conservation subject rather than an aquarium fish at this time.
Conservation
Hysteronotus megalostomus is assessed as Least Concern (LC) on the IUCN Red List, with the assessment published in 2022 based on a 2018 evaluation. The LC listing is supported by an estimated extent of occurrence of approximately 260,0 mi², which exceeds the thresholds for threatened categories under criterion B, and by the species' occurrence within several protected areas including the APA Sul, APA Carste da Lagoa Santa, and land adjacent to the Parque Nacional Serra do Cipó.
Despite this formal status, the species faces significant and ongoing habitat pressures. The cerrado headwater streams of the upper São Francisco basin are heavily impacted by agricultural expansion, livestock grazing, artisanal and commercial mining, and removal of riparian and gallery forest vegetation. The population trend is listed as unknown, and the species is noted as rare within its range. The cerrado biome has lost more than half its original extent to land conversion, and the small riachos that H. megalostomus depends upon are among the most vulnerable habitats in the basin to hydrological disruption and sedimentation. The Least Concern assessment should be read with these threats in mind rather than taken as an indication that the species is secure.