Taxonomy & naming
Hasemania negodagua was originally described by Flávio Lima and Peter Gerhard in 2001. FishBase currently places the species in the genus Hasemania, and it is treated here accordingly; earlier literature may list it under Hyphessobrycon, reflecting the fluid generic boundaries that persist across the family Characidae. The valid name as used here is Hasemania negodagua (Lima & Gerhard, 2001), with the parenthetical authority indicating that the species was moved from its original generic placement.
Hasemania is a small genus of South American tetras historically distinguished from Hyphessobrycon by the consistent absence of an adipose fin — a feature H. negodagua shares. The type locality is the upper Paraguaçú River basin in the state of Bahia, Brazil, in the Chapada Diamantina highland region. This basin drains eastward into the Atlantic and is hydrologically isolated from the major drainages of the São Francisco and Amazon systems, making endemism at species level common here.
Morphology
Hasemania negodagua is a small tetra, reaching a maximum of about 1 in standard length. The body is slender and laterally compressed, with two dorsal spines and eight to nine soft dorsal rays; the anal fin carries four spines and fourteen to seventeen soft rays. Vertebral count runs from 29 to 32. The species lacks an adipose fin entirely, which distinguishes it from most Hyphessobrycon and aligns it with the Hasemania group.
Sexual dichromatism is pronounced. Males are dark, with a high concentration of chromatophores on the posterior body that gives them a noticeably blackish or charcoal appearance; the distal portions of the dorsal and anal fins are white, producing a crisp contrast. Females are considerably paler throughout, with a rounded abdomen and without the dark posterior flush or white fin tips. This strong dimorphism makes sexing straightforward in adults.
Habitat
The species is endemic to the upper Paraguaçú River basin in Bahia, Brazil, within the Chapada Diamantina — a highland plateau of sandstone tablelands, gallery forests, and clear headwater streams that feeds the river toward the Atlantic coast. The IUCN estimates the extent of occurrence at roughly 12,345 mi², a small area for a wild vertebrate population. Habitat is freshwater inland wetlands: rivers, streams, and permanent freshwater marshes.
Field observations show that individuals establish small territories of roughly 98.5 in² with overhead or lateral cover — submerged wood, leaf litter, or rocky structure. The fish is benthopelagic, living close to the substrate in sheltered microhabitats. The Chapada Diamantina streams are typically clear and relatively cool for the Brazilian northeast, with stable flow in permanent reaches; seasonal flooding and dry-season contraction affect marginal wetlands within the range.
Feeding
Stomach content analysis of 14 wild specimens documented an eclectic diet that spans both aquatic and terrestrial pathways. Insects dominated, appearing in 85.6% of stomachs; filamentous algae occurred in 50% of specimens. Scales (possibly from acacari, suggesting facultative scale-grazing) were recorded at 21.3%. Less frequent items included insect pupae, ants, dipteran larvae, wasps, earthworms, and Lepidoptera larvae, each at approximately 7%.
This breadth marks Hasemania negodagua as a generalist omnivore that takes whatever food is available at the water surface, in the water column, and from the bottom. The algal component suggests some grazing on biofilm or macroalgae in addition to the predominantly animal diet. In the aquarium the species will accept good-quality dried foods but benefits from varied live and frozen items — Daphnia, bloodworm, brine shrimp, and small fruit flies or other terrestrial insects — to replicate the allochthonous input that appears important in the wild.
Mating
Males of Hasemania negodagua are territorial and behaviourally assertive. In the field they hold small defended patches with cover, and rival males are repelled through chasing and fin displays — the dark posterior coloration and bright white fin edges likely function as visual signals during these interactions. Females, which are paler and do not hold territories, enter male patches when receptive.
Breeding in the aquarium has been observed to follow a typical open-water egg-scattering pattern: spawning is vigorous and conspicuous, with activity levels that hobbyists have compared to zebra danios — unusually energetic for a tetra of this size. Spawning appears to be stimulated by soft, slightly acidic water in line with the clear headwater conditions of the Chapada Diamantina.
Breeding
Hasemania negodagua is an egg-scatterer with no parental care. Confirmed captive spawning shows eggs scattered and immediately at risk of predation from the parents themselves; there is no nest-building and no guarding of eggs or fry. Removing the adults or the eggs to a separate container after spawning is therefore necessary for successful rearing.
Little detailed captive husbandry guidance had been published for the species before 2020, so finer points of clutch size, incubation period, and fry requirements remain to be systematically documented. By analogy with similar small tetras, spawning mops or fine-leaved plants such as Java moss make suitable spawning substrates. Fry are presumed to require very fine first foods — infusoria or commercial liquid fry diet — before moving to newly hatched brine shrimp as they grow.
In the aquarium
Hasemania negodagua is a high-energy tetra, reported in hobby experience to be more active than most comparably sized tetras and on a level with zebra danios. This activity, combined with territorial behaviour in males, means the species does best in a well-structured aquarium with plenty of cover — wood, dense planting, and leaf litter can break sightlines and allow subdominant fish breathing room. A group of six or more, with a mix of males and females, distributes aggression more evenly than a pair or a single male.
Soft, slightly acidic water — matching the clear headwater streams of the Chapada Diamantina — is the appropriate baseline. The fish is small enough for a tank of 23.5 in or so for a modest group, though more space supports better behaviour. Little specialist husbandry literature existed before the species reached the hobby around 2020, so keepers should treat it as a soft-water, active micro-tetra and adjust from there. Its strong sexual dimorphism makes it visually interesting in a planted display, where the dark males with white-edged fins show well against green vegetation.
Conservation
Hasemania negodagua is assessed as Near Threatened (NT) under criterion B1b(iii) on the IUCN Red List, assessed 17 November 2018 and published in 2023. The Near Threatened classification reflects the species' small extent of occurrence (approximately 12,345 mi²), its restriction to a single river basin, and a continuing decline in habitat quality driven by residential and commercial development — housing projects, tourism infrastructure, and real estate expansion — alongside logging and wood harvesting in the Chapada Diamantina catchment.
The Chapada Diamantina is one of Brazil's major ecotourism destinations, and unregulated visitor pressure has contributed to stream degradation in parts of the range. Agricultural expansion adds further stress. The population trend is listed as unknown, and the species is not considered severely fragmented at present, but the trajectory of habitat quality is downward. Because Hasemania negodagua is endemic to a single, already-stressed drainage with no satellite populations, the conservation window may be narrower than the Near Threatened category alone suggests. Formal protection of key stream habitats within the Chapada Diamantina and enforcement of land-use restrictions would be the most direct conservation measures available.