Taxonomy & naming
Nematocharax costai was formally described by Bragança, Barbosa & Mattos in 2013 in the journal Vertebrate Zoology (63(1): 3–8), based on specimens from the Cambiriba Stream, Balneário Guaíra, within the middle Contas River basin in the state of Bahia, Brazil. The genus Nematocharax had been erected earlier to accommodate a distinctive characid with elongated, branched dorsal, anal, and pelvic fin rays and a two-rowed arrangement of premaxillary teeth in adults. The 2013 authors argued that their Bahian material differed consistently enough from the pre-existing Nematocharax venustus to warrant a new species.
In 2015, however, Menezes and colleagues published a broader morphological revision of the genus in Zootaxa (3920(3): 462–464) and concluded that the diagnostic characters separating costai from venustus were not consistent across the available material. Under that analysis, Nematocharax was confirmed as a monotypic genus and N. costai was placed in synonymy with N. venustus — meaning the valid species name for this fish is Nematocharax venustus (Eigenmann, 1987). For the purposes of this entry the name N. costai is retained to document the original description and its type locality.
Morphology
In life and in the original description, specimens attributable to Nematocharax costai shared the genus's most striking feature: conspicuously elongated, branched rays in the dorsal, anal, and pelvic fins that give the fish a graceful, almost filamentous fin fringe. Adults show two rows of teeth on the premaxilla, a character state regarded as a synapomorphy of the genus. Body depth is moderate and the overall build is slender, consistent with the active midwater existence of small stream characids.
FishBase records a maximum size of 2 in standard length for males of the species entry. The species has 33 vertebrae according to FishBase. The synonymisation with N. venustus implies that colour pattern and meristic counts overlap sufficiently with that taxon that no single external feature reliably separates the two, and definitive identification of museum material rests on the original type series.
Habitat
The type locality is the Cambiriba Stream at Balneário Guaíra, Município de Iguaí, in the middle basin of the Contas River — a north-east Brazilian river draining into the Atlantic in southern Bahia. The collection point sits at approximately 14°36'16"S, 40°06'08"W at an elevation of 1138 ft. FishBase describes the habitat as a sandy-bottomed, fast-running stream roughly 23 ft wide and 3 ft deep, with pale brown, turbid water and marginal vegetation along the banks.
The Contas River basin lies in the caatinga-cerrado transition zone of north-eastern Brazil, a region of seasonal rainfall and intermittent or reduced stream flows during the dry season. The benthopelagic habits attributed to the genus suggest that specimens occupy the lower-to-middle water column in these clear-to-turbid fast-flowing microhabitats, taking shelter among marginal roots and vegetation.
Feeding
No specific dietary data for Nematocharax costai has been published independent of the original description. By analogy with other small stream characids of similar body form and habitat, the genus likely consumes small aquatic invertebrates — insect larvae, microcrustaceans — as well as terrestrial insects and organic particles drifting downstream. The elongated fin rays do not suggest a specialised feeding adaptation and the dentition (two-rowed premaxillary teeth) is consistent with a generalised insectivore-omnivore diet.
In an aquarium context, fish of this genus would be expected to accept small live and frozen invertebrate foods readily and to take fine dried foods once acclimated.
Mating
No published observations of mating behaviour for Nematocharax costai or N. venustus are available in the accessible literature. For a small, active stream characid of this body plan, egg-scattering open spawning among fine-leaved aquatic plants or over sandy substrate is the expected reproductive strategy, consistent with the broader pattern across the family Characidae. There is no indication of parental care in any closely related genus.
The elongated pelvic fin rays seen in adults of both sexes may play a role in courtship signalling, though this has not been formally described.
Breeding
No captive-breeding records specific to Nematocharax costai or N. venustus are documented in the hobbyist or scientific literature available to this project. Given the species' restricted known range, very small maximum size, and origin in fast-moving, moderately turbid Bahian streams, it has almost certainly never entered the ornamental trade in meaningful numbers.
Any successful captive attempt would be expected to follow the standard characid model: well-conditioned adults in soft, slightly warm water with fine-leaved plant spawning media, removal of adults after spawning, and small first foods for the tiny fry. Data sparse.
In the aquarium
Nematocharax costai — or the broader N. venustus of which it is now considered a synonym — does not appear to be established in the hobby. The genus was described from a narrow geographic range in north-eastern Brazil and has not surfaced in the ornamental trade literature reviewed for this entry. Its small size (2 in SL), peaceful characid disposition, and the attractive elongated fin rays of the genus would make it an appealing nano or community aquarium subject were it to become available.
Should specimens be obtained, the type locality data suggest a preference for clean, well-oxygenated water with moderate current, a sandy substrate, and relatively neutral water chemistry at temperatures consistent with the highland Bahian stream environment (roughly 72–79 °F). Tankmates should be small and non-aggressive.
Conservation
Nematocharax costai has not been evaluated by the IUCN Red List, and because it is now treated as a junior synonym of N. venustus, it is unlikely to receive an independent assessment. The broader N. venustus likewise carries no current IUCN status. The type locality — Cambiriba Stream in the middle Contas River basin — lies in the semi-arid Bahian interior, a region where small streams are subject to agricultural abstraction, deforestation of riparian margins, and intermittent drought stress. No targeted population surveys of Nematocharax in this system appear to have been published.
The genus's apparent restriction to a small number of Bahian stream systems, combined with the pressures on Atlantic Forest and caatinga watercourses more broadly, warrants future attention. The ornamental fish trade is not a documented pressure given the species' absence from the hobby.