Taxonomy & naming
Hasemania diastatos was described by F.C.P. Dagosta, M.M.F. Marinho, and P. Camelier in 2014, based on material from highland stream systems on both flanks of the Serra Geral de Goiás. The original publication appeared in Neotropical Ichthyology, and the species was initially placed in Hyphessobrycon Durbin before subsequent reassignment to Hasemania Ellis. The authority is therefore written in parentheses: (Dagosta, Marinho & Camelier, 2014), following the Catalog of Fishes convention for names moved from their genus of original description. Catalog of Fishes (Eschmeyer, CAS) records the currently accepted combination as Hasemania diastatos.
Hasemania itself is a small genus of compact, often plainly coloured characids distributed across river basins in eastern and central Brazil, belonging to the large and internally complex family Characidae. The genus was named to honour John D. Haseman, an early twentieth-century American collector whose expeditions to South America produced important ichthyological material. Within Hasemania, diastatos is most closely allied to H. brumado, H. negodagua, and H. parvellus, from which it is distinguished principally by the absence of a concentrated humeral chromatophore patch, by 15–18 branched anal fin rays, and by the sexually dimorphic caudal peduncle spot present in males — characters detailed in the 2014 diagnosis.
Morphology
Hasemania diastatos is a small, slender fish; the largest confirmed specimen measured 1 in standard length, and FishBase records the maximum at 1 in SL, making it one of the smaller members of its genus. The body is fusiform and laterally compressed, as typical for Hasemania, with a vertebral count of 31–32.
The fins are hyaline to slightly yellowish and lack the dark pigment patches that characterise some congeners. The most diagnostic external feature is the absence of a humeral spot — a dark smudge behind the opercle common in related species. In adult males, the dorsal and anal fins are elongated relative to females, and a well-defined, round dark spot marks the caudal peduncle; this spot is rounded and clearly demarcated, distinguishing males of diastatos from the caudal marks seen in allied species. Females lack the elongated fin rays and caudal peduncle spot, making sexual dimorphism reasonably apparent in mature specimens.
Habitat
The species occupies clear-water creeks at altitudes of 1411–3068 ft on both slopes of the Serra Geral de Goiás, a highland escarpment separating the São Francisco and Tocantins drainages in the states of Bahia, Goiás, and Tocantins. Confirmed localities include tributaries of the rio Grande in the São Francisco basin (east of the divide) and the rios São Domingos and Sono in the upper and middle Tocantins basin (west of the divide).
The IUCN assessment describes the microhabitat as clear-water creeks with sandy substrates, submerged aquatic vegetation, fast-flowing current, and dense riparian vegetation consisting of grasses, herbs, and shrubs. This habitat type is characteristic of the cerrado-transition zone of central Brazil, where gallery forest ribbons clear, oxygen-rich watercourses across open savanna landscapes. The split distribution across two major basins separated by the serra is biogeographically notable and was discussed by the describing authors in the context of the species' history.
Feeding
Stomach-content analysis documented in the IUCN assessment reveals a mixed allochthonous and autochthonous diet: plant fragments, organic debris, and a variety of invertebrates — insect larvae from the orders Diptera and Trichoptera (particularly the micro-caddisfly family Hydroptilidae), and terrestrial insect fragments including ants (Hymenoptera: Formicidae), orthopterans, and other incidentally ingested terrestrial arthropods.
This diet profile is consistent with the species' clear-water, fast-current stream habitat, where drifting invertebrates and allochthonous inputs from the riparian zone form major food resources. Like many small characids of similar ecology, diastatos likely forages opportunistically on whatever invertebrate material the current delivers, supplemented by fine plant and detrital matter. At least one specimen has been recorded parasitised by an isopod in the urogenital cavity, an incidental finding noted in the IUCN assessment data.
Mating
Specific reproductive behaviour has not been studied in Hasemania diastatos, and no detailed field observations of courtship or spawning are available in the literature. The species is a member of Characidae, within which egg-scattering open spawning without parental care is the ancestral and predominant reproductive mode. The marked sexual dimorphism — elongated dorsal and anal fin rays and a distinct caudal peduncle spot in males — strongly suggests that fin displays play a role in male-to-male competition or female assessment, as documented in other sexually dimorphic small characids.
Based on the habitat (clear-water streams with submerged vegetation and sandy substrate) and the biology of closely related Hasemania species, spawning most likely occurs among fine-leaved aquatic plants or over sandy substrate, with adhesive or semi-adhesive eggs scattered during brief courtship chases. Water temperature in the known range (broadly tropical highland, approximately 72–79 °F) would set the pace of egg development, though no empirical incubation data exist for this species.
Breeding
No aquarium breeding accounts for Hasemania diastatos appear in the accessible literature, and the species is essentially absent from the international ornamental trade. General principles drawn from Hasemania and small characid breeding apply by inference: egg-scattering over fine-leaved plants or spawning mops, small adhesive eggs receiving no parental attention, and adults that will readily consume their own spawn if not removed.
For any keeper who might acquire specimens, a separate, dimly lit breeding tank with clumps of fine-leaved plants or synthetic mops, slightly acidic to neutral soft water reflecting the clear-water stream origin, and conditioning on live or frozen invertebrate foods would represent a reasonable starting point. Because the species inhabits highland streams at up to 3068 ft elevation, slightly cooler temperatures than standard tropical aquaria — perhaps 72–75 °F — may be more appropriate than the 79–82 °F used for lowland characids. All of these recommendations are extrapolated from related species; direct documentation is lacking.
In the aquarium
Hasemania diastatos is not an established aquarium fish and is unlikely to be encountered in the trade. Its tiny adult size (under 1 in SL) and clear-water highland stream origin suggest it would be suited to a small, well-filtered species tank or a carefully designed nano aquarium rather than a community setup. The clear, fast-flowing, vegetation-fringed streams it inhabits indicate a preference for well-oxygenated water with moderate current, a sandy or fine-gravel substrate, and cover provided by live or artificial plants.
Water chemistry from the known range — clear highland streams with sandy substrate — is consistent with soft to moderately mineralised, slightly acidic to neutral conditions, though the species spans two major basin systems and may have some tolerance for variation. The main attraction for a specialist keeper would be the sexual dimorphism: males in good condition show both the elongated dorsal and anal fin rays and the rounded caudal peduncle spot that distinguish this species from its congeners. Given its restricted range and data-sparse conservation profile, any captive breeding work would carry genuine value.
Conservation
Hasemania diastatos is assessed as Least Concern (LC) on the IUCN Red List, with the assessment published in 2022 based on a November 2018 evaluation by assessors from Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio). The LC listing reflects the species' occurrence across multiple stream systems in two major drainages and the absence of evidence of a population decline sufficient to trigger a threatened category.
Nevertheless, the IUCN assessment identifies the expansion of soya cultivation in western Bahia as a potential future threat, given that intensive soya farming carries high water demand and can affect stream hydrology and water quality in the cerrado region. The species' highland streams fall within or adjacent to several protected areas, including the Área de Proteção Ambiental Bacia do Rio de Janeiro, APA Jalapão, Parque Estadual do Jalapão, and Estação Ecológica Serra Geral do Tocantins, which offer some degree of habitat protection. The IUCN assessment notes that population trends are unknown and flags research needs in distribution limits, life history, and ecology — the basic biological data for this species remain sparse.