Taxonomy & naming
Hemigrammus ataktos was described in 2014 by Marinho, Dagosta, and Birindelli in the journal Neotropical Ichthyology, based on material from the rio Tocantins basin in central Brazil. The valid name, as recorded in Eschmeyer's Catalog of Fishes, is Hemigrammus ataktos Marinho, Dagosta & Birindelli, 2014; the authority is not parenthetical because the species was described in the genus in which it remains.
The genus Hemigrammus Gill, 1858 is one of the most species-rich among the American characids, grouping small, laterally compressed tetras with a partial or absent lateral line and typically a forked caudal fin. The family Characidae is used here in the sense of the Catalog of Fishes, the governing authority for valid names on this site. The specific epithet ataktos (Greek adjective: disordered, irregular) was chosen by the describing authors to draw attention to the variability in lateral-line scale perforation that distinguishes H. ataktos from otherwise similar congeners — a subtly diagnostic character that requires close examination under magnification.
Morphology
Hemigrammus ataktos is a small, elongated tetra reaching a maximum recorded standard length of 1.5 in. The body is slender and laterally compressed in the typical Hemigrammus fashion. Meristic counts include 11–12 dorsal soft rays and 33–34 vertebrae. The lateral line is incomplete and variably perforated — the character from which the species takes its name.
The colour pattern is the clearest field mark: a well-defined black midlateral stripe extends from the eye to the median caudal rays, creating a stark horizontal band along the body. Sexually mature males are further distinguished by elongated anteriormost rays of the dorsal, pelvic, and anal fins — an extension not seen in females or juveniles. FishBase notes this species as a benthopelagic freshwater fish of tropical environments, with a trophic level estimated at 3.2, consistent with a small invertebrate-and-microorganism diet in common with other small characids.
Habitat
Hemigrammus ataktos is endemic to the rio Tocantins drainage, a major river system draining central Brazil into the Atlantic via the Amazon estuary. The type locality is in the middle Tocantins River. Biogeographic work confirms the species as restricted to this basin, with no records outside it.
The Tocantins is a large, predominantly clear-to-white-water river system with seasonally fluctuating water levels. Like many small characids of the region, H. ataktos is assumed to associate with marginal vegetation, submerged woody debris, and shallower flooded environments during high-water periods. Specific microhabitat data for this species are sparse; given its small size and benthic-pelagic habits noted by FishBase, it likely occupies slower-moving or sheltered stretches rather than the fast main channel.
Feeding
No detailed stomach-content studies have been published for Hemigrammus ataktos. Based on its trophic level estimate of 3.2 and its affiliation with the genus Hemigrammus — whose members are generally opportunistic micropredators and omnivores — the species almost certainly feeds on small aquatic invertebrates, microcrustaceans, insect larvae, and particulate organic matter in the water column and at the substrate surface.
In captivity, small Hemigrammus are readily maintained on a varied diet of high-quality micro-pellets and crushed flake supplemented with frozen or live foods such as Daphnia, Cyclops, and newly hatched brine shrimp. A varied diet supports condition and, in males, the display intensity of the elongated fin rays that distinguish them.
Mating
Hemigrammus ataktos has not been documented breeding in captivity in published literature, but as a member of Hemigrammus it almost certainly follows the egg-scattering reproductive strategy universal in the genus. Males with their elongated dorsal, pelvic, and anal fin rays are likely to engage in competitive display and fin-spreading behaviours before gravid females, as is typical for sexually dimorphic Hemigrammus species.
Spawning in Hemigrammus generally occurs in early morning in the presence of fine-leaved vegetation or roots, with pairs or small groups of fish scattering adhesive eggs that catch on plant surfaces. Water chemistry for spawning induction in related species typically involves soft, slightly acidic water at temperatures of around 75–79 °F, though specific conditioning parameters for H. ataktos have not been reported in the literature.
Breeding
No aquarium breeding accounts have been published for Hemigrammus ataktos. Based on the standard Hemigrammus reproductive pattern, eggs are expected to be small, adhesive, and deposited among fine-leaved plants or spawning mops, with hatching occurring within approximately 24–36 hours under warm conditions and fry becoming free-swimming a few days later.
Parental care is absent in Hemigrammus: adults should be removed after spawning to prevent egg predation. First foods for the tiny fry would include infusoria and commercial liquid fry foods, followed by micro-worm and newly hatched brine shrimp as the juveniles grow. Given the rarity of this species in the hobby, captive breeding efforts have apparently not been reported publicly.
In the aquarium
Hemigrammus ataktos is not established in the ornamental fish trade and is therefore very rarely encountered outside specialist collections or ichthyological circles. Its small adult size (under 1.5 in SL), attractive midlateral stripe, and the fin elongation of mature males make it an aesthetically interesting species, but its rarity means practical husbandry experience in captivity is essentially undocumented.
If kept, it would logically be maintained in the manner of other small Hemigrammus species: a well-planted aquarium with subdued lighting and dark substrate to bring out the stripe, soft to moderately soft slightly acidic water reflecting its Tocantins provenance, and a shoal of six or more individuals to encourage natural behaviour. It should be housed with similarly sized, non-aggressive tankmates. Until captive-bred populations become established, any specimens in the trade would be wild-caught; sourcing responsibly and from documented legal origins would be essential given the lack of information on the species' population dynamics.
Conservation
Hemigrammus ataktos is assessed as Least Concern (LC) on the IUCN Red List, with the assessment completed in November 2018. The listing reflects the species' reasonably wide distribution across the Tocantins basin and the absence of identified major threats to the population at the time of assessment.
Nonetheless, the Rio Tocantins has been significantly altered by large-scale hydroelectric development, most notably the Tucuruí Dam, one of the world's largest hydroelectric reservoirs. The downstream and upstream effects of dams on small endemic characids — including habitat fragmentation, altered hydrology, and changes to seasonal flood pulses — are well documented in the broader literature, and future reassessments of species like H. ataktos may need to account for cumulative basin-level pressures. Its status as a range-restricted Tocantins endemic means that monitoring population trends and habitat condition within the basin remains prudent.