Taxonomy & naming
Hemigrammus tocantinsi was described in 2010 by Tiago P. Carvalho, Vinicius A. Bertaco, and Fernando C. Jerep in the journal Neotropical Ichthyology, based on material collected from the Rio das Pedras, a small tributary draining the Chapada dos Veadeiros highlands of Goiás State, Brazil. The holotype (MCP 44444) is a 1 in SL male from the type locality at Fazenda Córrego Fundo, Cavalcante, Goiás (13°44'34"S, 47°26'33"W). Eschmeyer's Catalog of Fishes treats this name as valid within the family Characidae.
The specific epithet tocantinsi refers to the Rio Tocantins drainage, which defines the species' known range. Hemigrammus is a large, taxonomically complex genus of small characins distributed across tropical South America; the group is diagnosed by the presence of scales on the lower lobe of the caudal fin, a character that separates it from the closely related genus Hyphessobrycon, which lacks them. H. tocantinsi was placed in Hemigrammus on the basis of meristic and morphometric analysis in the original description.
Morphology
Hemigrammus tocantinsi is a small, laterally compressed characin with a maximum recorded standard length of 1.5 in. The body form is typical of the genus: moderately elongate, with a subterminal mouth, a small adipose fin behind the dorsal, and a forked caudal fin.
The species displays pronounced sexual dichromatism in life. Adult males are coloured red-orange, a hue that suffuses much of the body, while females are yellowish. Males also develop small bony hooks on rays of the anal and pelvic fins once they exceed approximately 1 in SL — a feature seen in several other Hemigrammus species and interpreted as a reproductive adaptation. Additional sexual differences include proportional variation in fin size and body shape between the sexes.
Habitat
The species is known from the upper Rio Tocantins basin in Goiás State, Brazil. The type locality lies within the Chapada dos Veadeiros, a high-altitude cerrado plateau at the headwaters of the Tocantins system. The original description places the fish in small, shallow streams (less than 3 ft depth) with semi-lentic and lotic microhabitats and riparian vegetation fringing the banks.
The associated fauna recorded at the type locality includes Aspidoras albater, Astyanax sp., Characidium stigmosum, Corumbataia veadeiros, and Trichomycterus sp., all characteristic of the cerrado highland ichthyofauna. Water parameters for the region are not detailed in available sources, but the Chapada dos Veadeiros streams are generally clear, swift, and oligotrophic, draining over ancient quartzite and sandstone geology.
Feeding
Stomach contents examined during the original description showed that H. tocantinsi feeds primarily on allochthonous insects, with Diptera pupae dominating the gut contents alongside other terrestrial and aerial insects falling or blown into the stream. Autochthonous (aquatic) insects and plant matter also occur in small quantities, suggesting an opportunistic strategy that draws on whatever is locally available at the surface or within the water column.
In the aquarium this diet profile translates readily to small live and frozen invertebrates — Daphnia, brine shrimp, and finely minced bloodworm — supplemented with good-quality small flake or micro pellets. As with other allochthonous insect feeders, the fish likely station themselves near the surface or in moderate flow to intercept drifting prey.
Mating
H. tocantinsi shows the typical Hemigrammus reproductive strategy: open egg-scattering with no parental care. The spermatozoa of males have been described as aquasperm with a spherical-to-ovoid nucleus — the non-inseminating morphotype common across the family — indicating that fertilisation is external and eggs are released freely into the water column or among vegetation rather than via internal fertilisation as seen in some advanced characins.
Males develop bony fin hooks on the anal and pelvic fins at maturity, which are thought to function during spawning contact, helping the male maintain position alongside the female during the brief moment of egg and sperm release. Sexual colour differences — red-orange males versus yellowish females — presumably also play a role in mate assessment and courtship, though detailed behavioural observations in either field or aquarium settings have not been published.
Breeding
As an egg-scattering open spawner with aquasperm morphology, H. tocantinsi almost certainly follows the pattern typical of Hemigrammus: conditioned adults scatter small adhesive eggs among fine-leaved plants or over a substrate, with hatching occurring within a day or two at tropical temperatures and the tiny fry becoming free-swimming shortly after. No parental care is provided and adults should be separated from eggs to prevent predation.
Specific captive-breeding data for this species are not documented in available literature. Hobbyists working with closely related small Hemigrammus species typically use soft, slightly acidic water, dense planting or spawning mops, and conditioning on live or frozen foods to trigger spawning. Fry are very small at first and require infusoria-sized foods before graduating to newly hatched brine shrimp nauplii.
In the aquarium
Hemigrammus tocantinsi remains uncommon in the hobby and is rarely available commercially; most aquarists are unlikely to encounter it outside specialist circles interested in the cerrado ichthyofauna. Its care requirements, inferred from its habitat and from parallel species in the genus, are broadly in line with other small Hemigrammus: clean, well-oxygenated water, a temperature in the low-to-mid 20s Celsius, and a tank with some structure — fine-leaved plants, leaf litter, or dark substrate — to showcase the red-orange colouration of males.
At 1.5 in SL it is a nano-scale fish suited to smaller aquaria, and like most characins it is best kept in a group so that males display against one another. Because it originates from shallow, clear highland streams with allochthonous insect inputs, a varied diet featuring live or frozen invertebrates will maintain condition and colour better than an exclusively prepared-food regimen.
Conservation
Hemigrammus tocantinsi is assessed as Least Concern (LC) on the IUCN Red List, with the assessment dated 7 November 2018. The Chapada dos Veadeiros region, while part of the broader cerrado biome under general pressure from agricultural expansion and deforestation, includes significant protected areas: the Chapada dos Veadeiros National Park is a UNESCO World Heritage Site, and the type locality falls within or near this conservation zone.
The species' restriction to the upper Tocantins headwaters means its range is geographically limited, and ongoing monitoring of water quality, riparian vegetation, and hydrology in the Goiás cerrado streams will be important to confirm that the Least Concern assessment remains valid as pressures on the Brazilian cerrado intensify.