Taxonomy & naming
Hemigrammus arua was described by Flávio C.T. Lima, Wolmar B. Wosiacki, and Luciano F. Ramos in 2009. The type locality is within the Araguaia River drainage, one of the major tributaries of the Tocantins–Araguaia basin in central Brazil. The valid name as listed in Eschmeyer's Catalog of Fishes is Hemigrammus arua Lima, Wosiacki & Ramos, 2009.
Hemigrammus is the largest genus of small characid tetras in South America, encompassing well over 50 described species distributed widely across the Amazon, Orinoco, Paraná, and adjacent drainages. The genus has long been recognized as likely non-monophyletic, and several revisionary studies have reorganized close relatives; the placement of H. arua within the genus follows the original description. The species epithet arua refers to the Araguaia River, the drainage from which the type material was collected.
Morphology
Hemigrammus arua is a small tetra in the typical mould of its genus: a slender to moderately deep-bodied, laterally compressed fish with a forked tail, adipose fin, and the characteristic small mouth of a mid-water microinvertebrate feeder. Precise size data from the original description and FishBase records place it in the small range typical for Hemigrammus, with adults likely not exceeding a few centimetres in standard length.
Beyond the characters used to distinguish it from congeners in the original description — which would involve counts of lateral-line scales, fin rays, vertebrae, and details of pigmentation — published morphological accounts accessible for general reference are sparse. Data on coloration in life are not well documented in the available literature for this species.
Habitat
The Araguaia River basin, which forms part of the broader Tocantins–Araguaia system draining central Brazil into the Atlantic via the lower Amazon estuary, is characterized by a mosaic of cerrado (savanna woodland) and Amazon-edge habitats. The river itself cycles between a dry season when sandbanks and beaches emerge and a wet season when floodplains inundate broadly, creating a seasonally dynamic environment for small fishes.
Small Hemigrammus species in similar Amazonian and central Brazilian drainages typically occupy marginal shallows, floodplain lakes, and forested stream edges, sheltering among aquatic vegetation, submerged roots, or leaf litter. Water conditions in the Araguaia system are generally warm and soft to moderately soft, with pH leaning toward acidic, though specific locality data for H. arua have not been published in secondary sources reviewed here.
Feeding
No specific dietary study has been published for Hemigrammus arua. By analogy with the broader genus and with small characids from comparable Araguaia basin habitats, it is presumed to feed opportunistically on small aquatic invertebrates — microcrustaceans, insect larvae, and similar zooplankton — supplemented by terrestrial invertebrates falling onto the water surface and possibly algae or plant material.
In the aquarium, small Hemigrammus species are generally undemanding and accept good-quality micro-pellets, finely crushed flake, and small live or frozen foods such as Daphnia, cyclops, and newly hatched brine shrimp. A varied diet is advisable to maintain condition.
Mating
No detailed study of reproductive behaviour has been published for Hemigrammus arua. The genus as a whole is composed of egg-scattering, open spawners that shed small adhesive eggs among fine-leaved vegetation or over the substrate, with no parental care. Spawning in Hemigrammus is typically triggered by water changes, softer water, and rising temperature, and males generally display by chasing receptive females through vegetation.
Seasonal spawning is likely in the wild, tied to the Araguaia's pronounced wet-dry cycle, which creates cues analogous to the onset of the rainy season that trigger reproduction in many characids across the Amazon basin.
Breeding
Hemigrammus arua has not been documented in captive breeding in the hobbyist literature reviewed here. Based on the pattern established throughout the genus, it would be expected to scatter small, adhesive eggs among fine-leaved plants or spawning mops, with adults showing no care for eggs or fry and readily consuming both if not separated.
For Hemigrammus species generally, standard breeding procedure involves conditioning a small group of adults with live and frozen foods, then moving them to a dedicated breeding tank with soft, slightly acidic water and fine-leaved plants or a spawning mop. Eggs typically hatch within 24–36 hours at tropical temperatures, and the tiny fry require infusoria or commercial liquid fry food as first foods before graduating to newly hatched brine shrimp.
In the aquarium
Hemigrammus arua is not currently available in the ornamental fish trade and no aquarium husbandry accounts have been published. As a small Hemigrammus from a soft-water Amazonian drainage, it would be expected to follow the general care template of the genus: a peaceful schooling fish best kept in groups of six or more, suited to a planted aquarium with soft, slightly acidic to neutral water and temperatures in the range typical for central Brazilian tetras.
Given the lack of hobby records, any aquarist who encounters this species would be working largely from first principles and by analogy with closely related species. The Araguaia basin is periodically represented in the trade by other small tetras, but H. arua itself remains a scientific species rather than a hobby one at present.
Conservation
Hemigrammus arua has not been assessed by the IUCN Red List as of the time of writing. No targeted conservation evaluation is available for the species.
The Araguaia River basin faces significant anthropogenic pressure, including watershed deforestation for agriculture and cattle ranching, channelization, and the construction of hydroelectric infrastructure on the Tocantins–Araguaia system. These pressures affect the small-fish communities of the basin broadly, including small characids such as Hemigrammus. Without a formal assessment, the conservation status of H. arua specifically remains unknown, and the species' range within the Araguaia system has not been mapped in detail in secondary sources.