Taxonomy & naming
Acinocheirodon melanogramma was described by Luiz Roberto Malabarba and Stanley H. Weitzman in 1999 and represents the sole species in the genus Acinocheirodon. The genus is placed in the subfamily Cheirodontinae within the family Characidae. Eschmeyer's Catalog of Fishes (CAS) recognises Acinocheirodon melanogramma Malabarba & Weitzman, 1999 as the valid name; the authors placed it in a new genus, separate from the broadly conceived Cheirodontinae genera known at the time.
The type material was collected from tributaries of the Rio São Francisco and the Rio Jequitinhonha in Minas Gerais, Brazil. Both drainages run north-eastward through the Brazilian Highlands before reaching the Atlantic, and they share a broadly similar character fauna, though they drain into separate coastal systems. The subfamily Cheirodontinae has been substantially revised over the past three decades, and Acinocheirodon has been confirmed as a valid, distinct genus on morphological grounds.
Morphology
Acinocheirodon melanogramma is a small fish, recorded to a maximum of approximately 1.5 in standard length. The body is slender and fusiform, typical of the small-bodied Cheirodontinae, and laterally compressed in the manner of most characids. Overall colouration is silvery, with a dark, well-defined lateral stripe running from behind the operculum to the base of the caudal fin — the feature that gives the species its name.
Fin details and precise meristic data were established in the original description. Like most small cheirodontine characids, the species has a small adipose fin, a forked caudal fin, and a relatively unremarkable fin complement that places it squarely within the subfamily's typical body plan. Sexual dimorphism is not well documented in the literature, but in small characids females are typically deeper-bodied when gravid.
Habitat
The species is benthopelagic in freshwater tropical environments, inhabiting the smaller tributaries and streams of the São Francisco and Jequitinhonha river basins in the state of Minas Gerais, Brazil. These rivers drain the central Brazilian Highlands and carry clear to lightly turbid water over sandy and gravelly substrates, often shaded by riparian gallery forest.
The São Francisco basin is one of Brazil's great river systems, running roughly 2,559 mi from its headwaters in the cerrado and highland regions of Minas Gerais to its mouth on the semi-arid north-eastern coast. The Jequitinhonha drains a smaller highland area to the south-east before reaching the Atlantic in southern Bahia. Both drainages share a transitional cerrado-Atlantic Forest character and support considerable endemic fish diversity. Acinocheirodon melanogramma occupies the upland tributaries of these systems rather than the large main-channel rivers.
Feeding
Acinocheirodon melanogramma belongs to a subfamily whose members are predominantly insectivorous and microinvertebrate feeders. The natural diet almost certainly includes small aquatic invertebrates, zooplankton, and surface-fallen insects, consistent with the benthopelagic habits recorded for the species in the São Francisco and Jequitinhonha tributaries.
Specific gut-content studies have not been widely reported for this species in the available literature. In the aquarium, small cheirodontine tetras of this type generally accept fine dry foods — small-grade flake and micro-pellets — as well as live and frozen invertebrates such as Daphnia, Moina, and newly hatched brine shrimp. A varied diet supports condition and natural behaviour.
Mating
As a member of the Cheirodontinae, Acinocheirodon melanogramma is assumed to be an egg-scattering, open-water spawner with no parental care, following the typical pattern of the subfamily. Males likely display to females using colour and finnage, and spawning probably takes place among fine-leaved vegetation or over open substrate in the wild.
Detailed courtship observations have not been published in the accessible literature for this species. Its restricted distribution and rarity in captivity mean that hobbyist accounts are similarly sparse. The species' small adult size suggests spawning groups may be active and that spawning events involve the release of adhesive or semi-adhesive eggs among aquatic plants.
Breeding
Breeding in captivity has not been widely documented for Acinocheirodon melanogramma, and the species is uncommon in the ornamental trade. By analogy with other small cheirodontine tetras, it likely spawns by scattering adhesive eggs among fine-leaved plants or spawning mops, with no parental investment; adults should be considered capable of consuming their own eggs and removed after spawning.
Incubation at tropical temperatures typical of the São Francisco uplands — likely in the low-to-mid 20s Celsius — would be expected to yield fry within a few days. First foods for the newly free-swimming fry would be infusoria-grade items and rotifers before progression to newly hatched brine shrimp. In the absence of captive-breeding accounts, keepers should treat the species as they would any similarly sized characid from clear highland streams: clean water, fine-leaved spawning medium, and separation of adults from eggs.
In the aquarium
Acinocheirodon melanogramma is a rarity in the hobby. It is not established in the ornamental trade and is unlikely to be found outside of specialist collections or fish imported as bycatch in broader shipments from the São Francisco drainage. Hobbyists keeping this or similar cheirodontine species should follow the general guidelines for small, clear-water Brazilian tetras: soft to moderately hard, slightly acidic to neutral water; temperatures in the low-to-mid 20s Celsius; subdued lighting over a dark substrate to bring out the lateral stripe.
As a small schooling characid it should be kept in groups of at least six to ten individuals, in a well-filtered aquarium without large or boisterous tankmates. Gentle flow, some fine-leaved plant cover, and open mid-water swimming space suit the species' natural habitat well. The small adult size means tankmates should be selected carefully to avoid predation; nano-fish companions and small peaceful rasboras or other tetras of similar stature are appropriate.
Conservation
Acinocheirodon melanogramma is assessed as Least Concern (LC) on the IUCN Red List, with the assessment recorded in FishBase as evaluated in November 2018. The species is endemic to Brazil, restricted to the São Francisco and Jequitinhonha river basins in Minas Gerais, and while its range is limited compared to more widespread Neotropical characids, its populations were considered stable at the time of assessment.
Nonetheless, the São Francisco basin faces significant ongoing pressures from agricultural expansion, water extraction for irrigation, deforestation of riparian zones, and sand-mining in smaller tributaries — threats that affect the entire fish community of the upland streams. The Jequitinhonha valley is similarly affected by mining and land-use change. For a monotypic genus endemic to highland tributaries of two Brazilian drainages, any sustained habitat degradation carries heightened risk. Aquarists holding the species — however rare in captivity — are advised to source tank-bred stock wherever possible and to report observations to citizen-science databases.