Taxonomy & naming
Hemigrammus apiaka was originally described as Aphyodite apiaka by Alexandre Hilsdorf Esguícero and Oswaldo Takeshi Oyakawa de Castro in 2017, published in Copeia (volume 105, issue 4, page 758). The type locality is recorded in Eschmeyer's Catalog of Fishes as the ressaca (floodplain wetland) of Lago Jenipapo, Rio Aripuanã, Rio Madeira basin, Nova Aripuanã Municipality, Amazonas State, Brazil (06°09'08"S, 60°13'29"W). The holotype is deposited at INPA (catalogue number INPA 52541), with additional paratypes at INPA and LIRP.
The species was transferred to Hemigrammus following a phylogenomic revision by Melo and colleagues in 2024, which reorganised numerous characid genera and consolidated Aphyodite within the subfamily Pristellinae of the family Characidae. Eschmeyer's Catalog of Fishes (CAS) treats Hemigrammus apiaka as the current valid name. The species epithet apiaka refers to the Apiaká, an indigenous people of the Rio Juruena and upper Tapajós region of western Brazil — a patronym honouring a native Amazonian nation.
Phylogenomic studies confirm that Hemigrammus, in its post-revision circumscription, belongs to the hyperdiverse clade Characidae, which encompasses several thousand Neotropical freshwater species. H. apiaka is among the most recently described members of the genus and remains poorly represented in museum collections relative to better-known congeners.
Morphology
Hemigrammus apiaka is a small, slender characid reaching approximately 1 in standard length — firmly in the micro-tetra size class. Like other members of Hemigrammus it carries the family hallmarks of a laterally compressed, fusiform body, a single dorsal fin, a small adipose fin, and a forked caudal fin. Detailed meristic and morphometric data are documented in the original 2017 description.
Beyond the size record of 1 in SL from FishBase, published morphological detail in accessible secondary sources is limited, as the species is rarely encountered in the aquarium hobby and has attracted relatively little subsequent collector attention. The original description in Copeia is the primary source for its diagnostic characters; researchers and serious hobbyists seeking exact meristic counts and morphometric ratios are directed to that paper.
Habitat
The type locality of Hemigrammus apiaka — the ressaca wetlands of Lago Jenipapo on the Rio Aripuanã — places it in the floodplain fringe of the lower Rio Madeira basin, one of Amazonia's most species-rich river systems. Ressacas are seasonally inundated lowland wetlands: shallow, densely vegetated, and subject to dramatic water-level fluctuation between wet and dry seasons. This habitat type is characteristic of many small characids, which exploit the food and shelter of submerged terrestrial vegetation during high water and retreat to remnant pools and river channels as water recedes.
FishBase classifies the species as freshwater, benthopelagic, and tropical. The broader Madeira Brazilian Shield ecoregion (FEOW ecoregion 321) is characterised by dense lowland rainforest, clear-flowing rivers draining ancient Precambrian basement, and a mosaic of fast-running channels, black- and clearwater lakes, and seasonal floodplains. Water in such Amazonian floodplain wetlands is typically soft and acidic, with temperatures in the range of approximately 75–79 °F reflecting the equatorial lowland climate, though the type locality has not been comprehensively characterised in the published literature.
Feeding
No dietary study specific to Hemigrammus apiaka has been published in the accessible literature. By analogy with congeners of similar size and habitat — small Amazonian Hemigrammus inhabiting floodplain wetlands — the species is presumed to be a micro-omnivore, consuming small aquatic invertebrates such as microcrustaceans, insect larvae, and zooplankton, as well as algal films and detrital matter encountered in the water column and at the substrate.
In the aquarium, micro-tetras of this body plan typically accept high-quality micro-pellets, finely crushed flake, and small live or frozen foods including baby brine shrimp, Cyclops, and Daphnia nauplii. The species' rarity in the hobby means little husbandry-specific feeding information exists; cautious extrapolation from similar small Hemigrammus is the best available guide.
Mating
No specific mating observations for Hemigrammus apiaka have been published. Hemigrammus is a genus of egg-scattering, open-water or plant-associated spawners with no parental care — the ancestral characid reproductive strategy. Courtship in small Hemigrammus typically involves males pursuing females through fine-leaved vegetation or open water, with spawning triggered by environmental cues including temperature, photoperiod, and water quality.
Because the species is essentially unknown in the aquarium trade, captive mating observations do not exist in the hobbyist literature. Any account of its specific courtship behaviour would require either direct field observation at the type locality or successful captive spawning by a collector who obtained wild specimens — neither of which appears to have been documented as of the date of this article.
Breeding
Hemigrammus apiaka has not been bred in captivity in any documented account. Based on the breeding biology of the genus, spawning would be expected to involve adhesive or semi-adhesive eggs scattered among fine-leaved aquatic plants or deposited over the substrate, with no subsequent parental care. Incubation and fry development times are unknown for this species specifically.
For aquarists who might obtain specimens — as scientific collectors or through the specialist wild-fish trade — the approach used for other small Hemigrammus from soft, acidic Amazonian waters would serve as a reasonable starting framework: slightly acidic, warm, soft water; a heavily planted or mop-equipped breeding tank; separation of adults after spawning; and small live foods such as infusoria or rotifers for newly hatched fry. All such details remain speculative until captive breeding is documented.
In the aquarium
Hemigrammus apiaka is not available in the mainstream aquarium trade and is not known to be kept by hobbyists at any scale. It is described as a species only rarely kept in aquaria, its 2017 description being recent and its distribution restricted to a remote section of the Rio Aripuanã accessible primarily to ichthyological collectors.
Should specimens become available, the species' size (approximately 1 in SL), floodplain wetland origin, and phylogenetic placement suggest it would require care broadly consistent with other small, soft-water Amazonian tetras: warm water (approximately 75–79 °F), soft and slightly acidic to neutral conditions, a well-planted aquarium, and a peaceful community of similarly sized fish. A schooling disposition should be assumed and a group of at least six individuals would be appropriate. The species' obscurity means no hobbyist care guides exist; prospective keepers would be working with limited information.
Conservation
Hemigrammus apiaka has not been assessed by the IUCN Red List and carries a status of Not Evaluated (NE). The species is known from the type locality at the ressaca wetlands of Lago Jenipapo, Rio Aripuanã, Amazonas State, Brazil — a geographically restricted record from a single collecting event. Whether the species has a broader range within the Rio Aripuanã or the lower Madeira basin is unknown; the published literature does not document additional records.
The Madeira basin as a whole is subject to significant anthropogenic pressure, including deforestation in its headwaters, the impacts of the Jirau and Santo Antônio hydroelectric dams on the lower Madeira, and agricultural and ranching expansion across Rondônia and Amazonas states. The effect of these pressures on the specific floodplain wetland habitat of H. apiaka is not documented, but the sensitivity of ressaca ecosystems to changes in seasonal flood dynamics means that dam-driven hydrological alteration is a plausible concern. A formal assessment of the species' distribution, abundance, and threat status would be a useful contribution to Amazonian ichthyological conservation.