Taxonomy & naming
Characidium iaquira was described in 2020 by Angela M. Zanata, Willian M. Ohara, Osvaldo T. Oyakawa and Fabio F. Dagosta, part of a wave of Characidium species split out of the genus's historically broad, poorly resolved species complexes as researchers worked through Amazonian headwater material collection by collection. It belongs to the family Crenuchidae — the South American darters, which also includes the sailfin characins Crenuchus and Poecilocharax — within the order Characiformes. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the standing authority for the valid name and the genus placement.
The species epithet iaquira comes from Tupi, meaning "green," an allusion to the fish's iridescent green body colouration. The type locality is the upper rio Juruena, part of the rio Tapajós basin in Mato Grosso, Brazil — a remote, fast-flowing headwater system far from the collecting routes that produced most of the genus's older, more widely known species. As with many recently split Characidium, older aquarium-trade or museum records from the same drainage may still be labelled under a broader, since-revised species name; C. iaquira should not be assumed to appear under any older combination without confirmation from a current specimen.
Morphology
The holotype (MZUSP 125780) measures 2.5 in standard length, with a paratype at 2 in SL — consistent with the genus's typical small, slender-bodied build. Characidium iaquira carries the diagnostic riffle-dweller's toolkit: enlarged, robust, wing-like pectoral fins that the fish uses to pin itself against the current on rocky or gravel substrate, a slim, faintly banded body, and — distinctively for this species — an iridescent green sheen overall, with a unique v-shaped dark mark along the caudal fin visible in medium and large individuals.
Like other darters, C. iaquira perches rather than swims continuously: it rests propped on its pectoral and pelvic fins on the stream bed, using short, quick darts to reposition, feed, or evade disturbance, then settles again. This perching posture and the robust paired-fin morphology are shared adaptations across rheophilic Characidium species living in fast-flowing riffle habitats, and they are the easiest field mark separating a darter from a generic small tetra-shaped characin.
Habitat
Characidium iaquira is known from the upper rio Juruena, a major headwater tributary within the rio Tapajós basin — itself one of the largest southern tributaries of the Amazon. It is a rheophilic species, meaning it is specifically adapted to fast-flowing, well-oxygenated riffle sections of the stream rather than the slower backwaters or floodplain habitats favoured by many other small Amazonian characins.
Specific water chemistry for the type locality has not been published in the accessible literature, but the Tapajós basin generally runs warm, with a mix of clearwater and blackwater-influenced tributaries typical of the Amazon lowlands (broadly around 75–79 °F). Given the very recent description and remote headwater location, detailed limnological data for this exact stream reach is not yet available; keepers and researchers should treat any specific pH or hardness figures for this species as provisional rather than confirmed.
Feeding
Characidium species are micro-predators rather than grazers or generalist omnivores. In the wild, congeners such as C. fasciatum feed primarily on chironomid midge larvae and other small aquatic insects picked from the riffle substrate — a diet consistent with C. iaquira's habitat in fast-flowing, insect-rich stream sections, though stomach-content data specific to this species has not been published.
In the aquarium, darters in this genus readily take frozen bloodworm, brine shrimp, Daphnia and Cyclops, and they frequently refuse dry flake food altogether, reflecting their preference for small live or meaty invertebrate prey over surface-feeding on flake. Keepers should plan to offer small live or frozen foods rather than relying on flake as a staple.
Mating
No species-specific courtship account for Characidium iaquira has been published; this is a very recently described species from a remote headwater system, and behavioural data of this kind typically lags years behind a formal description. In better-studied congeners such as C. fasciatum, males are reported to chase females during spawning, with pairs or small groups becoming active over the substrate rather than performing an elaborate display.
External sexual dimorphism has not been documented for C. iaquira specifically. Data is sparse enough that keepers should not assume a particular sexing method without direct observation of a captive group.
Breeding
Characidium darters are substrate spawners that scatter eggs over the stream bed rather than building a nest or brooding young, and documented parental care in the genus is minimal to absent — this contrasts with their Crenuchidae relatives Crenuchus and Poecilocharax, the sailfin characins, which are cave- or leaf-spawners with brief male care of the clutch. For C. fasciatum, the best-documented congener, breeding has occurred in aquaria with essentially no parental investment after spawning.
No aquarium or captive-breeding account exists for Characidium iaquira itself; as a species described in 2020 from a remote Brazilian headwater, it is not established in the hobby and there is no record of it having been bred in captivity. Any breeding approach would need to be extrapolated cautiously from better-known Characidium species rather than treated as confirmed for this fish.
In the aquarium
Characidium iaquira is not currently established in the aquarium trade — it was described only in 2020 from a remote part of the rio Tapajós basin, and there is no record of live specimens being available to hobbyists. The notes here are extrapolated from the care needs documented for other, better-known Characidium darters and from this species' known rheophilic habitat.
As a small riffle-dwelling darter, C. iaquira would be expected to need strong water flow and high dissolved oxygen — a powerhead or spray-bar current and a fine gravel or rounded-stone substrate mimicking a stream bed, rather than the still, planted setups suited to many other small characins. Congeneric Characidium are typically kept in groups of five or more for security, since a lone darter tends to stay hidden. Given its micro-predator diet, any prospective keeper should plan on small live or frozen foods rather than flake, and should expect this to remain a specialist's fish rather than a mainstream trade species for the foreseeable future.
Conservation
Characidium iaquira has not been formally assessed by the IUCN Red List; as a species described only in 2020, it currently carries no status and should be treated as Not Evaluated / Data Deficient pending a formal review. Its type locality, the upper rio Juruena in the rio Tapajós basin, lies in a remote part of central Brazil, and specific habitat pressures on the population have not been documented in the available literature.
More broadly, the rio Tapajós basin faces watch-list pressures typical of southern Amazonian headwaters — hydroelectric development, agricultural expansion and associated sedimentation — but there is no published data connecting these pressures specifically to C. iaquira's population status. Given how recently the species was described and how little targeted survey work exists for it, its conservation picture should be considered essentially unknown rather than reassuring.