Taxonomy & naming
Characidium nana was described in 2015 by Mendonça and Netto-Ferreira as part of a wave of new darter-characin descriptions from the Brazilian Shield, published in Seriously Fish's coverage of "new darter characins from Brazil." It belongs to the family Crenuchidae, the South American darters, within the order Characiformes — a family that also holds the far larger-bodied sailfin characins (Crenuchus, Poecilocharax), placing this tiny stream-bed species and its showy blackwater relatives under the same taxonomic roof despite very different lifestyles.
The genus Characidium itself is large and taxonomically active, with new micro-endemic species still being described from headwater and tributary streams across the Amazon basin as recently as the mid-2010s; Characidium nana is one of these recent additions. The specific epithet nana is Latin for "dwarf," a direct reference to the species' unusually small adult size relative to its congeners — several of which reach two to three times the length. As with many recently described, narrow-range Characidium, detailed taxonomic and ecological data beyond the original description remain sparse.
Morphology
Characidium nana is a slender, elongate, cylindrical little fish typical of the genus, with a subterminal mouth and cryptic barring along the flanks that breaks up its outline against sand and gravel substrate. The holotype measures 1 in standard length (approximately 1 in), and the species is noted in the literature as one of the smallest members of Characidium, a genus whose members are themselves mostly modest in size compared with their free-swimming characin relatives.
Detailed comparative morphometric data — fin ray counts, precise colour-pattern variation between populations, or confirmed maximum size beyond the type material — are not available from the sources consulted here; data sparse. Like other Characidium, it almost certainly shows the genus's characteristic resting posture, propped on its pelvic and anal fins against the substrate between short darting movements.
Habitat
The type locality is the Rio 13 de Maio, a stream roughly 25 mi from Castelo dos Sonhos in Pará state, Brazil, and the species' broader distribution spans the Rio Xingu and Rio Tapajós drainages of northern Brazil — both major southern tributary systems of the Amazon on the Brazilian Shield. These are the kind of clear-flowing, structurally complex headwater and tributary streams typical for small-bodied Characidium: sand-and-gravel beds, leaf litter, submerged wood and marginal vegetation providing both cover and the darting fish's foraging ground.
Genus-level care literature places Characidium species in water around 72–79 °F with a pH of roughly 6.0–7.5, consistent with the moderately soft, mildly acidic-to-neutral stream conditions typical of Xingu and Tapajós tributaries. Confirmed site-specific water chemistry for this exact species has not been recovered from the sources consulted; the figures above are genus-level guidance, not a Characidium nana-specific reading.
Feeding
Characidium are micro-predators, not grazers or flake-eaters by inclination: in the wild the genus takes aquatic and terrestrial insects and other small invertebrates picked directly off the substrate or snatched from the drift. Characidium nana, at barely 1 in, is sized to a correspondingly tiny prey base — micro-crustaceans, small insect larvae and similar invertebrate fare.
In the aquarium, genus-level guidance recommends frozen bloodworm, brine shrimp, Daphnia and Cyclops as staples, with flake or pellet food tolerated by some individuals but often refused or ignored in favour of live and frozen fare. Given this species' diminutive size, foods should be sized accordingly — microworm, newly hatched brine shrimp nauplii and small cyclops are a better match than standard bloodworm-sized fare for adult community fish.
Mating
No species-specific courtship or spawning account for Characidium nana was located in the sources consulted; data sparse. Within Crenuchidae more broadly, breeding behaviour is not uniform: the cave- and leaf-spawning sailfin characins (Crenuchus, Poecilocharax) show brief male parental care, while Characidium itself is understood to scatter eggs over the substrate rather than guard a nest site, more in keeping with typical small-characin spawning than with its cave-spawning cousins.
No external sexual dimorphism has been documented for this species in the sources reviewed here. Aquarists working with the genus generally report that condition and readiness are judged by behaviour and a gravid female's fuller profile rather than by any reliable colour or fin difference between the sexes.
Breeding
Characidium as a genus are understood to be substrate egg-scatterers rather than nest-guarders — unlike their Crenuchidae relatives Crenuchus and Poecilocharax, which are cave- or leaf-spawners with brief male care of the clutch. No documented breeding record specific to Characidium nana was found in the sources consulted here; data sparse, and this species should be treated as effectively unbred in the hobby until a first-hand account surfaces.
Genus-level hobbyist guidance for triggering Characidium spawns in a species tank describes soft, acidic water, fine-leaved plants such as Java moss to catch scattered eggs, gentle but steady water movement, and a sponge-protected filter inlet to protect eggs and fry from being drawn in. There is no reported parental care once eggs are scattered, so eggs and fry are typically left to fend for themselves or are separated out to protect them from adult fish, including the parents.
In the aquarium
At barely 1 in, Characidium nana is a genuine nano fish, and it should be sized and stocked accordingly — a small, quiet, well-planted aquarium with a soft sand or fine gravel base suits its bottom-perching, short-hop locomotion far better than an open, high-flow community tank. Because it is a micro-predator that often ignores dry foods, keepers should be prepared to offer live or frozen micro-foods regularly rather than relying on flake alone, and tankmates should be limited to other small, peaceful nano species that will not outcompete it at feeding time or see it as prey in the other direction.
Water should be kept in the moderately soft, mildly acidic-to-neutral range typical of the genus (roughly 72–79 °F, pH 6.0–7.5), with gentle filtration and a sponge-covered intake, both to protect a fish this small from being pulled into equipment and to mimic the clear tributary flow of its native Xingu and Tapajós streams. As a recently described, narrow-range species with essentially no aquarium track record documented in the sources reviewed here, it should currently be considered a specialist's fish rather than a readily available trade staple.
Conservation
Characidium nana has not been formally assessed by the IUCN Red List and currently carries no conservation status; it is treated here as Not Evaluated, consistent with its recent description (2015) and narrow, poorly surveyed native range in Pará state, Brazil. Many recently described, single-drainage Characidium species share this status simply because too little population or threat data exists for a formal assessment, not because they are known to be secure.
No species-specific threat information was located in the sources consulted; data sparse. More broadly, small-bodied headwater fish across the Xingu and Tapajós drainages face pressure from regional deforestation, mining-related sedimentation and, in the Xingu basin specifically, large hydroelectric development — general watch items for the region's stream fauna rather than documented, species-specific findings for Characidium nana.