Taxonomy & naming
Characidium mirim was described in 2013 by Netto-Ferreira, Birindelli and Buckup, part of a wave of taxonomic work that has substantially expanded the recognised diversity within Characidium over the past two decades. It belongs to the family Crenuchidae — the South American darters and sailfin characins — within the order Characiformes, a family whose two subfamilies split cleanly along ecological lines: the shy, cave- and leaf-spawning sailfin characins (Crenuchus, Poecilocharax) prized as gentle blackwater nano fish, and the far more speciose Characidiinae, the stream-bed darters, of which Characidium mirim is one of roughly sixty-plus described species.
The species epithet "mirim" comes from Tupi, the indigenous Brazilian language family that has supplied many South American place and species names, and simply means "small" — an apt and economical name for what FishBase records as the smallest known species in the genus. The type locality is the Rio das Mortes drainage in Mato Grosso, Brazil, part of the upper Araguaia-Tocantins system. As with many recently described, narrow-range Characidium, the species is best treated cautiously in the trade: darters sold simply as "Characidium sp." or "darter tetra" may or may not be this species, and firm identification rests on the original description rather than on aquarium-shop labelling.
Morphology
Characidium mirim is minute even by the standards of a genus full of small fish: FishBase gives a maximum size of 1 in standard length, making it the smallest species so far described in Characidium. Like other members of the genus it has the classic darter build — an elongated, subcylindrical body, a flattened underside adapted for perching on the substrate, and pectoral and pelvic fins that are used almost like props to hold station on rock or wood rather than purely for swimming.
Data on colouration and fine morphological detail for this specific species is sparse in the sources available here beyond the original taxonomic description; broadly, Characidium species show the genus-typical pattern of a dark lateral stripe or series of blotches over a pale to olive body, camouflage suited to a life spent resting on stream-bed substrate rather than schooling in open water. No reliable size or pattern-based sexual dimorphism is documented for this species in the available sources.
Habitat
Characidium mirim is known from the Rio das Mortes drainage in Mato Grosso, Brazil, at roughly 15–16°S — part of the wider pattern for Characidiinae darters, which as a group are strongly associated with fast-flowing South American streams, rapids and waterfalls rather than slow, still water. Crenuchid darters generally favour clear to lightly tannin-stained water over the sand, gravel, rock and leaf-litter substrates typical of these habitats, with the current keeping the water well oxygenated.
Detailed site-level water chemistry specific to this species is not available in the sources here, but darters in the genus more broadly are found in soft, mildly acidic to near-neutral water, generally in the pH 6.0–7.5 range, consistent with the clearwater and blackwater tributary systems typical of the Araguaia-Tocantins and neighbouring basins.
Feeding
FishBase records a trophic level of roughly 3.1 for Characidium mirim, placing it as a micro-predator that feeds on small invertebrates rather than as a grazer or omnivore. This matches the broader pattern documented for the genus: Characidium darters take insects and their larvae — chironomid (bloodworm) larvae in particular — plus other small aquatic invertebrates picked from the substrate and drifting in the current.
In the aquarium, darters in this genus are reported to often ignore dry flake food and do best on small live or frozen fare — frozen bloodworm is specifically recommended as an ideal staple, alongside other small live foods such as microworm and baby brine shrimp for a fish this size. Given its tiny 1 in adult length, any prepared food offered would need to be correspondingly small.
Mating
No species-specific courtship description for Characidium mirim is available in the sources consulted here. For the broader genus, Characidium species are reported to spawn naturally in aquaria given the right conditions — well-oxygenated, flowing water and fine-leaved plants or spawning mops as an egg-scattering substrate — but detailed courtship behaviour is not well documented in the hobby literature. No external sexual dimorphism is confirmed for this species from the available sources; data is sparse.
Breeding
Characidium, as a genus within the Characidiinae, are egg-scatterers rather than nest-builders or mouthbrooders: eggs are released over the substrate or among fine-leaved plants and spawning mops, with no parental care documented. This is distinct from their Crenuchinae cousins (Crenuchus, Poecilocharax), the cave- and leaf-spawning sailfin characins that show brief male guarding of the clutch — that mode does not apply to Characidium.
No breeding account specific to Characidium mirim itself is available in the sources here; given the species' very recent description (2013), tiny adult size and narrow, poorly documented native range, it should be assumed effectively unbred in the hobby, and any spawning attempt would need to follow the general Characidium pattern of flowing, well-oxygenated water and fine egg-scattering substrate rather than a confirmed species-specific protocol.
In the aquarium
Characidium mirim is not established in the aquarium trade and data on husbandry specific to this species is sparse; what follows draws on general care guidance for Characidium darters as a genus. At a maximum size of 1 in SL this is a genuinely nano fish, suited only to a small, quiet tank with sand or fine gravel and some current to mimic its native rapids and streams — not the community-tank staple that the unrelated, larger tetras of this family's popular imagination might suggest.
Genus-level guidance recommends keeping darters in groups of five or more, in tropical water around 72–79 °F and mildly acidic to neutral pH (roughly 6.0–7.5), with good oxygenation and flow. Characidium are bottom-perching micro-predators that often refuse dry flake, so keepers should plan on small live or frozen foods such as bloodworm from the outset. As with any newly described, narrow-range species, a keeper encountering fish sold as "Characidium mirim" should treat the identification with some caution given how easily small Crenuchidae are mislabelled in the trade.
Conservation
The IUCN Red List assesses Characidium mirim as Least Concern, an assessment dated 2018, with no documented population threats noted in the assessment available here. This should be read against the species' narrow known range in the Rio das Mortes drainage of Mato Grosso — a Least Concern rating for a recently described, geographically restricted species often reflects limited survey effort and an absence of known threats rather than confirmed abundance across a wide range.
No trade-pressure or habitat-specific conservation concerns are documented for this species in the sources consulted here; as with many small-range, recently described Characidium, ongoing habitat pressures common to Brazilian Cerrado and Amazonian-margin drainages — deforestation-linked sedimentation and water-quality change — would be the general long-term watch items for the wider region, though none are specifically documented for this species.