Taxonomy & naming
Characidium clistenesi was described in 2016 by the Brazilian ichthyologists Fernando A. G. Melo and Vinícius C. Espíndola in Zootaxa, based on a holotype (MZUSP 120530, an adult male) and a series of paratypes collected from the rio Capivara, a tributary of the rio Paraguaçu near Lençóis in the Chapada Diamantina region of Bahia. It belongs to the family Crenuchidae — the South American darters, which also includes the popular aquarium sailfin characins Crenuchus and Poecilocharax — within the order Characiformes. The genus Characidium itself is large and taxonomically dense, with dozens of small, superficially similar rheophilic species scattered across South American drainages, many of them, like this one, described only in the last two decades from restricted headwater systems.
The species epithet honours a collector or colleague named Clistenes, in the customary practice of Latinizing a personal name for a taxonomic dedication; the original description should be consulted for the precise etymological note, which was not captured in the sources available for this article. There is no evidence of a valid-name change since the 2016 description, and the species is not associated with any established common or trade name — it remains known essentially only by its scientific binomial.
Morphology
Characidium clistenesi is a small, elongated fish with a moderately convex dorsal profile, typical of the genus's streamlined, current-adapted body plan. The original description reports a maximum standard length of 1.5 in across a type series of 30 specimens, with the holotype male measuring 1 in SL and paratypes ranging 1 in SL — in other words, an adult fish that tops out well under 2 in. The lateral line is complete, running the full length of the body with 32–36 perforated scales.
Colouration is understated rather than showy: a dark brown-to-beige dorsum shading to a pale beige or whitish belly, with two diagnostic dark blotches that help distinguish it from close congeners. A vertically elongated humeral blotch sits just behind the head over the posttemporal and supracleithrum bones, and a rounded peduncular blotch marks the base of the middle caudal-fin rays; in this species the two blotches are connected by a dusky band running along the flank, a pattern noted as conspicuous in the original description. As in most Characidium, there is no evidence of dramatic sexual dichromatism.
Habitat
This species is endemic to the upper rio Paraguaçu drainage in the Chapada Diamantina, a highland plateau region of Bahia in northeastern Brazil where cooler, rocky headwater streams cut through a transitional zone between Atlantic Forest remnants and the semi-arid Caatinga biome. The type locality, the rio Capivara near Lençóis, is representative of the streams the species inhabits: clear, fast-flowing, well-oxygenated water running over rock and coarse substrate — precisely the rheophilic habitat that the genus Characidium is adapted for across its range.
Species-specific water chemistry data were not captured in the available sources, but related Characidium species in similar Brazilian highland streams are documented in roughly 64–81 °F and pH 5.5–7.5, with moderate hardness. Given the elevation and semi-arid transition setting of the Chapada Diamantina, this species likely runs toward the cooler end of that range rather than warm Amazonian lowland norms; that inference should be treated as provisional pending species-specific measurements.
Feeding
Characidium clistenesi has not been the subject of a dedicated diet study, but the genus as a whole is well characterised as micro-predatory: darter characins pick tiny aquatic invertebrates, insect larvae and small crustaceans from the streambed and water column rather than grazing algae or detritus, and they frequently refuse dry flake food entirely in captivity. There is no reason from the available evidence to expect this species to differ from that pattern.
In the aquarium, keepers of Characidium generally report success only once live or frozen foods are offered — microworm, baby brine shrimp, chopped bloodworm and small cyclops are the staples cited for the genus — with prepared foods accepted, if at all, only after a fish has settled and learned to recognise them as food.
Mating
No courtship behaviour has been documented for Characidium clistenesi specifically. Across the genus, spawning is not preceded by the elaborate pairing or cave-selection rituals seen in its cousins Crenuchus and Poecilocharax; Characidium species are egg-scatterers, and available accounts describe no distinct courtship display beyond the general activity and chasing associated with readiness to spawn in flowing water.
Breeding
Characidium as a genus are substrate egg-scatterers: eggs are released over gravel or sand in flowing water with no nest construction and no documented parental care, in clear contrast to their crenuchid relatives Crenuchus spilurus and Poecilocharax, which are cave or leaf spawners with brief male guarding of the clutch. Nothing in the available sources indicates that Characidium clistenesi departs from this genus-wide pattern.
There are no records of this species being bred in captivity. Given its very restricted native range, tiny adult size, and near-total absence from the aquarium trade, it should be considered essentially unbred and unavailable to hobbyists at this time; anyone encountering it would be working from first principles rather than an established protocol.
In the aquarium
Characidium clistenesi is not established in the aquarium hobby, and the available sources include no aquarium-specific care record for this particular species. What can be said comes from the genus: darter characins are small, nano-scale fish suited to a modestly sized, well-filtered tank with strong water movement and high oxygenation to mimic their rheophilic stream habitat, plus a sand or fine gravel substrate they can perch on and dart across rather than swim continuously through open water. They are shy, bottom-oriented fish that do best alone, in a species group, or in a very quiet community of similarly unassuming tankmates — boisterous or fin-nipping companions are a poor match.
Because it is a micro-predator that may ignore flake, any keeper acquiring this species should be prepared to offer small live or frozen foods from day one. Given its endemic, narrow-range status and complete absence of a trade history, it is far more likely to be encountered in an ichthyological collection than a fish store, and any specimen in the hobby should prompt real scrutiny of its origin and legality of export.
Conservation
Characidium clistenesi has not been evaluated by the IUCN Red List; its conservation status is Not Evaluated. This is consistent with a great many recently described, single-drainage Characidium species, which frequently go years or decades without a formal assessment even when their entire known range is a single small watershed.
The species' endemism to the upper rio Paraguaçu in the Chapada Diamantina is itself a conservation-relevant fact: restricted-range headwater endemics in this region are inherently vulnerable to habitat degradation from agriculture, water extraction and land-use change in the surrounding Caatinga-transition landscape, even absent a formal threat assessment. No population trend or specific threat data were available in the sources consulted for this article.