Taxonomy & naming
Characidium cacah was described in 2020 by Zanata, Ribeiro, Araújo-Porto, Pessali & Oliveira-Silva, one of a wave of new Characidium species recognised from Brazilian river systems in recent years as the genus has been worked through in detail. It belongs to the family Crenuchidae, the South American darters, which also contains the sailfin characins Crenuchus and Poecilocharax prized in the blackwater nano-fishkeeping hobby; Characidium itself comprises the bottom-dwelling "true" darters rather than the mid-water sailfins.
The type locality is the rio das Velhas sub-basin, part of the upper rio São Francisco basin in Minas Gerais, Brazil, making C. cacah a range-restricted endemic of that drainage. The species is diagnosed from its congeners by a combination of meristic and colour-pattern characters — an incomplete lateral line, the absence of an adipose fin, 12 circumpeduncular scales, a completely scaled isthmus, a thin and inconspicuous midlateral dark stripe, and the absence of a conspicuous peduncular or humeral blotch, with a basicaudal spot that is only variably marked. As with much of the genus, identification in the field rests on these fine-grained characters rather than on bold, obvious markings.
Morphology
Characidium cacah has the typical darter body plan: an elongate, subcylindrical shape built for perching and short bursts of movement along the substrate rather than sustained swimming. The original description notes an incomplete lateral line and the lack of an adipose fin, both useful diagnostic features, along with 12 circumpeduncular scales and a fully scaled isthmus. Colour pattern is understated — a thin, inconspicuous dark midlateral stripe runs along the flank, without the strong peduncular or humeral blotches seen in some related species, and the spot at the base of the caudal fin is only weakly and variably expressed.
Specific measurements such as holotype standard length were not captured from the source material reviewed for this profile. Species in the genus Characidium are generally small, with related species such as C. fasciatum reaching only around 2.5 in; C. cacah is presumed to fall in a similar small size range typical of the genus, though a confirmed maximum length for this species specifically is not available.
Habitat
Characidium cacah is known only from the rio das Velhas sub-basin, a tributary system of the upper rio São Francisco basin in Minas Gerais, southeastern Brazil. Like other members of the genus, it is a stream-dwelling fish, associated with flowing freshwater habitat rather than still lakes or floodplain lagoons — darters characteristically hold station on or near the substrate in areas with some current, using rocks, gravel, sand or leaf litter as perching and cover structure.
Detailed water-chemistry data specific to C. cacah's home streams was not available in the sources reviewed. Genus-wide, Characidium species are generally kept and found in soft, warm, moving water, and hobby accounts of related darters note a clear preference for oxygenated, flowing conditions over stagnant water.
Feeding
Characidium species are micro-predators rather than generalist grazers: in the wild they take small aquatic invertebrates, insect larvae and other tiny prey items picked from the substrate and surrounding current, and in captivity related species are known to often ignore dried flake food entirely, doing best on small live or frozen foods. No diet-specific data has been published for C. cacah itself, but this pattern is consistent across the genus and is the safest working assumption for this species.
Aquarists keeping related darters report the best results offering microworm, baby brine shrimp, small cyclops and similarly sized live or frozen fare, with prepared foods accepted inconsistently if at all — a point worth flagging for any future keeper, given that C. cacah has essentially no aquarium-trade history to draw on.
Mating
No courtship or spawning behaviour has been documented for Characidium cacah specifically; the species was described in 2020 from preserved museum material and does not appear to have entered the aquarium hobby, so there is no hobbyist breeding record to draw on. For the wider family Crenuchidae, the sailfin genera Crenuchus and Poecilocharax are known cave- and leaf-spawners with a period of male courtship and site selection, but Characidium itself is understood to be a more conventional substrate egg-scatterer, without the cave-spawning courtship display of its sailfin relatives.
External sexual dimorphism has not been described for this species. In C. fasciatum, a related and better-known darter, females have been noted to appear fuller-bodied when in breeding condition — a pattern that may extend to C. cacah, but this has not been confirmed.
Breeding
There is no published or hobby breeding record for Characidium cacah. Based on what is known of the genus, Characidium species are generally understood to be substrate egg-scatterers, releasing eggs over the streambed or among fine debris without building a nest and without the sustained parental care seen in their sailfin relatives Crenuchus and Poecilocharax, which guard a small clutch in a cave or on a leaf. This should be treated as a genus-level inference rather than a confirmed account for C. cacah specifically — the species has not been bred in captivity as far as documented sources show.
Given the total absence of aquarium history, anyone attempting to breed this species would be working from first principles drawn from related Characidium rather than from any established protocol.
In the aquarium
Characidium cacah has no documented presence in the aquarium trade; as a species described only in 2020 from a restricted Brazilian drainage, it has not been reported as available to hobbyists. Any care guidance therefore has to be extrapolated from the genus as a whole rather than from direct experience with this species.
Genus-wide, Characidium darters do best in soft, warm, well-oxygenated water with some current, a fine substrate and cover such as smooth rocks, driftwood and leaf litter to mimic a stream bed, and they are micro-predators that may need to be weaned onto small live or frozen foods rather than flake. They are generally peaceful, unobtrusive bottom-dwellers suited to a quiet community rather than boisterous tankmates. Prospective keepers should treat any C. cacah offered in the trade with caution, given how recently the species was described and how little is documented about its specific husbandry needs.
Conservation
Characidium cacah has not been formally assessed by the IUCN Red List as of this writing and carries a status of Not Evaluated. This is unsurprising for a species described as recently as 2020 and known only from a restricted range within the rio das Velhas sub-basin of the upper rio São Francisco basin in Minas Gerais, Brazil — range-restricted, recently described stream fish from this region are frequently un-assessed simply because formal review has not yet caught up with taxonomic description.
As an apparent single-drainage endemic, C. cacah would be expected to face the general pressures common to small Brazilian stream fish — habitat degradation, water-quality change and land-use pressure within its home sub-basin — but no specific conservation assessment or population data is available to confirm the scale of any threat.