Taxonomy & naming
Characidium summus was described by Angela M. Zanata and Willian M. Ohara in 2015, part of a wave of new darter-characin descriptions from headwater drainages of the upper rio Madeira basin in Rondônia, Brazil. It belongs to the family Crenuchidae — the South American darters, which also includes the sailfin characins Crenuchus and Poecilocharax — within the order Characiformes, and to the large genus Characidium, which numbers dozens of small, cryptically patterned, bottom-perching species across South and Central America.
What sets C. summus apart from its congeners, and what the original description singles out, is a combination of characters unusual within Characidium: it lacks an adipose fin (present in the overwhelming majority of the genus) and it carries a complete lateral line, and the authors also noted an unusual mid-water behaviour distinguishing it from the typical strictly benthic, perch-and-dart habit of the genus. The specific epithet "summus" is Latin for "highest" or "uppermost", a reference to the species' occurrence at unusually high altitude for the genus in the upper reaches of the rio Pacaás Novos.
Morphology
Characidium summus is a small, slender, torpedo-shaped darter reaching a documented maximum of about 1.5 in standard length, consistent with the diminutive build typical of the genus. Like other Characidium it has a subterminal, slightly underslung mouth suited to picking food items off the substrate, and cryptic, mottled patterning along the flanks that breaks up its outline against a stony or leaf-littered stream bed.
The two features that distinguish it from most of its relatives are structural rather than colour-based: the absence of an adipose fin, present in the great majority of Characidium species, and a complete lateral line running unbroken along the body — many congeners have an interrupted or partial lateral line. Data on external sexual differences are not documented for this species.
Habitat
The species is known only from the upper rio Pacaás Novos, part of the rio Madeira basin, in Rondônia State, Brazil — its type locality and, based on available records, its entire known range. It occurs at unusually high altitude for the genus, above roughly 1,100 metres, in blackwater habitat over a rocky bottom, including small isolated puddles on the order of 1–4 metres wide and 1.5 metres deep as well as river backwaters.
Water chemistry at the type locality is acidic blackwater; FishBase records a documented pH as low as roughly 3.6, with the practical range likely running from about 3.6 up to 5.0, and a maximum recorded temperature around 72 °F — cooler than the typical lowland Amazonian norm, consistent with the species' headwater, higher-altitude setting. Given the narrow, single-drainage range, this is a habitat specialist rather than a widespread generalist.
Feeding
No diet study specific to Characidium summus has been published, but a trophic level of roughly 3.3 (±0.3) is recorded on FishBase, a value consistent with a micro-invertebrate feeder rather than a herbivore or a fish-eating predator. That fits the broader pattern for Characidium and Crenuchidae generally: small-bodied darters that pick aquatic insect larvae, micro-crustaceans and other tiny invertebrates off the substrate and out of the water column, and that generally ignore dried or flake foods in captivity.
In the aquarium, congeneric Characidium species are reported to do best on small live and frozen foods — bloodworm, brine shrimp, Daphnia and Cyclops — rather than flake, and there is no reason from the available data to expect C. summus to differ; specific husbandry accounts for this species are not documented, so this inference is extended cautiously from its genus.
Mating
No courtship behaviour has been documented for Characidium summus specifically. Within Crenuchidae more broadly, the pattern varies sharply by genus: the sailfin characins Crenuchus and Poecilocharax are cave or leaf spawners with brief male care, while Characidium as a genus scatters eggs over the substrate without the elaborate courtship of its sailfin relatives. Nothing in the published record indicates which pattern, if either, applies precisely to C. summus, and no external sexual dimorphism has been described for it.
Breeding
Breeding data for Characidium summus itself is absent from the literature — the original description and FishBase both note no reproductive information for the species. Within the genus, Characidium generally are egg-scatterers over the substrate rather than nest builders or mouthbrooders, without the sustained parental care seen in the genus's sailfin cousins Crenuchus and Poecilocharax; hobby accounts of commoner Characidium species describe spawning over fine-leaved plants such as Java moss in softly acidic water, but this has not been confirmed for C. summus and should be treated as an inference from the genus rather than a documented fact for this species.
In the aquarium
Characidium summus has essentially no documented presence in the aquarium trade; general hobby literature on "darter characins" covers commoner Characidium species and explicitly notes that C. summus itself is rarely, if ever, kept or discussed. What can be said comes from the genus at large: Characidium are bottom-dwelling nano fish, surprisingly capable jumpers that require a tightly fitted lid, and micro-predators that often refuse dried foods in favour of small live or frozen fare.
Given its documented water chemistry — strongly acidic, blackwater, and cooler than typical Amazonian lowland norms — a prospective keeper working from the species' known habitat rather than assumption would look to replicate soft, very acidic water at moderate rather than tropical-maximum temperatures, with a rocky or leaf-littered substrate and calm, sheltered spots to perch. This remains an extrapolation from habitat data, not an established husbandry protocol, since none has been published for this species.
Conservation
Characidium summus has not been formally assessed by the IUCN Red List and carries no evaluated conservation status; it is treated here as Not Evaluated. It is known from a single drainage — the upper rio Pacaás Novos in the rio Madeira basin, Rondônia, Brazil — at high altitude for the genus, which by itself is a narrow-range profile of the kind that often warrants conservation attention once formally assessed.
No population trend, threat assessment or trade data are documented for this species. Its restricted, headwater blackwater habitat and lack of any evident aquarium trade presence suggest that any conservation concern, if it exists, would stem from localized habitat pressure within its single known drainage rather than from collection for the hobby.