Taxonomy & naming
Characidium tapuia was described in 2018 by Zanata, Ramos and Oliveira-Silva as part of a revision of Characidium species in the rio Parnaíba basin of northeastern Brazil. It belongs to the family Crenuchidae — the South American darters, which also includes the sailfin characins Crenuchus and Poecilocharax — within the order Characiformes. The genus Characidium itself is large and taxonomically active, with new species still being described from previously under-sampled river basins across South America, of which C. tapuia is one recent example.
The species is distinguished from its congeners by a broad dark lateral stripe running from the snout to the caudal peduncle, and by the absence of dark vertical bars on the body in specimens larger than about 1 in standard length — a pattern that separates adult C. tapuia from related Characidium species that retain barring at that size. The epithet "tapuia" references a historical term used in Brazil for indigenous or non-Tupi-speaking peoples of the interior, tying the name to the species' inland, upland home in the Parnaíba drainage.
Morphology
Characidium tapuia has the typical build of its genus: a slender, elongated, faintly cylindrical body built for holding position against current and darting rather than for sustained open-water swimming. The genus as a whole tops out under 4 in (roughly 1.5 in standard length for most species), and there is no indication C. tapuia departs from that pattern.
The most distinctive external feature is the broad, dark lateral stripe that runs continuously from the snout through the eye to the base of the caudal fin. Younger fish (under about 1 in SL) may show dark vertical bars crossing this stripe, but these bars are absent in larger, more mature individuals, leaving a clean, uninterrupted stripe as the adult pattern — a useful field mark for separating this species from close relatives in the same basin.
Habitat
Characidium tapuia is endemic to the rio Parnaíba basin of northeastern Brazil, where it has been recorded from the upper and middle portions of the drainage. The Parnaíba is one of the largest river systems draining directly to the Atlantic from Brazil's semi-arid northeast, and its headwater and mid-basin streams typically run clear to lightly turbid over sand, gravel and rock, with the moderate current that sit-and-wait darters like Characidium favour.
As with most Characidium, C. tapuia is a bottom-oriented stream fish rather than a still-water or floodplain species; it perches on the substrate in flowing sections and makes short darting movements to feed or evade disturbance rather than cruising in midwater. Detailed water-chemistry data specific to this species have not been captured here; treat any numbers as inferred from the genus rather than measured for this population.
Feeding
Characidium tapuia is a micro-predator, in keeping with the genus. Characidium species forage as sit-and-wait predators, perching on rocks or sediment and darting out to snatch small aquatic and terrestrial invertebrates — insect larvae (including chironomid/midge larvae), small crustaceans and other tiny prey items carried past by the current.
In the aquarium, Characidium species typically ignore flake and dry foods and need to be offered live or frozen foods sized for a small mouth: frozen bloodworm, brine shrimp, Daphnia and cyclops are all readily taken by the genus. Keepers should expect to feed accordingly rather than assume a darter will compete for standard community fare.
Mating
Specific courtship behaviour has not been documented for Characidium tapuia, but the genus breeds as substrate-spawning pairs: a male and female position side by side low in the water column over open sediment or gravel rather than building any kind of nest or bubble structure. General Characidium accounts describe modest sexual dimorphism — females tend to look plumper-bodied with a clear (unmarked) dorsal fin, while males often show small spots on the dorsal fin — though this has not been confirmed specifically for C. tapuia.
Breeding
Characidium as a genus are egg-scatterers, not mouthbrooders, bubble-nest builders or cave spawners. A paired male and female spawn side by side over the bottom; eggs are non-adhesive to only lightly adhesive and settle among rocks and coarse sediment rather than being placed in a prepared nest. Reported clutch sizes for the genus run to roughly 150 eggs, which hatch quickly — around 30 to 40 hours at warm tropical temperatures.
There is no parental care after spawning: once the eggs are released and fertilised, the adults do not guard, fan or tend them. This mode has not been confirmed specifically for C. tapuia, but it matches the pattern reported across Characidium generally, and there is no reason from the available material to expect this species to differ.
In the aquarium
Characidium tapuia has not been established in the hobby to any documented extent, and specific aquarium care data for this species is sparse — most of what follows is inferred from Characidium care in general and should be treated as a starting point rather than a confirmed protocol. As with other darters, a tank should replicate a flowing stream-bed environment: sand or fine gravel substrate, smooth rock and driftwood to perch on, moderate current and good oxygenation, since these are active, current-adapted fish rather than still-water dwellers.
Characidium do best kept in a group — commonly cited as five or more — which reduces stress and lets the naturally shy, territorial-toward-each-other darting behaviour play out without any one fish being singled out. Given the micro-predator diet, tankmates should be other small, peaceful species that will not out-compete a bottom-perching darter for live and frozen foods, and very small or fry-stage tankmates should be avoided given the species' predatory feeding style.
Conservation
Characidium tapuia has not been formally assessed by the IUCN Red List and currently carries no conservation status (Not Evaluated). It is known only from the rio Parnaíba basin, and while it has been recorded across the upper and middle portions of that drainage rather than from a single restricted locality, its status has not been quantified with population or trend data.
As an endemic of a single major river basin in Brazil's semi-arid northeast, C. tapuia is realistically exposed to the general pressures facing Parnaíba basin fishes — water abstraction, damming and land-use change in the surrounding catchment — but no species-specific assessment of these threats is available. A formal IUCN evaluation would be needed to say more.