Taxonomy & naming
Diapoma obi was described by Casciotta, Almirón, Piálek, and Říčan in 2012 in the journal Zootaxa (3391: 39–51), originally under the genus name Cyanocharax — making the full original combination Cyanocharax obi. The type material was collected from the arroyo Paranay-Guazú, a tributary of the upper río Paraná drainage in Misiones province, northeastern Argentina, and the description represented the first record of the genus Cyanocharax from Paraná basin tributaries.
In 2015 the species was transferred to Diapoma by Koerber and colleagues in CLOFFAR (Checklist of the Freshwater Fishes of Argentina), bringing the valid combination to Diapoma obi (Casciotta, Almirón, Piálek & Říčan, 2012) — the parenthetical authority reflecting the change in genus. Phylogenetic context was further explored by Vanegas-Ríos et al. in a 2018 paper in the Journal of Fish Biology (93: 830–841) covering an identification key and evolutionary framework for the genus and its allies. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Van der Laan) is the governing authority for the valid name.
Diapoma belongs to the subfamily Stevardiinae within Characidae, a diverse radiation of small South American tetras. The genus contains a modest number of described species distributed across the southern cone river systems of South America, all sharing the compressed body form and interrupted or alternating lateral line that characterise the group.
Morphology
Diapoma obi is a small, slender characid with a maximum recorded standard length of 2.5 in, typical of the genus. The body is laterally compressed with a depth of 34.5–40.8% of standard length — a moderately deep profile for a small tetra. The lateral line is interrupted or formed by alternating scales rather than a continuous series, a diagnostic character shared across the genus. The anal fin carries 22–24 branched rays, with a mode of 24, a count that contributes to distinguishing this species within Diapoma.
The epithet 'obi', meaning 'blue' in Guaraní, was chosen for the body coloration visible in fresh specimens — a steel-blue or blue-grey lateral sheen that is characteristic of the living fish and would have been apparent in the type series at collection. As with many small characids, coloration may fade or change in preserved material, making the live or freshly fixed fish a better reference for the colour that motivated the name.
Habitat
Diapoma obi is known from tributaries of the río Paraná basin in Misiones province, Argentina, with the type locality at the arroyo Paranay-Guazú. Misiones sits in the subtropical Atlantic Forest zone of northeastern Argentina, where the Paraná and its tributaries drain a landscape of red basaltic soils and dense rainforest.
FishBase records the species as benthopelagic and freshwater, associated with turbid, rapidly flowing water conditions that follow heavy rainfall — the collecting notes from the type locality describe bottom substrates of mud, sand, and stones typical of small, fast Paraná-system streams. Water chemistry data specific to D. obi are not available in published sources; regional small-stream tetras from this part of the Paraná drainage typically occupy warm subtropical water with temperatures in the low-to-mid 20s Celsius and slightly acidic to neutral pH, though local geology can moderate these values considerably.
Feeding
No published dietary studies specific to Diapoma obi have been located. By analogy with other small Stevardiinae and the broader genus Diapoma, the species is presumed to be an opportunistic micro-predator and omnivore, taking small invertebrates — aquatic insect larvae, microcrustaceans, and similar prey — along with algae, detritus, and organic matter encountered near the bottom and in the water column of fast-flowing stream habitats.
In aquarium conditions, species of this size and body form generally accept small live and frozen invertebrates such as Daphnia, Artemia nauplii, and chironomid larvae, along with fine-grained prepared foods. Data sparse for this species specifically.
Mating
No published observations of mating behaviour specific to Diapoma obi are available. Like other small characids in the family Characidae, it is presumed to follow the general pattern of egg-scattering open spawning in which males pursue females through fine-leaved aquatic vegetation or other structured substrates, with adhesive eggs deposited among plant material and no parental investment beyond fertilisation.
Sexual dimorphism in Diapoma species is generally modest, and the cues governing mate choice, seasonality of spawning, and intraspecific signalling are undocumented for this species. Further field and aquarium study would be needed to characterise reproductive behaviour in detail.
Breeding
Breeding has not been documented in the published literature for Diapoma obi, and the species does not appear to be established in the aquarium hobby. Based on the biology of closely related small South American characids, egg-scattering spawning over fine-leaved vegetation or substrate materials with adhesive eggs and no parental care would be the expected mode.
Any aquarist encountering this species would likely apply the standard approach used for small Stevardiinae: a separate spawning vessel with fine-leaved plants or synthetic mops, slightly cooler or fluctuating temperature as a trigger, removal of adults post-spawning to prevent egg predation, and rearing of fry on infusoria and micro-foods before graduating to brine shrimp nauplii. Data sparse; confirmation from kept specimens is lacking.
In the aquarium
Diapoma obi is not established in the ornamental fish trade and no aquarium husbandry records have been published for the species. It is known primarily from ichthyological collections and the scientific literature describing its taxonomy, morphology, and type locality.
Should specimens become available, the natural habitat — turbid, fast-flowing subtropical streams in Misiones — suggests care requirements broadly similar to other small south-cone tetras: warm temperatures in the low-to-mid 20s Celsius, neutral to slightly acidic or neutral water, good oxygenation and moderate to strong flow, and a tank with fine-leaved vegetation or leaf litter to replicate the structured bottom environment. A schooling arrangement with conspecifics or similar small tetras would be appropriate given the gregarious tendencies typical of the genus. Data sparse.
Conservation
Diapoma obi has not been formally assessed by the IUCN Red List and carries a status of Not Evaluated (NE). No conservation data specific to the species — population estimates, threat assessments, or monitoring records — have been published.
The species is restricted to tributaries of the Paraná basin in Misiones province, a region that has experienced significant deforestation and agricultural intensification, with consequent effects on stream hydrology, sedimentation, and water quality in smaller watercourses. Whether these pressures affect D. obi populations is unknown. Its small, potentially narrow range within the Paraná system would make it sensitive to local habitat degradation, and a formal IUCN assessment drawing on modern survey data would help clarify its status. Until such data exist, the species should be regarded as data deficient with respect to conservation standing.