Taxonomy & naming
Diapoma guarani was originally described by Valentin Mahnert and Jacques Géry in 1987 under the combination Hyphessobrycon guarani. The parenthetical authority — (Mahnert & Géry, 1987) — reflects this later transfer to a different genus. Eschmeyer's Catalog of Fishes records the current valid combination as Diapoma guarani and places it in the family Characidae.
The genus Diapoma belongs to the large and taxonomically complex subfamily Stevardiinae within Characidae, an assemblage of small South American tetras whose boundaries have been substantially revised by molecular phylogenetics over the past two decades. Thomaz et al. (2015) provided the phylogenetic framework that moved guarani and its relatives from Hyphessobrycon into Diapoma; the species is sometimes referenced in the literature under the notation 'comb. nov.' reflecting that reclassification. Diapoma is a small genus of roughly a dozen formally described species, all confined to the La Plata and associated drainages of subtropical South America.
Morphology
Diapoma guarani is a small, elongate tetra reaching a maximum standard length of approximately 2 in, though many specimens are smaller. The body is slender and laterally compressed, with the streamlined form typical of mid-water schooling characids.
Coloration is described as a pale yellow-whitish ground colour overlaid by a prominent dark-brown longitudinal stripe running along the body. The fins are reddish with black distal margins — a combination that distinguishes it from several congeners within the genus Diapoma. Overall it is a modestly marked fish rather than a brightly coloured one, fitting the pattern of many small stevardiine tetras that rely more on subtle patterning than vivid pigmentation.
Habitat
The species is distributed in the upper Paraná River basin, with confirmed records from Argentina, Brazil, and Paraguay. The upper Paraná is a large, subtropical drainage system descending from the Brazilian highlands, and encompasses a broad range of habitat types from fast-flowing highland streams to slower lowland reaches and associated floodplain waters.
Detailed microhabitat data for Diapoma guarani specifically are sparse in the published record. As a stevardiine of the Paraná basin it is presumed to occupy clear to slightly turbid streams and rivers with moderate current, areas of vegetation, and substrates of gravel or sand. The subtropical latitude means the fish experiences seasonal temperature variation, with cooler winters and warmer summers across its range — a factor relevant to husbandry.
Feeding
Diapoma guarani is an omnivore in the broad sense typical of small characids. Stevardiine tetras generally feed on small invertebrates — aquatic insects and their larvae, microcrustaceans — as well as algae, plant matter, and organic detritus available in their stream environments.
Specific dietary data for this species in the wild are not well documented in the available literature. In captivity, small tetras of this group typically accept fine flake and micro-pellet foods, frozen foods such as Daphnia, brine shrimp nauplii, and bloodworm, and will graze on algae. A varied diet covering both animal protein and plant-derived matter is advisable.
Mating
Like other members of the Stevardiinae and small characids generally, Diapoma guarani is presumed to be an egg-scattering, open spawner with no parental care. The family Characidae includes a wide range of reproductive strategies, but small stevardiine tetras typically deposit adhesive or semi-adhesive eggs among fine-leaved aquatic plants or on submerged surfaces, with no courtship territory held and no guarding of eggs or fry after spawning.
Specific mating behavior for this species has not been described in the primary literature available. Males of stevardiine tetras frequently possess glandular tissue associated with the caudal fin or a 'bursicle' organ involved in chemical signalling during courtship — a derived feature of the Stevardiinae — though the details for Diapoma specifically require further research.
Breeding
No detailed captive breeding accounts for Diapoma guarani have been published in the accessible literature. Based on the biology of closely related small stevardiine tetras, the expected pattern is egg scattering over fine-leaved plants or spawning mops, with adhesive eggs and no parental care. Adults should be expected to consume both eggs and fry if left together after spawning.
Breeders working with this or related small Diapoma species would likely apply the same approach used for small South American tetras generally: condition adult fish on varied live and frozen foods, provide fine-leaved vegetation or synthetic mops as a spawning substrate, remove adults or eggs promptly after spawning, and raise fry on infusoria or commercial liquid fry food before transitioning to newly hatched brine shrimp. The subtropical origin of the species suggests a slightly cooler breeding temperature than typical Amazonian tetras may be appropriate.
In the aquarium
Diapoma guarani is not an established aquarium fish and is rarely, if ever, encountered in the hobby trade. The genus Diapoma as a whole remains largely unknown outside specialist circles focused on stevardiine diversity. This means that practical husbandry information is very limited, and the notes below are extrapolated from related taxa rather than from documented experience with this species specifically.
As a small, schooling tetra from a subtropical drainage, it would likely do best in a group of six or more individuals in a planted aquarium with open swimming space. Water conditions reflecting the Paraná basin — temperatures ranging from roughly the low 20s to the upper 20s Celsius, neutral to slightly acidic or alkaline pH depending on the local conditions of its collection site — would be appropriate starting parameters. It is not a species readily sourced from the trade; specimens would most likely come through specialist importers or scientific collection.
Conservation
Diapoma guarani is assessed as Least Concern (LC) on the IUCN Red List. The Least Concern designation indicates that the species is not considered to face any immediate threat of extinction at the population level given its distribution across the upper Paraná basin in three countries.
The upper Paraná basin has, however, experienced significant habitat modification over the past century, including large hydroelectric impoundments (notably Itaipu), agricultural land conversion, and water quality degradation. These pressures affect many native fishes of the system to varying degrees. For a small, narrowly-studied stevardiine like Diapoma guarani, the full picture of local population trends is not well resolved, and the Least Concern assessment is partly a reflection of data limitations as much as confirmed population health. Continued survey work in the region remains important for accurately tracking the status of small, less conspicuous fish of the Paraná fauna.