Taxonomy & naming
Diapoma dicropotamicum was described by Luiz R. Malabarba and Stanley H. Weitzman in 2003, in a paper that simultaneously established the genus Diapoma and described six new species within it. The work identified a group of miniature South American characids sharing a suite of morphological characters and placed them together under the new generic name. Eschmeyer's Catalog of Fishes (Catalog of Fishes, Fricke, Eschmeyer & Fong) records the valid name as Diapoma dicropotamicum (Malabarba & Weitzman, 2003); parenthetical authority indicates that the species was originally described in a combination different from the one currently used, or as part of a genus establishment where placement shifted.
Diapoma belongs to the subfamily Stevardiinae within Characidae — a diverse radiation of small to medium-sized freshwater tetras whose phylogenetic relationships have been progressively refined by molecular and morphological studies. A 2015 review of Stevardiinae phylogeny (published in PLOS ONE) confirmed Diapoma's placement within this subfamily alongside many other miniature characid genera from South and Central America. A subsequent genus-level revision published in Zoosystematics and Evolution identified approximately 16 valid species within Diapoma, distributed across river systems in Argentina, Brazil, Paraguay, and Uruguay, with most species being single-basin endemics — a pattern consistent with the fine-scale isolation that characterises small stream-dwelling tetras in southeastern South America.
This fish was long listed as Diapoma dicropotamicus; Eschmeyer's Catalog of Fishes places the species in Diapoma, and the current combination Diapoma dicropotamicum is followed here.
Morphology
Diapoma dicropotamicum is a small, slender-bodied characid reaching approximately 1.7 in standard length (SL) according to FishBase, though the original description notes that most individuals are smaller than 0.98 in SL, suggesting that the larger recorded size may represent the upper limit of the natural range of variation. The body form is typical of miniature stevardiine tetras: elongate and laterally compressed, with the proportions of an active, benthopelagic fish occupying the middle and lower water column in clear-water habitats.
As with most Diapoma species, D. dicropotamicum lacks the bright colours that characterise many popular aquarium tetras. The original description places it within a group of morphologically similar small characids distinguished from related genera by combinations of fin-ray and scale counts, dentition, and the configuration of internal anatomical structures used in stevardiine systematics. Detailed meristic and morphometric data were provided in the 2003 original description and subsequently evaluated in the Pensoft genus revision; hobbyist literature on this species is essentially absent given its rarity in the trade.
Habitat
Diapoma dicropotamicum is endemic to the rio Jacuí drainage of southern Brazil, a system that drains the southern Brazilian Highlands into the Lagoa dos Patos basin. The type locality described in the original 2003 paper references the Upper Negro basin on the Uruguay and Brazil border region, and FishBase records the distribution as the rio Jacuí drainage — both references point to the southern portions of the Brazilian hydrological network that feed ultimately into the Río de la Plata system.
The original description characterises the preferred microhabitat as lentic stretches of rivers and streams with clear waters — meaning slow to still zones within moving-water systems rather than fast riffles. Shallow depths with slow to moderate flow, clear water, and presumably fine substrate and aquatic vegetation are consistent with the ecology of other miniature stevardiine tetras. Water parameter data specific to this species are not available in published sources, but the broader Jacuí and Upper Negro drainages in subtropical southern Brazil typically experience cooler seasonal temperatures than tropical Amazonian systems, with water that tends toward soft and slightly acidic in the granite and basalt catchments of the southern Brazilian Highlands.
Feeding
No species-specific dietary data for Diapoma dicropotamicum have been published in the accessible literature, and the species is sufficiently rare in captivity that hobbyist feeding observations are essentially unavailable. By analogy with related miniature stevardiine tetras and the general biology of small, benthopelagic characids, D. dicropotamicum is presumed to be a micropredator feeding opportunistically on small invertebrates — insect larvae, microcrustaceans, and other zooplankton — found in and around the benthic and mid-water zones of clear-water streams.
In an aquarium context, if the species were to become available, it would likely accept fine-grained prepared foods alongside small live or frozen offerings such as Daphnia, copepods, and newly hatched brine shrimp, following the feeding behaviour documented for similarly sized tetra relatives.
Mating
No direct observations of courtship or mating behaviour in Diapoma dicropotamicum have been published. Breeding biology within the genus Diapoma and the broader Stevardiinae subfamily suggests that species in this group are egg-scattering spawners with no parental care, following the general characid pattern. Spawning is likely to occur among fine-leaved aquatic vegetation or submerged substrates in shallow, clear-water lentic zones, consistent with the habitat description from the original species account.
Stevardiine tetras as a clade do not exhibit the specialised internal fertilisation or viviparity found in some other characid lineages; D. dicropotamicum is presumed to follow an orthodox open-water or plant-associated egg-scattering mode, with adhesive or semi-adhesive eggs deposited without subsequent parental attention. Sexual dimorphism, if present, has not been described in the literature for this species.
Breeding
Diapoma dicropotamicum has not been bred in captivity under documented conditions, and no published breeding protocols exist for this species or, as far as the available literature indicates, for most Diapoma species. The genus is not represented in the ornamental fish trade to any meaningful extent, and the biological research has focused on taxonomy and systematics rather than reproductive biology.
Based on the general biology of miniature characids and the stevardiine subfamily, breeding, if achieved, would likely follow the standard open-spawning egg-scattering pattern: adhesive eggs scattered among fine-leaved plants or substrate, no parental care, and adults removed promptly to prevent egg predation. Incubation and larval development timings are unknown but would be expected to fall within the range typical of small subtropical characids — a few days to a week at appropriate temperatures, followed by a brief yolk-sac stage before free-swimming fry require fine first foods.
In the aquarium
Diapoma dicropotamicum is not available in the mainstream ornamental fish trade and is unknown in hobby fishkeeping outside of occasional wild-caught specimens obtained by specialist collectors with access to its southern Brazilian range. There are no published husbandry accounts for the species, and its care requirements must be inferred from habitat data and the biology of related stevardiine tetras.
The subtropical origin of D. dicropotamicum — the rio Jacuí drainage of southern Brazil experiences genuine seasonal cool periods — suggests that it would do best at temperatures somewhat cooler than those typical for tropical tetras, potentially in the range of 64–75 °F rather than the 75–82 °F favoured by Amazonian species. Clear, well-oxygenated water with low to moderate hardness and a slightly acidic to neutral pH would reflect the likely natural conditions. A well-planted tank with subdued lighting and a peaceful community of similarly sized fishes would suit its ecological profile. Any aquarist fortunate enough to obtain specimens should treat this as a rare and scientifically significant fish and consider contributing observations to the published record.
Conservation
Diapoma dicropotamicum has not been assessed by the IUCN Red List and carries no formal conservation status. As a single-basin endemic — restricted to the rio Jacuí drainage of southern Brazil — the species has a naturally limited range, and any significant degradation of that watershed would affect its entire known population. The broader genus Diapoma, with approximately 16 valid species each largely restricted to individual drainages in southeastern South America, represents a group whose conservation biology has received essentially no targeted attention.
The rio Jacuí system in the state of Rio Grande do Sul, Brazil, drains a heavily agricultural and increasingly urbanised catchment. Sedimentation, water abstraction, riparian clearing, and pollution associated with intensive agriculture and urban expansion are known pressures across southern Brazilian river systems. Whether D. dicropotamicum is abundant within its native drainage or genuinely scarce is not known from published sources. Its absence from trade and from hobbyist records suggests limited encounter rates, though this may reflect collector effort rather than population size. A formal IUCN assessment using available locality data from the original description and any subsequent surveys would be a useful first step in understanding its conservation situation.