Taxonomy & naming
Diapoma alegretense was described by Luiz Roberto Malabarba and Stanley H. Weitzman in 2003, in a monograph that erected the genus Diapoma and diagnosed six new species from southern Brazil, Uruguay, and Argentina, published in Comunicações do Museu de Ciências e Tecnologia da PUCRS, Série Zoologia. The valid name as accepted in Eschmeyer's Catalog of Fishes is Diapoma alegretense (Malabarba & Weitzman, 2003); the parenthetical authority reflects the original placement in a new genus at the time of description.
Diapoma belongs to the large, diverse family Characidae and is positioned within the subfamily Stevardiinae, a clade of small South American characids characterised by specialised reproductive anatomy and predominantly riverine, South American distributions. The genus unites species sharing a suite of internal morphological characters, including modifications of the gill arches and pectoral girdle that distinguish them from superficially similar small characids.
Morphology
Diapoma alegretense is a small, slender-bodied fish reaching a maximum recorded standard length of 1.5 in. Like other members of Diapoma and the broader Stevardiinae, it has the typical characid body plan: a compressed, fusiform shape, a terminal mouth, and a small adipose fin posterior to the dorsal. The overall impression is of a streamlined, active swimmer adapted to moderate-current environments.
Coloration in life is not extensively documented in the available literature, but members of Diapoma typically display silvery body flanks with varying degrees of lateral pigmentation, and may show a mid-lateral stripe or dusky chromatophore patterning along the flank. Sexually dimorphic features characteristic of Stevardiinae males — including specialised scales and hooks on fins — may be present; specific details for this species remain sparsely documented.
Habitat
The species is known from tributaries of the rio Ibicuí basin, itself a tributary of the rio Uruguay drainage, in the state of Rio Grande do Sul in southern Brazil. The epithet alegretense references Alegrete, the municipality in that region associated with the type locality.
IUCN assessors describe the preferred habitat as clear water with moderate flow and sandy or rocky substrates — the kind of clean, well-oxygenated stream environment typical of the upper Uruguay drainage system. The subtropical climate of Rio Grande do Sul produces seasonal temperature fluctuations, with cooler winter conditions at these latitudes contrasting with warm summers; FishBase characterises the species as benthopelagic and subtropical.
Feeding
FishBase records a trophic level of 2.8 for Diapoma alegretense, placing it among low-level consumers that feed primarily on plant material, algae, and small invertebrates rather than as a predator of other fish. This trophic position is broadly consistent with other small Stevardiinae in clear Neotropical streams, which commonly take small aquatic insects, microcrustaceans, and fine organic matter from the water column and substrate.
Detailed dietary studies specific to this species are not available in the cached sources, and the low vulnerability index assigned by FishBase (10 out of 100) reflects both its small size and its position as a lower-order consumer in the food web.
Mating
Specific mating behaviour in Diapoma alegretense has not been described in the available literature. Members of the subfamily Stevardiinae are generally egg-scatterers that spawn among aquatic vegetation or substrate without providing parental care — the broad reproductive pattern shared across most small characids.
Stevardiine males of many species possess specialised hook-like structures on the anal-fin rays and modified scales associated with courtship and spawning; whether such features occur in D. alegretense is not confirmed in the sources available. Spawning in subtropical Rio Grande do Sul species is likely to be influenced by seasonal temperature cues, with increased activity during the warmer months.
Breeding
No captive breeding accounts for Diapoma alegretense are documented in the available sources, and the species is not established in the ornamental fish hobby. Based on the general biology of Stevardiinae, reproduction is expected to involve the deposition of adhesive eggs among fine-leaved aquatic vegetation or over substrate, with no parental care following spawning.
Fry would presumably require small live foods in the first days post-hatching, as is typical for small characids of this body size. Data on clutch size, incubation period, and larval development remain unrecorded for this species.
In the aquarium
Diapoma alegretense is not a species encountered in the ornamental fish trade, and no hobbyist keeping accounts are available. Its small size (to 1.5 in SL), clear-water stream origins, and apparent tolerance of subtropical temperature ranges suggest it would be suited to a well-filtered, oxygenated aquarium with moderate water movement and a varied diet of small live or frozen invertebrates supplemented with fine prepared foods.
Any keeper working with wild-caught material should replicate the clear, moderately flowing conditions of the rio Ibicuí tributaries — clean water, good circulation, sandy or fine-gravel substrate — and be prepared for a temperature range spanning cooler subtropical winters. The species is likely best kept as a small school, as is typical for characids of similar size and ecology, but dedicated husbandry guidance is absent from the current literature.
Conservation
Diapoma alegretense was assessed as Least Concern (LC) on the IUCN Red List in an evaluation completed on 4 March 2021 and published in 2023 (assessor: Lima, F.). The assessment notes a stable, moderately common population within its range and identifies no significant threats at the time of review.
The species' restriction to tributaries of the rio Ibicuí basin means its range is geographically limited within the rio Uruguay watershed; however, the stability of the population and the absence of documented pressures place it comfortably within the Least Concern category under current criteria. Continued monitoring of water quality and land-use change in southern Rio Grande do Sul would be prudent given broader pressures on Neotropical stream fauna.