Taxonomy & naming
Diapoma nandi was formally described in 2018 by Jorge Andrés Vanegas-Ríos, María de las Mercedes Azpelicueta, and Luiz Roberto Malabarba in the Journal of Fish Biology. The type locality is the Piray-Miní stream, a tributary of the Río Paraná drainage in Argentina. The species is placed in the genus Diapoma within the subfamily Stevardiinae, a diverse lineage of South American characins within the family Characidae.
The diagnosis presented in the original description distinguishes D. nandi from congeners by a combination of meristic and morphological characters: a terminal mouth, two unbranched and eight branched dorsal-fin rays, one unbranched and six branched pelvic-fin rays, the absence of a caudal-fin organ, a hyaline (unpigmented) adipose fin, and an incomplete lateral line. These characters collectively place it within the stevardiine cluster of genera and separate it from its nearest Diapoma relatives.
The genus Diapoma is part of a broader Stevardiinae radiation spread across the drainages of southern South America. Because the species was described directly into Diapoma and has not been moved, no parenthetical authority is used in the valid name. Eschmeyer's Catalog of Fishes is the governing authority for the valid name Diapoma nandi Vanegas-Ríos, Azpelicueta & Malabarba, 2018.
Morphology
Diapoma nandi is a small characin reaching a maximum recorded standard length of 2.5 in in males and unsexed specimens, with females reaching up to 2 in SL — modest even by tetra standards. The body form is typical of the stevardiine genus Diapoma: slender, moderately compressed, and benthopelagic in habit.
Key morphological features include a terminal mouth orientation, an incomplete lateral line (a diagnostic character in the genus), and a hyaline adipose fin. The fin-ray counts — D ii,8; V i,6 — are stated in the original description and help distinguish D. nandi from other Diapoma species. The absence of a caudal-fin organ, a secretory structure present in some allied stevardiines, is another diagnostic feature. Colour details in the available literature are sparse; the species likely displays the subdued, silvery to lightly pigmented pattern common to small-stream stevardiines of the Paraná basin.
Habitat
The known natural range of Diapoma nandi centres on the Piray-Miní stream in northeastern Argentina, within the broader Río Paraná basin. Occurrence data from GBIF confirm the species within the Paraná drainage; it is described as freshwater and benthopelagic in habit, associating with the bottom and lower water column in stream environments.
The Piray-Miní and its parent system, the upper Río Paraná, are subtropical to warm-temperate rivers characterised by moderate to warm water temperatures, somewhat turbid flows in the main channels, and a complex of smaller tributary streams with varying substrate and vegetation. As a small-stream stevardiine, D. nandi is presumed to favour the slower, more vegetated reaches of its tributary habitat rather than the main river channel, though detailed micro-habitat data are not available in the current literature.
Feeding
Detailed dietary studies for Diapoma nandi have not been published. Based on its small body size, benthopelagic habit, and the feeding ecology described for stevardiines generally, it is inferred to be an omnivore targeting small invertebrates — microcrustaceans, midge larvae, and other benthic or epibenthic invertebrates — as well as opportunistic plant or algal material.
The original description characterises the likely diet as composed of small prey items consistent with omnivory, which is the general pattern within Stevardiinae. Until dedicated stomach-content analyses are published for this species, diet should be considered inferred from the subfamily-level pattern rather than directly documented.
Mating
No published observations of reproductive behaviour in the wild are available specifically for Diapoma nandi. As a member of the subfamily Stevardiinae, it is expected to practise egg-scattering spawning, releasing adhesive or semi-adhesive eggs among fine-leaved aquatic vegetation or over the substrate, with no parental care following spawning.
Stevardiines as a group are diverse in their reproductive specialisations, with some lineages displaying elaborate courtship involving gland-based pheromone signalling or caudal-fin organ secretions; the absence of a caudal-fin organ in D. nandi suggests it does not share those particular traits. Mate choice and courtship details for D. nandi remain undocumented.
Breeding
Captive breeding records for Diapoma nandi are not available in the hobbyist or scientific literature reviewed. Based on its stevardiine affinity and small size, breeding in captivity would be expected to follow the open-water egg-scattering mode typical of the subfamily: adults spawning among fine-leaved plants or over substrate, depositing adhesive eggs, and offering no parental care thereafter.
Were the species to become available in the hobby, standard small-tetra breeding protocols — a conditioning diet of live or frozen foods, a densely planted spawning tank, separation of adults after spawning to prevent egg predation, and very fine first foods for the fry — would represent the logical starting approach. At present the species is essentially unknown in the ornamental trade.
In the aquarium
Diapoma nandi is not established in the ornamental fish trade and is not currently available to hobbyists. Its small maximum size (under 2.5 in SL), peaceful stevardiine temperament, and Paraná-basin origin suggest it would be suited to a subtropical or warmly temperate planted aquarium if it were to enter the hobby — comparable in requirements to other small Paraná-region tetras.
Water parameters appropriate to its natural range in the Río Paraná basin would include temperatures broadly in the 68–79 °F range, neutral to slightly acidic or alkaline pH, and moderate water movement. As with other small characins, a school of six or more individuals, fine-leaved planting, subdued lighting, and peaceful tankmates would likely bring out natural behaviour. Because the species is currently only known from scientific collections, practical care data are absent; any information here must be treated as inference from related taxa.
Conservation
Diapoma nandi has not been assessed by the IUCN Red List; its conservation status is Not Evaluated (NE). The species was described only in 2018, and newly described small-stream fishes from the Paraná basin frequently have assessment gaps simply because of the time lag between formal description and IUCN review.
The Piray-Miní stream and broader upper Paraná basin face ongoing pressures from agriculture, deforestation, and water-quality degradation associated with the highly modified landscapes of northeastern Argentina. Small, possibly localised tributary endemics in such systems are inherently vulnerable to these pressures even when their population status is unknown. Targeted surveys to establish the actual range extent of D. nandi within the Paraná drainage, combined with formal IUCN assessment, would be valuable first steps in understanding any conservation need.