Taxonomy & naming
Moenkhausia diamantina was described in 2007 by Benine, Castro, and Santos in the journal Neotropical Ichthyology (volume 5, issue 3, pages 259–262). The holotype measures 2.5 in standard length, and 42 paratypes ranging from 1–2.5 in SL were examined in the original description. The type locality is the rio Toalhas, a tributary of the rio Paraguaçu in Bahia, Brazil; the species was collected from seven tributaries across the upper and middle portions of that basin.
The valid name follows the original placement in Moenkhausia Eigenmann, 1903, a large Neotropical genus in the family Characidae. Catalog of Fishes records the authority as Benine, Castro & Santos, 2007 without parentheses, as the species was described directly in Moenkhausia. The genus is distinguished from close relatives by characters including the presence of a complete lateral line, absence of a humeral spot, and a subterminal mouth, though species-level identification within Moenkhausia can require close attention to meristics and colour pattern.
The specific epithet diamantina refers to the Chapada Diamantina — the Diamond Plateau — the elevated sandstone-and-quartzite tableland from which the rio Paraguaçu headwaters originate, a region of exceptional geological and biological interest in the Caatinga-Cerrado transition zone of northeastern Brazil.
Morphology
Moenkhausia diamantina is a moderately slender characid reaching a maximum recorded length of 2.5 in standard length. Body depth is relatively modest: the greatest body depth in the original description averages about 43.8% of standard length, giving the fish a somewhat elongate outline compared with deeper-bodied Moenkhausia species. Head length averages approximately 27.3% SL, and the caudal peduncle is relatively narrow at around 12.0% SL.
The species is distinguished from congeners by its body colour pattern combined with the number and arrangement of predorsal median line scales and its anal-fin ray count. Like most Moenkhausia it carries the family's standard Characiform body plan: a forked caudal fin, adipose fin, a relatively complete lateral line, and small cycloid scales. The exact details of fin-ray counts and scale series are the diagnostic meristics documented in the original description; the colour pattern, described from preserved and fresh specimens, includes details of lateral pigmentation that differentiate it from other rio Paraguaçu characids.
Habitat
The species is endemic to the upper and middle rio Paraguaçu basin within the Chapada Diamantina region of Bahia, northeastern Brazil. It has been collected from seven tributaries, all blackwater or clearwater streams draining the quartzite plateau and foothills of the Chapada. These stream habitats are characterised by sandy or rocky substrates, relatively low mineral content, and acidic to slightly acidic water chemistry — conditions typical of streams draining ancient, nutrient-poor geological formations.
The Chapada Diamantina lies in the transition between the dry Caatinga scrubland and Cerrado savanna biomes. Streams in the region are subject to strong seasonal variation in water level and flow, swelling markedly during the rainy season and shrinking in the dry months. Water temperature in upland Bahia streams can be relatively moderate by Neotropical standards given the elevation. The combination of geographic isolation — the Paraguaçu system is hydrologically distinct from the major São Francisco drainage to the north — and the extreme habitat specificity of the Chapada tributaries has produced a notably endemic ichthyofauna, of which M. diamantina is one element.
Feeding
No dedicated study of the feeding ecology of Moenkhausia diamantina has been published. FishBase records a trophic level of 3.4 for the species, consistent with an omnivore that incorporates both animal prey and plant or detrital material. Characids of this size and body form in blackwater Neotropical streams typically consume small invertebrates — microcrustaceans, aquatic insect larvae, surface-fallen terrestrial insects — alongside algae, fine organic particles, and seeds.
In the aquarium, Moenkhausia species at this size accept a wide range of small foods. Commercially prepared micro-pellets and fine-grade flake provide a stable base diet, while frozen or live Daphnia, Artemia nauplii, and small bloodworm provide the protein and enrichment needed to maintain condition and breeding readiness. Feeding small amounts multiple times daily rather than a single large meal suits the continuous foraging behaviour typical of active small tetras.
Mating
Moenkhausia diamantina has not been studied in detail in the wild or in captivity, but all Moenkhausia for which reproduction is documented are egg-scattering open spawners following the standard Characiform pattern. Males of the genus typically engage in active pursuit of females, with courtship consisting of chasing and parallel swimming near fine-leaved vegetation or other spawning substrates. There is no pair bond and no territorial defence of a spawning site.
Spawning in related Moenkhausia species such as M. pittieri occurs characteristically in the early morning hours, with the most active display and egg release taking place shortly after the lights come on. Males that are kept as a group rather than in isolation tend to display more intensely in the presence of rivals, which also accelerates female readiness.
Breeding
Based on the biology of the genus, Moenkhausia diamantina is expected to be a free-spawning egg scatterer that deposits adhesive eggs among fine-leaved aquatic plants or over spawning mops, with no parental care and active egg predation by the adults if they are not removed. In Moenkhausia generally, eggs hatch within 24–36 hours at warm temperatures, and larvae become free-swimming 3–4 days after hatching.
Breeding attempts in captivity would logically follow the standard approach for small Moenkhausia: condition a small group with live and frozen foods, provide fine-leaved plants or synthetic spawning mops in a dedicated breeding vessel, and remove the adults promptly after spawning is observed. First foods for the larvae are infusoria or commercial liquid fry preparations, followed by freshly hatched brine shrimp nauplii as the fry grow. No aquarium breeding reports specific to M. diamantina are available in the published literature or major hobby resources.
In the aquarium
Moenkhausia diamantina is an uncommon species in the hobby, and detailed husbandry observations specific to it are not well documented. By analogy with other small Moenkhausia from blackwater or clearwater Brazilian tributaries it would be expected to do well in soft, slightly acidic water at temperatures in the mid-to-upper 20s Celsius, reflecting the physical chemistry of its native Chapada Diamantina streams.
As a tetra of modest size — under 3 in — it is suitable for a community of similarly scaled, non-aggressive species. Keeping a group of six or more individuals is advisable to distribute any intraspecific chasing and to allow the fish to display natural shoaling behaviour. A planted aquarium with subdued or dappled lighting suits blackwater-stream tetras and is likely to show the species at its best. Given its restricted wild range and rarity in trade, specimens sourced from established aquarium-bred lines are preferable to wild-caught fish.
Conservation
Moenkhausia diamantina is assessed as Least Concern (LC) on the IUCN Red List, with the assessment completed in 2018. The LC designation reflects the absence of evidence for rapid population decline at the time of assessment, though the species' extremely restricted range — confined to tributaries of the upper and middle rio Paraguaçu within the Chapada Diamantina — means it is inherently vulnerable to any habitat degradation within that small area.
The Chapada Diamantina faces ongoing pressures including agricultural expansion in the surrounding lowlands, altered hydrology from water abstraction, and the downstream effects of land-use change on sediment loads and water chemistry in headwater streams. The region is partially protected by the Chapada Diamantina National Park, but stream habitats outside park boundaries receive limited direct protection. The narrow endemism of M. diamantina means that no population elsewhere could compensate for losses within its native tributaries, and its status warrants continued monitoring as development pressure in Bahia grows.