Taxonomy & naming
Moenkhausia dorsinuda was described by D. Zarski and J. Géry in 2002 in the journal Zoologische Abhandlungen (vol. 52, pp. 11–21), under the full title 'Moenkhausia dorsinuda sp.n. — ein neuer interessanter Salmler (Teleostei, Characiformes, Characidae) aus dem Einzugsgebiet des Río Iténez in Bolivien'. The type locality is the Río Ipurupuru, a tributary system draining into the Río Iténez (also known as the Río Guaporé) in the Bolivian lowlands, approximately 14°S–15°S. Eschmeyer's Catalog of Fishes treats the name as valid within Moenkhausia, the authority formatted without parentheses as the species was described directly into that genus.
The genus Moenkhausia Eigenmann, 1903 is one of the largest and most heterogeneous genera in Characidae, encompassing well over 70 nominal species distributed broadly across South American river systems. The genus has long been regarded as potentially polyphyletic, and molecular phylogenetic studies continue to revise its boundaries. M. dorsinuda has not been widely treated in broader Moenkhausia revisions, and its placement within genus-level groupings remains unresolved; it is held in Moenkhausia on the basis of the original description.
Morphology
Moenkhausia dorsinuda is a small, laterally compressed characid reaching a maximum recorded standard length of 3 in (approximately 2.8 in), though many individuals are smaller. The body form follows the typical Moenkhausia plan: moderately deep, fusiform, with a bluntly rounded snout and a terminal mouth.
The defining diagnostic character is the naked predorsal region — the stretch of body between the occiput and the origin of the dorsal fin lacks scales (or carries only rudimentary ones), in contrast to the fully scaled condition seen in many congeners. This feature, unusual within the genus, gave the species both its scientific name and a useful field identification character. Colouration is not well documented in the literature available; like many small Moenkhausia, the species is likely silvery-bodied with variable lateral or caudal pigmentation, but detailed colour notes from fresh specimens are sparse and are not reproduced here to avoid fabrication.
Habitat
The known range of Moenkhausia dorsinuda is the Río Ipurupuru drainage within the Río Iténez catchment of lowland Bolivia, at latitudes roughly 14°S–15°S. This region lies within the Beni savannas and adjacent humid lowland forest of the upper Amazon system, where the Río Iténez forms the border between Bolivia and Brazil before emptying into the Río Mamoré.
FishBase characterises the habitat as white-water river with low current and vegetated muddy banks — a typical Bolivian lowland stream environment with elevated suspended sediment, moderate turbidity, and dense marginal and submerged vegetation. Recorded water chemistry from this habitat includes a pH range of 6.6–7.2, indicating slightly acidic to near-neutral conditions consistent with the soft, low-mineral waters draining the sedimentary lowlands of the Beni. Temperature data for the Río Ipurupuru specifically are sparse, but lowland Bolivian streams at this latitude typically warm to 75–82 °F through the wet season and cool somewhat in the dry season.
Feeding
No dedicated feeding studies on Moenkhausia dorsinuda appear in the available literature. By analogy with well-studied congeners and with small characids occupying similar lowland stream habitats, the species is presumed to be an opportunistic omnivore taking small invertebrates — aquatic insect larvae, microcrustaceans, and fallen terrestrial insects — as well as algae, plant material, and organic detritus present at vegetated stream margins.
In the aquarium, small Moenkhausia species generally adapt readily to a varied diet of good-quality flake, micro-pellets, and small live or frozen foods such as Daphnia, Artemia nauplii, and bloodworm. Offering a mix that includes live or frozen components supports conditioning and maintains colour in species where full pigmentation is diet-dependent.
Mating
Specific mating observations for Moenkhausia dorsinuda have not been published. The genus as a whole follows the characin norm: egg-scattering open spawning with no pair bond or parental care, females broadcast eggs among fine-leaved plants or other fine substrate, and males pursue and court females with lateral displays and fin-spreading. In species monitored in aquaria, males distinguish themselves by slightly slimmer body profiles compared to ripe females, though the degree of external dimorphism in M. dorsinuda specifically is not documented.
Courting behaviour typical of the group involves active male pursuit in open water followed by tight parallel swimming as eggs are released and fertilised. Because spawning is not confined to a single event, sustained conditioning of groups in well-planted aquaria is the standard approach for encouraging reproductive activity.
Breeding
Moenkhausia dorsinuda has not been bred in captivity in any published account. General Moenkhausia breeding biology, applicable by inference, describes egg-scattering free spawners that deposit adhesive eggs among fine-leaved aquatic plants or spawning mops. Clutch sizes in comparable species can reach approximately 500 eggs per spawning; eggs are small and semi-adhesive, hatching within 24–48 hours at typical tropical temperatures, with the fry becoming free-swimming after a further 2–3 days once the yolk sac is absorbed.
Adults offer no care and will consume eggs and fry if left together, so breeders working with closely related species routinely remove adults after spawning or harvest eggs to a separate, well-aerated vessel. First foods for the fry must be appropriately minute — infusoria or commercial liquid fry foods initially, progressing to newly hatched brine shrimp as the fish grow. Soft, slightly acidic water in the lower-to-mid 20s Celsius is the typical conditioning target for lowland Bolivian stream characids.
In the aquarium
Moenkhausia dorsinuda is essentially absent from the commercial aquarium trade and is not regularly available through hobby channels. Its restricted, remote range in the Bolivian lowlands means wild-caught specimens rarely reach export points, and no captive-breeding programme for the species is known. Aquarists specifically seeking it would need to source through specialist collectors or conservation-linked breeding programmes.
Based on its habitat profile — a small, lowland white-water stream fish with near-neutral pH and warm temperatures — it would be expected to adapt well to a spacious, well-planted community aquarium with soft to moderately hard water, a temperature of around 75–82 °F, and subdued to moderate current. A group of six or more would suit the schooling character typical of the genus. Given its modest maximum size of around 3 in, it is compatible with other peaceful small to medium characids, corydoras, and small loricariids. The lack of trade availability means husbandry specifics remain to be established by the hobbyist.
Conservation
Moenkhausia dorsinuda is assessed as Data Deficient (DD) on the IUCN Red List, with the assessment dated 22 April 2014. The Data Deficient designation reflects the paucity of information on population size, range extent, and habitat trends rather than a confirmed absence of threat; a species can be genuinely threatened and still carry DD status when survey data are insufficient to evaluate that threat objectively.
The species is known from a single river system — the Río Ipurupuru in Bolivia's Beni lowlands — and, like many narrowly distributed Bolivian stream endemics, may be vulnerable to the land-use pressures affecting lowland South American drainages: forest clearance for cattle pasture and soy agriculture, associated siltation and water-quality changes, and increasing dry-season flow variability. Until dedicated surveys establish population baselines and clarify the full range of the species, its conservation needs cannot be properly assessed. The Data Deficient listing is a call for field work, not reassurance.