Taxonomy & naming
Knodus dorsomaculatus was formally described by K.M. Ferreira and A.L. Netto-Ferreira in 2010, with the description published in the Journal of Fish Biology (volume 77, issue 3, pp. 468–478). The holotype, MZUSP 102823, measures 2 in standard length and was collected from a tributary of the Teles Pires River in the Tapajós basin, state of Pará, Brazil. The Catalog of Fishes records the valid name as Knodus dorsomaculatus Ferreira & Netto-Ferreira, 2010.
The genus Knodus belongs to the large and incompletely resolved characid assemblage; as currently understood it encompasses small, streamlined tetras spread across Amazonian and Orinoco drainages. K. dorsomaculatus is distinguished from its congener K. pasco by several meristic and pigmentation characters: it has only 2 maxillary teeth (vs. 5–6 in K. pasco), 5 scale rows between the lateral line and the dorsal fin origin (vs. 7), and a narrower, 2–2.5 scale-wide dark longitudinal band running from the gill cover to the middle caudal fin rays. The dorsal fin ray formula is ii,9. These characters, combined with the diagnostic dark dorsal blotch, place it unambiguously within the genus while separating it from its nearest described relative.
Morphology
K. dorsomaculatus is a small, slender characin reaching a maximum recorded standard length of 2 in. Its body form is typical of the genus: fusiform and laterally compressed, built for mid-water and near-bottom life in flowing and still freshwater. FishBase classifies it as benthopelagic, occupying the water column from substrate level upward.
The defining feature is the conspicuous dark blotch at the base of the first five branched dorsal fin rays — the character from which the specific epithet dorsomaculatus ('dark-spotted back') is drawn. A dark longitudinal band, approximately 2 to 2.5 scales wide, extends from the posterior edge of the gill cover to the middle caudal fin rays, giving the flanks a bold, stripe-like pattern. Colouration is otherwise modest: a pale to silvery ground colour typical of small Amazonian characins. No data on sexual dimorphism are available in the accessible literature.
Habitat
The species is known from the Teles Pires River and its tributaries, which drain into the Tapajós system in Pará, Brazil. The Tapajós is a major clearwater tributary of the Amazon, rising on the Central Brazilian Plateau; the Teles Pires forms its upper headwaters. This is a tropical freshwater system with warm temperatures and, like most clearwater Amazonian rivers, moderately acidic to near-neutral water chemistry.
No detailed habitat microdata have been published for K. dorsomaculatus specifically, but the benthopelagic classification and the broader Knodus ecology suggest the species occupies shallow, near-shore zones — leaf litter margins, submerged vegetation, and stream bank habitats — at low to moderate depths. Its restricted range within a single river basin defines the key geographic constraint on the population.
Feeding
No specific dietary study of Knodus dorsomaculatus has been published. In the wider genus Knodus, species are generally micropredatory omnivores, taking small invertebrates, insect larvae, microcrustaceans, and particulate organic material from the water column and substrate. Small-bodied characins occupying similar ecological roles in Amazonian clearwater streams show a similar feeding profile.
In captivity — where this species is rarely kept — close relatives in the genus accept live and frozen microfoods such as Daphnia, baby brine shrimp, and bloodworm, as well as quality micro-pellets and crushed flake. Given the small adult size of under 2 in, food particle size must be kept appropriately small. Data for this species specifically remain sparse.
Mating
No published observations of courtship or mating behaviour exist for K. dorsomaculatus. Small Knodus species, like most small Neotropical characins, are presumed to be egg-scattering open spawners: males pursue females, and eggs are released and fertilised without attachment to a fixed substrate, scattering among fine-leaved plants or into the current. No pair bonding or territory defence has been documented in the genus.
Reproductive timing in the wild is likely linked to hydrological seasonality in the Teles Pires basin, with spawning peaking during rising or high-water periods when food and dispersal opportunities are greatest — the pattern observed across many Amazonian characins. Specific courtship signals, if any, have not been described.
Breeding
K. dorsomaculatus has not been reported bred in captivity in accessible literature. Based on typical Knodus and small-characin biology, spawning would involve egg scattering among fine-leaved aquatic vegetation, with small adhesive or semi-adhesive eggs released in modest numbers. Parental care is not expected — adults of egg-scattering characins typically ignore or consume their own eggs.
Should captive breeding be attempted, conditions approximating the warm, soft-to-moderately-soft clearwater of the Tapajós system — temperatures in the mid-to-upper 20s Celsius, slightly acidic to neutral pH, subdued lighting — would be the logical starting point. Dense fine-leaved plant cover or synthetic spawning mops would serve as spawning medium. Fry, if obtained, would require infusoria or rotifers as first foods given the small adult size.
In the aquarium
Knodus dorsomaculatus is not an established aquarium fish and is essentially absent from the hobby trade. Its Endangered status under IUCN criteria and restricted Amazonian range mean that wild-collected specimens are unlikely to reach the ornamental market legitimately, and no captive-breeding programme for this species is known to exist.
If kept, it would require a soft, warm, well-oxygenated setup mimicking clearwater Amazonian stream conditions: temperatures around 75–82 °F, pH 6.0–7.0, low hardness, and gentle to moderate flow. The small size demands appropriate tankmates — comparably sized, non-aggressive species — and a well-planted aquarium with shaded areas. As with most small streamside characins it would benefit from a group rather than solitary keeping. In practical terms it remains a species for the specialist, if obtainable at all.
Conservation
Knodus dorsomaculatus is assessed as Endangered (EN) on the IUCN Red List, evaluated on 18 May 2022 under criterion B1ab(i,iii,iv). This criterion flags a restricted geographic range (extent of occurrence below the threshold for EN under B1) combined with continuing decline inferred for the area of occupancy, habitat quality, and the number of locations or subpopulations. The restricted range is the fundamental vulnerability: a species known from a single river basin has no distributional buffer against local disturbances.
Threats in the Teles Pires basin include habitat loss through deforestation, agricultural runoff, and water extraction — pressures documented across much of the southern Amazon frontier. Large-scale hydropower development on the Teles Pires River, including the Teles Pires dam complex constructed in the 2010s, has altered the hydrology and connectivity of the basin substantially. No specific conservation or monitoring programme targeting K. dorsomaculatus is recorded in accessible literature.