Taxonomy & naming
Knodus longus was described by Axel Zarske and Jacques Géry in 2006. The species belongs to the genus Knodus Eigenmann, 1911, a group of small, slender characids placed within the large family Characidae. Eschmeyer's Catalog of Fishes recognises Knodus longus Zarske & Géry, 2006 as the valid name; the authority is not parenthetical because the species was described directly in Knodus and has not been moved to another genus.
The genus Knodus encompasses several dozen small, stream-dwelling tetras distributed across the Amazon and Orinoco drainages and adjacent Guiana Shield rivers. The group has historically been considered part of the Stevardiinae, though the internal phylogeny of this lineage and the boundaries of Knodus itself have been subject to ongoing revision. K. longus is one of the more recently described species in the genus and detailed type-locality information beyond the original description is not readily available in secondary sources.
Morphology
Knodus longus is a slender, laterally compressed characid with the elongate body form implied by its specific epithet (Latin longus, long). Like other Knodus, it displays the typical small-tetra plan: a streamlined fusiform outline, a forked tail, and a small adipose fin behind the dorsal. The flanks are silvery, often with a faint lateral stripe or stripe-like iridescence running from the operculum toward the caudal peduncle; precise meristic counts and colour-pattern details are recorded in the original description.
Data on maximum standard length are sparse in widely accessible secondary sources. Most Knodus species reach 1–2 in standard length; K. longus is expected to fall within or near that range based on genus-level body plans, but a confirmed size figure from the primary literature has not been located and a precise measurement is not stated here to avoid fabrication.
Habitat
Members of the genus Knodus are characteristic inhabitants of clear to moderately turbid streams, small rivers, and flooded forest edges in the Amazon and Orinoco basins and the rivers of the Guiana Shield. They typically occupy the middle and upper water column in flowing or gently moving water, often near marginal vegetation or overhanging riparian cover.
Specific habitat data for K. longus at the type locality are not available in secondary sources consulted. By analogy with congeners, the species is likely associated with soft, slightly acidic to near-neutral waters of low to moderate conductivity in a lowland Amazonian drainage, but water-parameter figures are not stated here without a confirmed source.
Feeding
Knodus species are generalised micro-predators and opportunistic omnivores that feed on small invertebrates, zooplankton, insect larvae, and organic surface particles in the wild. This feeding strategy is shared broadly across the small-characid guild and is consistent with the slender body form and terminal mouth of K. longus.
No specific gut-content studies for K. longus have been located. In captivity, congeners accept fine-grade flake, micro-pellets, and small live or frozen foods such as Daphnia, baby brine shrimp, and micro-worms without difficulty.
Mating
No published observations of courtship behaviour specific to Knodus longus are available. Across the genus Knodus and the broader Stevardiinae, spawning is characteristically open and non-guarding: males pursue receptive females and eggs are scattered among fine-leaved vegetation or over substrate with no parental attention. Adhesive or semi-adhesive eggs settle among plant material after release.
Sexual dimorphism in the genus is typically subtle — females may be slightly deeper-bodied when gravid — and courtship is brief. Because K. longus has not been reported in the hobby, captive observations do not supplement the field record.
Breeding
Knodus longus has not been bred in captivity in any documented account accessible through secondary sources. Based on closely related Knodus and general stevardine biology, the expected reproductive mode is egg scattering over fine-leaved aquatic plants or substrate, with small, adhesive or slightly adhesive eggs and no parental care; adults are presumed egg-eaters and would require separation from spawn.
Incubation time and fry development data are not available for this species. Aquarists working with congeners typically condition fish on live and frozen foods, raise temperature slightly into the upper range of the species' tolerance, and provide spawning mops or fine-leaved plants to collect eggs for separate rearing.
In the aquarium
Knodus longus is not established in the aquarium hobby and is unlikely to be encountered in the trade. No dedicated husbandry profile has been published in hobbyist literature for this species specifically. General care principles for small Knodus tetras apply: a well-filtered, well-oxygenated aquarium with soft to moderately hard, slightly acidic to neutral water, kept in the range typical for Amazonian stream tetras (roughly 72–82 °F), and maintained as a group of six or more individuals to support the species' schooling behaviour.
A long tank with open swimming space and areas of fine-leaved vegetation or floating plants would suit the genus. Compatible tankmates include similarly sized, peaceful characids and small catfishes from the same biogeographic region. Because population data and wild-collection pressure for K. longus are unknown, sourcing tank-bred specimens — should the species ever become available — would be preferable.
Conservation
Knodus longus has not been assessed by the IUCN Red List and carries no formal conservation status. The majority of small, recently described Amazonian tetras in genera such as Knodus fall into this gap: described from preserved museum specimens, they are seldom encountered in subsequent fieldwork and the size and trajectory of their wild populations are unknown.
The Amazon and its tributaries face continuing pressure from deforestation, agricultural runoff, mercury pollution from artisanal gold mining, and large-scale hydroelectric development — threats that affect stream-dwelling small characids broadly. Without a targeted assessment, the actual conservation situation of K. longus cannot be determined. Its obscurity in the hobby means no captive-assurance population exists.