Taxonomy & naming
Knodus geryi was described by Flávio Lima, Heraldo Britski, and Fábio Machado in 2004. The genus Knodus belongs to the family Characidae and is placed within the subfamily Stevardiinae (formerly treated under Tetragonopterinae), a diverse group of small Neotropical characins distributed widely across South American river basins. The type locality for K. geryi is the Rio do Peixe, a tributary of the upper Rio Paraguay basin in the state of Mato Grosso do Sul, Brazil; the holotype (MZUSP 83354, 2.5 in SL) is held at the Museu de Zoologia da Universidade de São Paulo.
Within Knodus, the species is set apart from congeners primarily by a lower scale count around the caudal peduncle — 10–12 scales versus 13–15 in most relatives — and by the distinctive paired caudal-fin blotches. Meristic details include 10 dorsal soft rays, 18–20 anal soft rays, and a vertebral count of 38–39. The valid name as governed by Eschmeyer's Catalog of Fishes is Knodus geryi Lima, Britski & Machado, 2004.
Morphology
Knodus geryi is a slender, moderately compressed characid reaching a maximum recorded standard length of 2.5 in. Like other members of Knodus it has the streamlined, fusiform body typical of small, current-adapted characins, with a small terminal mouth and a short lateral line.
The most immediately recognisable feature is the colour pattern: two large, symmetrical, dark blotches sit basally on the upper and lower lobes of the caudal fin, a character that is consistent and diagnostic within the genus. General body colouration in life is not fully documented from the literature in the cache, but the species follows the genus norm of a silvery to pale-yellowish ground colour with a lateral stripe of variable intensity. The 10–12 caudal-peduncle scales (versus the higher counts typical in congeners) are the key meristic diagnostic used by Lima et al. in the original description.
Habitat
The type locality is the Rio do Peixe in the upper Rio Paraguay drainage, Mato Grosso do Sul, Brazil — a clear, transparent river characterised by sections with rapids and waterfalls, with the collecting site described as a steep-sided channel approximately 5–8 metres wide and more than 2 metres deep. The water is described as transparent, suggesting low sediment load and a forested or well-vegetated catchment.
This type of fast, well-oxygenated, clear-water habitat is characteristic of many small Stevardiinae. Water-parameter data specific to the type locality are not captured in the available sources, but upper Rio Paraguay tributaries in Mato Grosso do Sul typically run warm (mid-to-high 20s Celsius) with near-neutral to slightly acidic pH and low to moderate hardness, reflecting the Cerrado and gallery-forest catchments of the region.
Feeding
FishBase records the diet of Knodus geryi as a combination of vegetal matter and insects, consistent with the opportunistic omnivory seen across Stevardiinae. In fast-flowing, clear-water habitats, small characins of this size class typically forage on allochthonous material — terrestrial insects and plant debris that fall into the river — as well as aquatic invertebrates, periphyton, and algae.
Detailed stomach-content analyses specific to this species are not available in the cited sources. In an aquarium context, a diet of good-quality small flake or micro-pellets supplemented with frozen or live foods such as Daphnia, brine shrimp nauplii, and Cyclops would align with the wild dietary guild.
Mating
No detailed field or laboratory studies of mating behaviour in Knodus geryi have been published in the sources available. The genus Knodus belongs to Stevardiinae, a subfamily in which the reproductive biology of most species follows the general characid pattern of broadcast egg-scattering with no parental investment.
Males of small Stevardiinae typically exhibit courtship pursuit of females, with spawning occurring among fine-leaved aquatic vegetation or over substrate. The species almost certainly produces adhesive or semi-adhesive eggs that settle among plants or gravel, as is typical of open-water egg-scattering characins at this body size. Specific courtship details for K. geryi await dedicated study.
Breeding
Captive breeding of Knodus geryi has not been documented in the hobby literature captured in the research cache, and this species appears to be essentially absent from the ornamental trade. Based on its placement in Stevardiinae and the general biology of small current-adapted characins, it would be expected to scatter small adhesive eggs among fine-leaved vegetation or over gravel, with no parental care and a short incubation period at tropical temperatures.
Should the species become available to aquarists, standard small-characin breeding protocols — a well-conditioned pair or group, a tank with spawning mops or fine-leaved plants, and separation of adults after spawning — would be a reasonable starting approach. First foods for larvae of species this size are typically infusoria or commercial micro-fry foods before graduation to newly hatched brine shrimp.
In the aquarium
Knodus geryi does not appear in the mainstream ornamental fish trade and is essentially unknown in the hobby. Practical Fishkeeping noted its description as a new species in 2004, but there is no evidence of commercial availability or established husbandry literature. Its habitat — a fast, clear, well-oxygenated river with rapids — suggests it would require good water movement and high dissolved oxygen in captivity.
As a small (2.5 in SL) characid from warm South American waters it would, in principle, suit a species tank or a community of similarly sized, peaceful fish maintained at tropical temperatures with moderate to strong current. Its close relatives in Stevardiinae are generally hardy and adaptable once established. Any specimens encountered would almost certainly be wild-caught, and care should be taken to match water quality to the clear, oxygen-rich conditions of the type locality.
Conservation
Knodus geryi is assessed as Least Concern (LC) on the IUCN Red List (2018). The LC assessment reflects a lack of documented major threats and a distribution within a river system that, at the time of assessment, was not under severe pressure. The upper Rio Paraguay basin in Mato Grosso do Sul does face ongoing pressures from agricultural expansion, deforestation of riparian zones, and water abstraction associated with Brazil's Cerrado agricultural frontier, but these have not been judged sufficient to qualify the species for a threatened category.
The species is not commercially fished and is not part of the ornamental trade, so harvest pressure is not a concern. Its restricted known range — a single river drainage — means that localised habitat degradation could pose a future risk, but current data do not support an elevated threat category. The IUCN assessment should be revisited as land-use change in the upper Paraguay catchment continues.