Taxonomy & naming
Hyphessobrycon notidanos was described by Carvalho and Bertaco in 2006 in the journal Neotropical Ichthyology (volume 4, number 3). The type locality is the Rio Doze de Outubro, a tributary of the Rio Juruena near Comodoro, Mato Grosso, Brazil (12°58′39″S 60°00′30″W), placing it within the upper rio Tapajós basin. The holotype (MCP 39511) is a male of 1 in standard length. The species is placed in the large, heterogeneous genus Hyphessobrycon within the family Characidae, which remains the accepted assignment under the Catalog of Fishes.
The specific epithet notidanos is derived from Greek, meaning 'having a pointed dorsal fin', a direct reference to the male's elongate dorsal-fin rays that are the most immediately visible diagnostic character of the species. Among the suite of meristic characters used in the original description, a dorsal fin count of iii,8 rays is noted as diagnostic, along with 17–20 anal-fin rays and a vertically elongate humeral spot. Because the species was described within Hyphessobrycon and the genus assignment has not changed, no parenthetical is used around the authority.
Morphology
Hyphessobrycon notidanos is a small, laterally compressed characid reaching approximately 1 in standard length (FishBase; aquarium collections typically show individuals up to 1 in). The body is comparatively deep, consistent with the broader Hyphessobrycon body plan. The caudal fin is forked, and a small adipose fin is present. The humeral spot is vertically elongate — another feature noted in the original description — and a longitudinal stripe runs from the orbital margin to the tip of the caudal fin. A dark stripe is also present on the four to five median caudal rays.
Two wild colour morphs are documented and occur together in the type locality: a red-bodied form with a blue iris, and a yellow-bodied form with a red iris (Aquarium Glaser). Both morphs share the same basic pattern of fin coloration and stripe placement; the body and iris colours are the distinguishing feature. Males carry the species' signature elongate dorsal fin, which can reach nearly to the adipose fin; females are generally smaller and less intensely coloured, lacking the elongate dorsal.
Habitat
The species is known only from the Rio Doze de Outubro and the immediate surrounding area within the upper Rio Tapajós drainage, Mato Grosso, Brazil. The original description records its occurrence in semilentic stretches of a shallow, clear-water river with a substrate of sand and scattered stones. Aquatic vegetation is scarce in the known habitat.
Water conditions at the type locality are consistent with clear-water Amazonian tributaries: soft to moderately mineralised, likely slightly acidic to neutral, and warm — FishBase records the type locality as tropical with temperatures around 79 °F. The restricted known range — a single tributary in a single drainage — means that very little ecological data beyond the original collection is available, and the species' full distributional extent remains uncertain.
Feeding
No published dietary study specific to Hyphessobrycon notidanos is available. As a small, active characid from a clear-water Amazonian stream, it is presumed to be a micro-predator of invertebrates in the wild — small insects, larvae, and crustaceans of the size appropriate to a fish of under 1 in.
In the aquarium, Aquarium Glaser's husbandry notes record that the species accepts all common small food sizes readily, which is consistent with the typical Hyphessobrycon feeding habit. Good-quality small flake or micro-pellets, supplemented with frozen or live Daphnia, Cyclops, baby brine shrimp, or micro-worms, will satisfy it in captivity. The species is not known to be finicky.
Mating
Hyphessobrycon notidanos is presumed to follow the egg-scattering open-spawning strategy typical of the genus and the broader Characidae. No detailed field study of mating behaviour at the type locality has been published. Males in good condition display their elongate dorsal fins, which are likely used as a visual signal to females and rival males, though no formal ethological description of this species' display has appeared in the available literature.
In captivity, males should be expected to court females by displaying extended fins and chasing; conditioning on live or frozen foods and providing fine-leaved plants or spawning mops will encourage spawning behaviour consistent with that seen in related Hyphessobrycon species.
Breeding
Breeding in captivity follows the standard Hyphessobrycon pattern: an egg-scattering open spawner with adhesive eggs deposited among fine-leaved plants or synthetic spawning mops and no parental care. The original description does not record the presence of a gill gland in males — a structure seen in some Characidae and sometimes associated with breeding pheromones — and no specialised breeding behaviour beyond typical scatter-spawning has been documented for this species.
For aquarium breeding, conditioning males and females separately on live and frozen foods before introducing them to a prepared spawning tank with a substrate of fine plants or mops and shallow, warm, slightly soft to neutral water is the recommended approach. Adults will consume their own eggs and should be removed after spawning. Fry are tiny and require infusoria or commercial liquid fry food as first foods, graduating to newly hatched brine shrimp as they grow.
In the aquarium
Hyphessobrycon notidanos is described as completely peaceful and easy to keep by hobbyists who have worked with it (Aquarium Glaser). Its small size — under 1 in — makes it suitable for nano or species-focused small-fish aquaria, and it does not damage plants. A group of six or more will allow the species to school naturally and encourage males to display, showing off the elongate dorsal and body colour to best effect.
Water conditions should reflect the species' clear-water Amazonian origins: warm (around 75–82 °F), soft to moderately soft, slightly acidic to neutral water with good filtration and low nitrates suits it well. A planted tank with dim to moderate lighting and dark substrate will bring out the red or yellow body colours. Tankmates should be similarly small and peaceful; it should not be kept with anything large enough to treat it as prey. The two naturally occurring colour forms — red-body/blue-iris and yellow-body/red-iris — can be kept together, and keeping both adds visual interest to the display.
Conservation
Hyphessobrycon notidanos is listed as Data Deficient (DD) on the IUCN Red List, with the assessment dating from 7 November 2018 (FishBase). A Data Deficient designation indicates that insufficient information is available to assess extinction risk directly, rather than that the species is not at risk. Given that the species is known only from a single tributary in a restricted area of Mato Grosso, and was described as recently as 2006, this limited distributional knowledge is the primary reason for the DD status.
No targeted conservation assessment or recovery plan is known to exist for this species. The upper Rio Tapajós region of Mato Grosso faces ongoing pressures from deforestation, agricultural runoff, and altered hydrology in parts of the Brazilian Cerrado-Amazon transition zone, though the direct impact on this species' specific locality has not been formally evaluated. Aquarists working with tank-bred stock reduce any pressure on the wild population, and documenting breeding successes in captivity is a practical contribution to knowledge of the species.