Taxonomy & naming
Creagrutus ignotus was described by Richard P. Vari and Allison S. Harold in 2001 in a systematic revision of the genus Creagrutus. The type locality is a tributary to the Rio Preto in the state of Mato Grosso, Brazil, within the upper Tapajós River drainage. The valid name, as recorded in Eschmeyer's Catalog of Fishes, is Creagrutus ignotus Vari & Harold, 2001; the species has not been placed in synonymy or transferred to another genus.
Creagrutus belongs to a diverse and taxonomically complex assemblage of small Neotropical characids. The genus was long placed among the incertae sedis groups within Characidae — taxa that could not be confidently assigned to a subfamily — a situation that persisted through several major reviews of the family. Modern phylogenetic analyses have progressively resolved these placements, but Creagrutus and its close relatives remain a focus of ongoing revisionary work. Vari and Harold's 2001 treatment was a major step in organising the genus, formalising many previously unnamed species from across northern and central South America.
Morphology
Creagrutus ignotus is a small fish, with a maximum reported standard length of 2 in (Reis et al.) and a size up to 2.5 in SL noted by FishBase. The body is elongate and moderately compressed, typical of the genus, with a pointed snout and a terminal to slightly subterminal mouth. Like other Creagrutus, it is a uniform, rather subdued fish in colouration — silvery on the flanks with darker pigmentation along the dorsal surface, a mid-lateral stripe that may be indistinct, and hyaline fins.
Creagrutus species as a group are difficult to distinguish externally, sharing a broadly similar body plan and colour pattern; their identification relies on a combination of meristic characters (scale and fin-ray counts), osteological features, and details of the lateral line system that are not always apparent in preserved or live specimens. No distinctive external features that separate C. ignotus from its congeners in the field have been widely publicised in the hobby or popular literature.
Habitat
The known distribution of Creagrutus ignotus is the upper Tapajós River basin in Mato Grosso, Brazil. The type locality is a small tributary to the Rio Preto, a stream within this drainage. The Tapajós system drains a large area of the Brazilian Shield, and its upper tributaries in Mato Grosso are characterised by clear, well-oxygenated water running over rocky and sandy substrates, typically at moderate elevations.
FishBase classifies the species as benthopelagic — occupying the water column just above the substrate — which is consistent with the foraging ecology of other Creagrutus. Specific water-chemistry data for the type locality of C. ignotus have not been widely documented; given its drainage, the water is likely soft and acidic to neutral, with low conductivity, as is typical for clearwater tributaries of the upper Tapajós. Data on its range are sparse, and whether it occurs beyond the Tapajós headwaters remains uncertain.
Feeding
No species-specific feeding studies have been published for Creagrutus ignotus. Members of the genus are generally considered omnivorous micropredators, feeding on small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, and surface-fallen insects — alongside some algal and plant material. This generalist strategy is characteristic of small characids occupying stream habitats across Amazonia.
In any aquarium setting, a species of this size and body form would be expected to accept small live and frozen invertebrate foods readily, alongside fine-grade prepared foods. However, C. ignotus is not an established aquarium fish, and observations on captive feeding behaviour are essentially absent from the literature.
Mating
No direct observations of mating behaviour have been published for Creagrutus ignotus. On the basis of its phylogenetic position and the biology of closely related small Neotropical characids, it is inferred to be an egg-scattering open spawner with no parental care. Males of Creagrutus and related genera do not show conspicuous secondary sexual characters, though subtle differences in fin morphology between sexes have been noted in some congeners.
In the wild, spawning in small-stream characids of this type likely coincides with seasonal hydrological cues — rising water levels at the start of the wet season often trigger reproductive activity in Amazonian fishes — but no field data specific to C. ignotus are available.
Breeding
Creagrutus ignotus has not been reported as bred in captivity in the aquarium literature. Extrapolating from the reproductive biology of the genus and of comparable small characids: spawning would be expected to involve the scattering of adhesive or semi-adhesive eggs among fine-leaved plants or over substrate, with no guarding of eggs or care of larvae by either parent.
As with most stream-dwelling small characids, incubation would likely be brief at tropical temperatures, with fry requiring very small first foods. Because the species is not in hobby circulation, no captive-breeding data — clutch size, incubation period, fry development rate — are documented.
In the aquarium
Creagrutus ignotus is not an established aquarium species and is not in regular trade. It has been collected only from its type drainage, and no specialist importation or captive-breeding programme has brought it into the hobby at any scale. For a hobbyist who did obtain specimens — from a scientific collection or a specialist collector working in Mato Grosso — husbandry would broadly follow that of other small stream-dwelling characids: a well-oxygenated tank with moderate flow, soft to neutral water, a sandy or fine-gravel substrate, and cover in the form of leaf litter or fine plants.
As a benthopelagic insectivore it would need small live or frozen invertebrate foods in addition to any prepared diet, and it would likely do best in a species or genus tank or with similarly sized, non-aggressive companions. Given the data gaps and the species' obscure status, it is best considered a fish of specialist and scientific interest rather than a mainstream aquarium subject.
Conservation
Creagrutus ignotus has not been assessed for the IUCN Red List. No conservation status, threat analysis, or population estimate has been published for this species. The lack of assessment is common for the majority of Creagrutus species, many of which are known from only a handful of specimens and restricted type localities.
The upper Tapajós basin faces pressure from agricultural expansion, gold mining, and associated siltation and water-quality degradation — threats that affect many stream-associated fishes in Mato Grosso. Whether C. ignotus is resilient to such changes, or whether it is highly localised and therefore vulnerable, is unknown. Additional fieldwork and collection effort in the upper Tapajós would be needed to determine the true extent of its range and population status.