Taxonomy & naming
Creagrutus ungulus was described by Richard P. Vari and Andrew S. Harold in their 2001 monograph on the genus Creagrutus, published in the Smithsonian Contributions to Zoology (volume 613). That work — spanning 239 pages and revising the entire genus — remains the foundational reference for species delimitation in Creagrutus. The valid name as governed by Eschmeyer's Catalog of Fishes is Creagrutus ungulus Vari & Harold, 2001. The type locality is the Soga Ravine, a tributary of the Madre de Dios River at approximately 12°53'S, 71°12'W in Manu Province, Peru.
Creagrutus belongs to the family Characidae, the largest family of freshwater fishes, and to a radiation of small, often cryptically coloured characids distributed across piedmont and foothill streams from Panama to Argentina. The genus is notably diverse and identification of individual species often requires careful meristic and morphological comparison — making field identification of Creagrutus to species level a task for specialists.
Morphology
Creagrutus ungulus reaches a maximum recorded length of 4.5 in total length (TL), making it one of the larger members of its genus, though most individuals are likely smaller. The body plan is that of a typical small-stream characid: moderately elongate and laterally compressed, with the rounded snout and terminal or subterminal mouth associated with invertebrate feeding in fast-flowing waters. Colouration in preserved material is not strongly marked, as is typical for the genus, but in life these fish show the silver flanks and darker dorsal surface common to clear-water stream characids.
A notable feature documented in the Vari and Harold 2001 revision is that Creagrutus is the first genus within its lineage known to produce breeding tubercles with keratinized caps — structures associated with tactile contact during spawning that are better known from cyprinids than from characids. This detail makes the genus of particular interest to students of fish reproductive biology.
Habitat
Creagrutus ungulus inhabits small, fast-flowing rainforest streams in the Madre de Dios and Urubamba river basins of Peru, at elevations of roughly 656–2297 ft above sea level. Per the original description, the preferred microhabitat is clear-water, swift-flowing streams with rocky substrates — the piedmont and foothill streams of the Amazonian Andes flanks, where gradient and flow are higher than in the main lowland channels. Part of the species' range falls within Manu National Park, one of the most biologically rich protected areas in South America.
Water temperatures at the type locality area have been recorded between 18.6 and 78 °F, reflecting the cooler conditions of foothill streams relative to the Amazonian lowlands proper. The clear water and rocky substrate suggest a well-oxygenated, oligotrophic environment typical of Andean piedmont drainages.
Feeding
FishBase records a trophic level of 2.2 for Creagrutus ungulus, placing it near the base of the food web as a consumer of primary producers, small invertebrates, and organic detritus rather than a predator of other fish. Related species in the genus, such as Creagrutus brevipinnis, have been documented feeding primarily on Diptera (flies and their larvae), algae, and seeds — a diet well suited to the benthopelagic foraging habit assigned to the species by FishBase.
In fast-flowing, rocky streams this translates to picking drifting invertebrates and surface-fallen matter from the current and gleaning algae and insect larvae from the substrate. The combination of animal and plant material makes Creagrutus an opportunistic omnivore that exploits whatever the stream delivers, a dietary flexibility common across the genus and the Characidae more broadly.
Mating
No specific courtship observations have been published for Creagrutus ungulus in the wild or in captivity. Based on what is known for the genus Creagrutus and the Characidae broadly, the species is expected to be an egg-scattering open spawner without parental care — the ancestral reproductive mode of the family. The keratinized breeding tubercles documented for Creagrutus by Vari and Harold suggest that tactile contact plays a role in spawning behaviour, with males and females pressing against one another as eggs are released and fertilised.
Breeding in related Creagrutus species is reported to occur in both wet and dry seasons, suggesting that spawning is not strictly tied to a single seasonal flood pulse but may instead track local microhabitat conditions such as water temperature and flow. No spawning photographs or aquarium breeding accounts for this species are known to the editors.
Breeding
Creagrutus ungulus has not been bred in captivity under documented conditions, and no published spawning accounts are available. By family-wide analogy with the Characidae, the species is expected to scatter small adhesive eggs among fine-leaved plants or over rocky substrate, with no parental care and both adults likely to consume the eggs if given the opportunity.
The notable presence of keratinized breeding tubercles in the genus — a feature Vari and Harold (2001) identified as unique among the relatives of Creagrutus at the time of description — suggests a more elaborate contact-based spawning than is typical for the most distantly related tetras, but the functional significance has not been studied in aquarium conditions. Aquarists wishing to attempt breeding would be advised to replicate the species' cool, well-oxygenated, clear-water habitat conditions (64–77 °F, neutral to slightly acidic pH) with ample current and a rocky or fine-leaved spawning medium.
In the aquarium
Creagrutus ungulus is essentially unknown in the aquarium hobby. It is not a species that appears in the trade under any common name, and captive husbandry notes are absent from the hobby literature. What can be inferred from its natural habitat applies: the fish comes from cool, fast-flowing, well-oxygenated Andean piedmont streams with clear water, rocky substrates, and temperatures in the 64–77 °F range, so an aquarium replicating those conditions — strong filtration and current, cooler temperatures than are typical for most tropical community tanks, neutral to slightly acidic water chemistry — would be most appropriate.
Being a small-stream species from relatively undisturbed habitats, Creagrutus ungulus would benefit from a quiet, species-focused setup rather than a busy community tank. Its benthopelagic lifestyle suggests it would appreciate open mid-water swimming space alongside substrate cover. Feeding should be straightforward given the genus's omnivorous tendency: small invertebrate foods, quality small pellets, and frozen Daphnia or bloodworm would likely all be accepted. The species' near-absence from commerce means any specimens encountered would almost certainly be wild-caught and should be sourced with that in mind.
Conservation
Creagrutus ungulus is assessed as Least Concern (LC) on the IUCN Red List, with the assessment last conducted in April 2014 and published in 2016 by assessors J. Chuctaya and E. Correa (reviewed by M. Tognelli). The assessment notes a stable population with no major identified threats. A significant portion of the species' range falls within Manu National Park in Peru, one of the largest protected areas in the Amazon basin and recognised as a UNESCO World Heritage Site, which provides substantial de facto protection for the foothill stream habitats the species occupies.
The Madre de Dios and Urubamba basins face ongoing pressures from illegal gold mining, agricultural expansion, and road development — threats that affect water quality and stream integrity across southeastern Peru. While these pressures have not yet placed Creagrutus ungulus in a threatened category, foothill stream specialists with restricted ranges are inherently vulnerable to catchment-scale habitat change, and the species warrants continued monitoring as land use in the region intensifies.