Taxonomy & naming
Creagrutus runa was described by Richard P. Vari and Alison S. Harold in their 2001 Smithsonian Contributions to Zoology monograph, a comprehensive phylogenetic study and revision of the Neotropical fish genera Creagrutus Günther and Piabina Reinhardt. That work, spanning 239 pages, remains the definitive treatment of the cis-Andean Creagrutus and established the valid placement of dozens of species, including C. runa.
The genus Creagrutus was erected by Günther in the nineteenth century and belongs to the order Characiformes. Its placement at the family level has been discussed in the literature — some classifications nest Creagrutus within Stevardiidae, a family elevated from the former Characidae subfamily Stevardiinae, while Eschmeyer's Catalog of Fishes and many treatments retain a broader Characidae arrangement. The top-level family recorded here follows the Catalog's authority. No currently accepted common name is widely applied to Creagrutus runa in the aquarium hobby or scientific literature.
Morphology
Creagrutus runa is a small, elongate characin reaching a maximum recorded standard length of 2.5 in. Like other members of the genus it has the general body plan typical of small stream-dwelling characins: a moderately compressed, fusiform body, a terminal mouth, and the characteristic adipose fin of the Characiformes positioned between the dorsal and caudal fins.
Detailed diagnostic characters for C. runa were established in the 2001 Vari and Harold revision and include meristic and morphometric data used to distinguish it from its congeners. In life, small Creagrutus species often show a pale or silvery ground colour with a diffuse lateral stripe and translucent fins. Specific colour notes for C. runa in life are not comprehensively documented in accessible sources; the data-sparse record reflects the species' obscurity outside specialist ichthyological literature.
Habitat
Creagrutus runa is recorded from the upper Negro River basin in South America. The Negro, a major left-bank tributary of the Amazon, drains an enormous area of northwestern Brazil, southern Venezuela, and eastern Colombia, and its upper reaches include a mosaic of blackwater streams, forest creeks, and flooded gallery forest habitats.
FishBase categorises the species as benthopelagic and freshwater, occupying tropical waters. Small Creagrutus in the Neotropics typically inhabit fast-flowing forest streams with sandy or gravelly substrates, often beneath riparian cover where leaf litter and root tangles provide shelter. Water conditions in the upper Negro system tend toward warm, soft, and acidic — low pH, minimal hardness, and conductivities far below temperate standards — characteristic of the nutrient-poor, tannin-stained blackwater drainages of the Amazon basin.
Feeding
Detailed dietary studies specific to Creagrutus runa are not available in accessible sources. As a genus, Creagrutus species are generally regarded as opportunistic carnivores or omnivores, taking small invertebrates from the water column and substrate. Gut-content analyses of related species in similar habitats have recorded aquatic insect larvae, small crustaceans, terrestrial invertebrates that fall onto the water surface, and occasionally small amounts of plant or algal material.
In the aquarium, small characins of this type are typically undemanding feeders that accept live and frozen invertebrates — Daphnia, brine shrimp nauplii, micro-worms — as well as high-quality micro-pellets. A varied diet is advisable to maintain condition, though specific feeding observations for captive C. runa are lacking in the literature currently reviewed.
Mating
No published observations of mating behaviour specific to Creagrutus runa are available. By analogy with related characins and the broader Stevardiidae-Characidae complex, mating likely involves egg-scattering over fine substrates or vegetation, with males pursuing ripe females. Courtship in small stream characins is often brief and opportunistic, without elaborate nest-building or long-term pair formation.
Sexual dimorphism in Creagrutus species, where described, is subtle; females are typically deeper-bodied when gravid. Data on reproductive seasonality for C. runa in the upper Negro basin are not documented in the sources reviewed.
Breeding
Captive breeding of Creagrutus runa has not been documented in accessible hobby or scientific literature. The species appears to be absent from the ornamental fish trade, and no breeding accounts are available to draw on.
Based on the biology of related small characins, spawning would be expected to follow an egg-scattering pattern: adhesive or semi-adhesive eggs deposited among fine-leaved plants or over substrate, no parental care, and adults that will consume eggs given the opportunity. Incubation and larval development periods are not recorded for this species. The sparse captive record means all breeding information must be regarded as inference from congeners rather than observed fact.
In the aquarium
Creagrutus runa is essentially unknown in the ornamental fish trade and is not kept as an aquarium fish in any documented mainstream context. It is a species of ichthyological and biogeographic interest rather than a hobby fish, and no husbandry recommendations specific to it are established.
For aquarists who encounter small Creagrutus species through the specialist trade or field collection, the blackwater origin of the upper Negro basin provides the main husbandry guidance: warm, soft, acidic water (low pH, minimal hardness, temperatures in the mid-to-upper 20s Celsius), subdued lighting, and a well-planted or well-decorated tank with hiding places. Given the data-sparse record, keepers should treat detailed care requirements as provisional and adjust based on observed fish behaviour and condition.
Conservation
Creagrutus runa is assessed as Least Concern (LC) on the IUCN Red List, with the assessment completed in March 2021. The LC designation reflects a distribution within the large and relatively intact upper Negro River basin, where major industrial-scale habitat destruction has not yet reached the levels seen in parts of the Amazon's southern and eastern drainages.
Nevertheless, the upper Negro basin faces ongoing pressures including illegal mining activity, forest clearance, and the broader trajectory of Amazon basin development. Species with restricted ranges within the basin, or those dependent on specific microhabitats, can be vulnerable even within an overall LC assessment. No targeted conservation programme for C. runa is documented; the species' welfare is bound to the broader fate of the blackwater forests and streams of the northwestern Amazon.