Taxonomy & naming
Creagrutus varii was described by Ribeiro, Benine, and Figueiredo in 2004, in a paper published in the Journal of Fish Biology (volume 64, issue 2). The type locality is the Rio São Marcos basin, part of the upper Rio Paraná system in Goiás State, Brazil. The valid name is recorded in Eschmeyer's Catalog of Fishes as Creagrutus varii Ribeiro, Benine & Figueiredo, 2004.
The genus Creagrutus belongs to the family Characidae and is placed within the subfamily-level assemblage sometimes treated as Stevardiinae or in related groupings — FishBase lists the species under Stevardiidae, reflecting ongoing debate over the circumscription of these clades within the order Characiformes. Morphological diagnosis of C. varii in the original description rests on characters of the jaw complex, including a slender anterior process of the maxilla, along with specific skeletal characters and a vertebral count of 37 to 38. The genus Creagrutus encompasses numerous small, slender characins distributed across South American river systems from Panama to southern Brazil, many of them narrowly endemic.
The specific epithet varii honours Richard P. Vari (1949–2016) of the Smithsonian Institution's National Museum of Natural History, one of the most prolific and influential students of South American freshwater fishes, whose revisions of Creagrutus and many other characiform genera transformed the systematics of the group.
Morphology
Creagrutus varii is a small, elongate, laterally compressed characin with a maximum recorded standard length of 1.5 in. Like other members of the genus it has the streamlined, fusiform body plan typical of free-swimming small characins, with a relatively small head and terminal mouth equipped with fine teeth on both jaws. Vertebral count in the type series is 37 to 38, a character used in the original description's diagnosis.
Specific external colour pattern details are not extensively documented in accessible literature for this species; as with many Creagrutus, the preserved coloration is likely a pale-to-silvery background with a dark lateral stripe or humeral spot, features common across the genus. Sexual dimorphism, if present, has not been documented in published sources available for this account.
Habitat
Creagrutus varii is known from the upper Rio Paranaíba basin, part of the upper Rio Paraná drainage, in Goiás State, Brazil. The type locality falls within the Rio São Marcos sub-basin. This region is characterised by cerrado (Brazilian savanna) and transitional forest catchments at elevated interior plateau elevations, with clear to slightly turbid, generally soft and acidic to near-neutral streams.
Water parameters specific to C. varii collection sites have not been published in sources available for this account. Congeners from comparable Brazilian interior basins, and the broader Creagrutus genus from tropical South American rivers, typically occur in warm, soft, moderately acidic freshwater. A related congener, Creagrutus beni, has been recorded at temperatures of 72–79 °F and pH 6.0–7.2 with hardness of 0–20 dH, which may approximate conditions in similar interior-basin habitats, though these parameters should not be taken as established data for C. varii itself. FishBase categorises the species as freshwater, benthopelagic, and tropical.
Feeding
No published dietary data specific to Creagrutus varii are available in sources reviewed for this account. The genus Creagrutus is broadly characterised as insectivorous and invertivore, taking small aquatic invertebrates, terrestrial insects falling onto the water surface, and microcrustaceans — a feeding guild typical of small characins occupying the mid-water and benthic-margin zones of South American streams.
In an aquarium setting, a fish of this size and body plan would be expected to accept small live and frozen foods such as Daphnia, brine shrimp nauplii, micro-worm, and similarly sized invertebrates, as well as fine-ground flake or micro-pellet foods. Definitive diet data await field studies of this species.
Mating
No published observations of mating behaviour in Creagrutus varii exist in the literature reviewed for this account. Based on the reproductive biology of the broader family Characidae and the genus Creagrutus, egg-scattering open-water spawning with no parental care is the expected mode. Males of small characins typically court females with lateral displays and brief chasing, with spawning occurring in open water or near fine-leaved vegetation.
Adhesive or semi-adhesive eggs are likely deposited among aquatic vegetation or over substrate, where they receive no further attention from the parents. This generalisation is drawn from congeners and family-level biology rather than direct observation of C. varii, and field or captive breeding records for this species remain unpublished.
Breeding
Captive breeding of Creagrutus varii has not been documented in the hobbyist or scientific literature reviewed for this account. As a small, wild-caught, narrowly endemic species from interior Brazil, it is essentially absent from the ornamental fish trade and is unknown in general hobby fishkeeping.
By analogy with the family and genus, spawning would be expected to involve egg-scattering over fine-leaved plants or substrate, with adhesive or semi-adhesive eggs and no parental care, requiring separation of eggs or adults to achieve fry survival in captivity. Water temperature for breeding would plausibly fall in the warm-tropical range typical of the basin. Reliable breeding data await captive work with this species.
In the aquarium
Creagrutus varii is not established in the ornamental fish trade and is not available to hobbyists through normal commercial channels. It is a scientifically described species known from museum specimens and field collections rather than from aquarium experience. Any account of its suitability as an aquarium fish would therefore be speculative.
If encountered in specialised collections, its small size (to 1.5 in SL), likely peaceful temperament typical of small characins, and presumed preference for warm, soft, slightly acidic water would make it broadly comparable in care to other small stream characins. Groups would likely be more comfortable than solitary individuals. Its conservation status as Vulnerable underlines that wild collection would be inappropriate; the species is noted here as a scientifically significant endemic, not as a recommended aquarium fish.
Conservation
Creagrutus varii is assessed as Vulnerable (VU) on the IUCN Red List under criterion B1ab(iii), based on an assessment dated 7 November 2018. The B1 criterion reflects a restricted extent of occurrence, and the b(iii) qualifier indicates continuing decline in the quality of its habitat. The species is confined to the upper Rio Paranaíba basin in Goiás State — a small portion of the upper Rio Paraná system — giving it almost no geographic buffer against local threats.
The upper Paraná plateau in Goiás is one of Brazil's most intensively agricultural regions, with extensive soy and cattle-ranching activities that affect stream water quality through sedimentation, agrochemical runoff, and riparian vegetation clearance. The basin has also been heavily modified by hydroelectric infrastructure. These pressures reduce the area and quality of suitable habitat for a benthic or benthopelagic small-stream fish such as C. varii, which appears to have a naturally limited distribution even before human modification. Targeted surveys and habitat protection within the Rio São Marcos sub-basin would be the most direct conservation interventions for this species.