Taxonomy & naming
Creagrutus gephyrus was described by James E. Böhlke and William G. Saul in 1975, in a paper treating the characid fish genus Creagrudite (then a synonym of Creagrutus) and describing new species from Amazonian Ecuador, published in the Proceedings of the Academy of Natural Sciences of Philadelphia (volume 127, no. 3: 25–28). The type locality is the Aguarico River at Santa Cecilia, Napo Province, Ecuador. The name Creagrutus gephyrus Böhlke & Saul, 1975 is the valid combination as currently recognised by Eschmeyer's Catalog of Fishes (Catalog of Fishes, CAS).
The genus Creagrutus is placed by some authorities within the subfamily Stevardiinae (formerly treated under Characidae sensu lato) or within Stevardiidae depending on the higher-level classification used; FishBase lists it under Stevardiidae > Creagrutinae. Regardless of the family-level assignment, Creagrutus is a large, primarily Andean-foothill genus of small characins. The definitive systematic treatment of the cis-Andean members, including C. gephyrus, is the monograph by Richard P. Vari and Andrew S. Harold (2001), published as Smithsonian Contributions to Zoology 613 (1–239), which provides detailed redescriptions and a phylogenetic framework for the group.
The genus name Creagrutus derives from the Greek kreagreytos, meaning 'to tear or rip flesh', a reference to the group's dentition. The specific epithet gephyrus is from the Greek gephyra, meaning 'bridge', likely alluding to the species' position bridging parts of the genus' distribution or to a geographic feature at the type locality; the precise reason for the epithet is not stated explicitly in the original description.
Morphology
Creagrutus gephyrus is a small characin reaching a maximum recorded standard length of 2.5 in (approximately 2.6 in), based on male or unsexed specimens in the FishBase database. Like other members of the genus it is a slender to moderately built fish with the typical characiform body plan: a laterally compressed, fusiform outline, a small adipose fin behind the dorsal, and a forked caudal fin. The mouth is subterminal, suited to picking food from the water column and from substrate in benthopelagic fashion.
No published photographs or drawings of C. gephyrus are widely available in accessible literature, reflecting the species' obscurity in both the trade and in hobbyist-facing ichthyological resources. Coloration details for living specimens are not captured in the sources available for this article; the species' appearance in life must be inferred from the genus-level pattern of silvery, lightly marked small characins. Sexual dimorphism has not been documented in sources accessible here.
Habitat
Creagrutus gephyrus inhabits freshwater rivers of the Andean piedmont in eastern Ecuador and northeastern Peru, primarily within the Napo River basin (Ecuador) and the Marañón and Amazon basins (Peru). The IUCN assessment (2016) places the species at elevations of 656–2625 ft above sea level — a mid-altitude Andean foothill zone where rivers are typically swift to moderately flowing, cool relative to lowland Amazonia, and carry sediment loads characteristic of white-water Andean drainages. The IUCN notes the species as occurring in slow to moderate flowing white-water rivers with a pelagic to benthopelagic lifestyle.
Within Ecuador the species is known from the Napo Province and has been recorded in the vicinity of Yasuní National Park and the Cuyabeno and Sumaco protected areas — habitats that support exceptional fish diversity, as documented by recent ichthyological surveys including the 2025 study by Escobar-Camacho and colleagues in the Biodiversity Data Journal. These protected areas provide a degree of buffer against habitat loss that many unprotected Andean foothill streams lack.
Feeding
The IUCN assessment describes Creagrutus gephyrus as insectivorous. The FishBase trophic level for the species is recorded at 2.4, consistent with a diet based on small invertebrates rather than plant material or higher trophic prey. As with other small Andean piedmont characins, the diet likely centres on aquatic insect larvae, terrestrial insects that fall onto the water surface, and other small invertebrates encountered in the water column and near the substrate.
No detailed gut-content studies for C. gephyrus specifically are cited in the available sources. Aquarium feeding data are similarly absent from accessible literature, as the species is rarely if ever maintained in captivity as a deliberate specimen. In a captive setting the diet of small, similar insectivorous characins typically transitions readily to small live and frozen invertebrates (Daphnia, Artemia, bloodworm, chironomid larvae) and fine-particle prepared foods.
Mating
No species-specific observations of mating behaviour in Creagrutus gephyrus have been documented in the literature accessible for this article. Like the overwhelming majority of small characins in the tribe or subfamily to which Creagrutus belongs, the species is presumed to be an egg-scattering open spawner without parental care, releasing adhesive or semi-adhesive eggs among fine-leaved aquatic plants or submerged vegetation in flowing water. Sexual signals, courtship display, or any special spawning behaviour specific to this species remain undescribed.
Seasonal reproductive cues in Andean foothill streams are typically tied to the hydrological cycle — rising water levels associated with wet-season flooding often trigger spawning in lowland and foothill characins — but no seasonality data have been published for C. gephyrus specifically.
Breeding
No captive breeding records for Creagrutus gephyrus are available in the literature or hobby sources surveyed for this article. The species does not appear to be held in captivity with any regularity, and no aquarium reproductive data exist in accessible form.
By analogy with the genus and with small cis-Andean characins generally, breeding would be expected to follow the egg-scattering pattern: adhesive eggs deposited among fine-leaved plants or on the substrate in moderate current, with no parental care and rapid fry development at cool to moderate temperatures. Fry would require very small initial foods. These inferences are speculative in the absence of documented breeding attempts.
In the aquarium
Creagrutus gephyrus is essentially unknown in the aquarium trade and is not deliberately kept as an aquarium fish. Its small size (to 2.5 in SL) would make it physically suitable for a modest-sized aquarium, but its availability is entirely absent from hobby channels, and its natural habitat — cool, moderately flowing Andean piedmont streams — would require attention to temperature and oxygenation that departs from standard tropical aquarium practice.
Any aquarist who encountered this species incidentally (for example, as bycatch in a collection of other Napo basin fishes) might house it in a well-oxygenated, flowing-water setup at temperatures toward the cooler end of the tropical range, with fine-leaved plants and a soft, sandy substrate. Diet of small live and frozen invertebrates would be appropriate. Data on compatibility with other species are lacking; small, non-aggressive tankmates of similar size would be the prudent choice.
Conservation
Creagrutus gephyrus is assessed as Least Concern (LC) on the IUCN Red List, in an assessment dated 25 April 2014 and published in 2016, conducted by Ortega Torres, Anderson, and Arguello. The assessment notes the species as frequent, with a population presumed stable. Its range spans two countries — Ecuador and Peru — and it occurs within multiple protected areas in Ecuador, including Cuyabeno Wildlife Reserve, Yasuní National Park, and Sumaco-Napo-Galeras National Park, all within the Napo River drainage.
Threats identified by the IUCN include agriculture, livestock, urbanisation, and pollution, but none of these is considered widespread enough across the species' range to drive a population decline at a scale that would warrant a threatened category. The principal conservation concern for Andean foothill characins as a group is the pace of Amazonian deforestation and agricultural encroachment on river catchments; C. gephyrus benefits from the protected-area coverage in its Ecuadorian range but faces less certain prospects in unprotected portions of its Peruvian range. No specific conservation measures have been implemented for this species.