Taxonomy & naming
Creagrutus saxatilis was formally described by Richard P. Vari and Andrew S. Harold in their 2001 phylogenetic revision of the Neotropical genera Creagrutus and Piabina, published in the Smithsonian Contributions to Zoology (no. 613). The type specimen (USNM 341480, a 2 in paratype) was collected from the Rio Maranhão in the upper Tocantins basin, Brazil. The valid name is recorded in Eschmeyer's Catalog of Fishes as Creagrutus saxatilis Vari & Harold, 2001.
Creagrutus belongs to a large and taxonomically complex group of small Neotropical characins. Some reference databases, including GBIF, classify the genus within Stevardiidae, a family that has been recognised as distinct from Characidae by certain recent molecular and morphological analyses; the IUCN assessment follows Characidae in the broad sense. This article follows that treatment as the top-level family designation.
The species has been recorded sympatrically with the closely related Creagrutus menezesi within the upper Tocantins basin, and the 2001 monograph provides the only detailed comparative morphological treatment of the two.
Morphology
Creagrutus saxatilis is a small, elongate characin reaching a maximum recorded standard length of approximately 3 in SL, based on the largest specimen measured in the original description and confirmed by subsequent FishBase records (3 in SL). In body form it is typical of the genus: moderately slender, laterally compressed, with the blunt snout and large eye common to small rheophilic characins navigating fast, rocky current.
The original description in Vari and Harold (2001) provides a full morphological account, including meristic data for both the holotype and paratypes from the Rio Maranhão. As with many Creagrutus species, colouration in life is not extravagantly documented in the published literature; preserved specimens show the subdued, silvery-to-brown patterning typical of small stream characins. Distinguishing features from sympatric congeners are set out in detail in the 2001 monograph, which remains the primary taxonomic resource for the species.
Habitat
Creagrutus saxatilis is endemic to the upper Tocantins River basin, in the states of Goiás and the Distrito Federal, central Brazil. The type locality is the Rio Maranhão, a headwater tributary of the Tocantins system. The IUCN Red List characterises the species' preferred environment as high-gradient rocky streams — reophilic (current-adapted) habitats with stony substrates, which is consistent with the specific epithet saxatilis (rock-dweller in Latin).
The upper Tocantins basin in this region is dominated by cerrado (Brazilian savanna) vegetation, and the streams draining it experience seasonal fluctuations in flow and temperature. The Serra da Mesa hydroelectric reservoir, constructed in the upper Tocantins basin in Goiás, has inundated portions of what would have been suitable habitat; the IUCN assessment specifically identifies this dam as a threat, citing uncertainty about how its impoundment has affected subpopulations upstream and downstream of the reservoir.
Feeding
The natural diet of Creagrutus saxatilis is not exhaustively documented, but one significant data point comes from the IUCN assessment: stomach-content analysis of at least one specimen revealed crushed seeds, indicating that the species acts as a granivore — consuming seeds, at least opportunistically — in the wild. FishBase records a trophic level of 2.3, placing the species in the lower-level consumer range consistent with a diet of small invertebrates, plant material, and seeds rather than predation on other fish.
Like most small stream characins in the Neotropics, C. saxatilis likely takes a wide variety of food items that drift past in current or settle on the rocky streambed. In the very rare event it is maintained in captivity, it would be expected to accept small live and frozen invertebrates and fine-particle prepared foods, though no aquarium feeding records are available in the published or hobbyist literature.
Mating
No specific observations of mating behaviour in Creagrutus saxatilis have been published. By analogy with other small Neotropical characins of similar ecology and body form, the species is expected to be an egg-scattering open spawner that releases adhesive or semi-adhesive eggs among submerged stones, gravel, or fine-leaved vegetation in current, with no parental care provided after spawning.
Many rheophilic characins time spawning to coincide with seasonal hydrological cues — rising water levels at the onset of the rainy season trigger reproductive activity and distribute eggs and larvae across the floodplain or upstream into cooler, oxygen-rich headwater habitats. Whether C. saxatilis follows this pattern in the Tocantins headwaters has not been confirmed; the data gap is reflected in the IUCN's Data Deficient assessment.
Breeding
No captive-breeding records for Creagrutus saxatilis appear in the scientific or hobbyist literature; the species is essentially unknown in the aquarium trade. Its reproductive biology can be inferred only by analogy with other small, current-adapted characins: egg-scattering over a hard substrate (stones, coarse gravel) or among fine vegetation, adhesive eggs left without parental attention, and fry that must find shelter in interstitial spaces within the rocky stream bottom during early development.
Given the species' restriction to the cerrado headwaters of the upper Tocantins and its Data Deficient conservation status, it is unlikely to appear in the aquarium hobby in meaningful numbers. Any captive maintenance or breeding work would represent valuable natural-history data and should be documented and contributed to scientific records.
In the aquarium
Creagrutus saxatilis is not an established aquarium species, and no care records exist in the hobby literature. Aquarists maintaining specimens would be working from first principles based on its natural history: a high-gradient, oxygen-rich rocky-stream environment suggests the need for good water movement and aeration, a temperature range appropriate to the Brazilian cerrado headwaters (approximately 72–79 °F), and a substrate or hardscape incorporating smooth stones and gravel.
As a small characin from flowing water, it would almost certainly require live or frozen invertebrate food to thrive in captivity, and tankmates would need to be chosen carefully for compatibility in a rheophilic setup. The data-sparse nature of this species means that any observations made by keepers — size at maturity, colour in life, reproductive behaviour — would be genuinely useful contributions to knowledge of the genus.
Conservation
Creagrutus saxatilis is assessed as Data Deficient (DD) by the IUCN Red List (assessment published 2022, assessment date 7 November 2018). The DD designation reflects not an absence of concern but an absence of sufficient data to determine threat status confidently. The IUCN assessment specifically flags the Serra da Mesa hydroelectric dam — the largest reservoir by surface area in Brazil at the time of its completion — as a significant potential threat, noting that the effects of its impoundment on C. saxatilis subpopulations upstream and downstream of the reservoir remain unquantified.
The species' restriction to the upper Tocantins basin in Goiás and the Distrito Federal, combined with the ongoing pressure of hydroelectric development and land-use change in the cerrado biome, makes it plausible that C. saxatilis faces meaningful population-level stresses. The population trend is unknown. Until targeted surveys establish baseline abundance and distribution data, the conservation situation of this small rocky-stream endemic will remain unresolved.