Taxonomy & naming
Creagrutus petilus was described by Richard P. Vari and Alec S. Harold in 2001 as part of their systematic revision of the genus Creagrutus, a large and taxonomically complex assemblage within the family Characidae. The Catalog of Fishes (Eschmeyer et al.) records the valid name as Creagrutus petilus Vari & Harold, 2001. The type locality is located in Brazil within the eastern Amazon basin.
The specific epithet petilus is a Latin adjective meaning slender or lean, an apt reference to the fish's gracile body form relative to some of its congeners. The genus Creagrutus takes its name from the Greek kreagreytos — meaning to tear or rip up the flesh — reflecting the predatory tendencies that characterise these small but active carnivores. With well over 70 species now assigned to the genus, Creagrutus represents one of the most species-rich Neotropical characid lineages, distributed across river systems from Panama south through much of South America.
Morphology
Creagrutus petilus is a small, laterally compressed characid reaching a maximum recorded standard length of 2 in. Like other members of the genus it is benthopelagic in habit, with a body form suited to life in and around the substrate of flowing water. The body is notably slender — the meaning of the specific epithet — with a pointed snout and a terminal to slightly subterminal mouth equipped with small teeth adapted for seizing invertebrate prey.
The species shares the typical characid complement of fins, including a small adipose fin between the dorsal and caudal. Coloration in life has not been extensively described in the literature available for this species; like many small stream characids it is likely silvery with subdued lateral pigmentation. The swim bladder is well developed and the mineral skeleton follows the standard apatite/hydroxylapatite composition of teleost fishes, with bilateral symmetry throughout.
Habitat
Creagrutus petilus inhabits upland freshwater streams within the eastern Amazon basin of Brazil. It is classified as a benthopelagic species, occupying the lower water column in flowing freshwater environments — the kind of clear, well-oxygenated headwater streams that characterise the elevated portions of the Amazon drainage before water descends to the lowland floodplain.
Ecological research on fish assemblages of the eastern Amazon region documents the broader community context in which Creagrutus species occur: these upland streams experience seasonal hydrological fluctuation, with high-water periods expanding available habitat and promoting breeding activity and resource exploitation across the fish assemblage. Specific physicochemical data for the precise localities where C. petilus occurs are not widely published, but Amazonian upland streams typically run soft, slightly acidic to neutral, and warm.
Feeding
As a member of Creagrutus — a genus whose name evokes predatory flesh-tearing — Creagrutus petilus is presumed to be a carnivore or insectivore, feeding on small aquatic invertebrates, insect larvae, and other benthic prey items encountered at or near the substrate. This feeding mode is consistent with its benthopelagic habitat preference and with the diet documented for related congeners throughout the Neotropics.
Detailed dietary studies specific to C. petilus are not available in the published literature captured by current sources. By analogy with other small stream Creagrutus species, the diet likely shifts seasonally in response to invertebrate availability and water level, with aquatic insect larvae forming the dietary backbone during low-water periods and terrestrial insects and other allochthonous input becoming more important during high water when the stream margin expands.
Mating
Creagrutus petilus reproduces sexually, as confirmed by morphological and biological records. Like other small characids, it is expected to be an egg-scattering, open spawner with no parental care — the ancestral reproductive mode throughout the Characidae. Males and females likely congregate in areas of suitable spawning habitat, releasing eggs and milt into the water column or over fine substrates and vegetation during periods of elevated reproductive activity.
Ecological work on eastern Amazon stream fish assemblages suggests that high-water seasons, when flooded margins provide expanded habitat and food resources, coincide with peak breeding activity in many stream characid species. Specific courtship behaviours for C. petilus have not been documented in the available literature, and details of mate selection or any sex-specific coloration or morphology remain uncharacterised for this species.
Breeding
Based on the reproductive biology of the Characidae broadly and Creagrutus specifically, Creagrutus petilus is an egg-scattering spawner depositing small, adhesive or semi-adhesive eggs among fine-leaved aquatic vegetation or over gravelly substrate in flowing water. No parental care is provided; adults scatter eggs and abandon them to hatch unattended, a strategy common across the family.
Breeding records specific to C. petilus in captivity are not documented in the published or hobby literature currently available. The species is not established in the ornamental trade, so aquarist experience with captive reproduction is essentially absent. Incubation duration and fry requirements can only be inferred from related small stream characids, where eggs typically hatch within a few days at tropical temperatures and fry require infusoria-scale first foods before accepting larger prey.
In the aquarium
Creagrutus petilus is not an established aquarium fish and is essentially absent from the ornamental trade. It has no recorded common name, reflecting its status as a species known primarily from scientific collections rather than hobbyist importations. Should it become available, its small size — under 2 in — and benthopelagic temperament suggest it would be suited to a species tank or a carefully curated community of similarly sized, peaceful South American stream fish.
The upland stream habitats it occupies in eastern Brazil suggest a preference for well-oxygenated, moderately cool to warm water with some current, soft to neutral chemistry, and a dark, natural substrate that replicates the leaf litter and gravel beds of headwater streams. Given the genus's predatory nature, very small tankmates such as neon tetras or microrasboras would likely be at risk. Detailed husbandry information is not available, and aquarists encountering this species would be working largely from first principles based on analogous stream characids.
Conservation
Creagrutus petilus is assessed as Least Concern (LC) on the IUCN Red List, with the assessment completed on 7 November 2018. The Least Concern designation reflects the absence of evidence for population-level declines significant enough to trigger a threatened category under IUCN criteria, given the species' occurrence across upland stream habitats in the eastern Amazon of Brazil.
While its overall status is stable, it is important to note that the eastern Amazon upland stream systems face ongoing pressure from deforestation, agricultural run-off, and changes in hydrological regimes associated with land-use change in Brazil's interior. Small-bodied stream endemics are inherently vulnerable to habitat degradation because their limited dispersal ability and specialised microhabitat requirements restrict the options for recolonisation following local disturbance. The Least Concern assessment should be read in this context: current evidence does not support a threatened listing, but the broader landscape pressures on Amazonian headwater systems warrant continued monitoring.