Taxonomy & naming
Creagrutus meridionalis was described by Richard P. Vari and Arthur S. Harold in 2001 as part of a major revisionary treatment of the genus Creagrutus within the family Characidae. The genus Creagrutus Günther, 1864 encompasses a large number of small to medium-sized characins distributed broadly across lowland South America, united by a suite of osteological and tooth-morphology characters that separate them from superficially similar genera. The original description documented phylogenetic hypotheses for the genus and characterised C. meridionalis by its tooth morphology and meristic characters.
The valid name, Creagrutus meridionalis Vari & Harold, 2001, is recorded in Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) as the accepted name; the authority is not parenthesised because the species was described in Creagrutus, where it remains. The specific epithet meridionalis is Latin for 'southern', referring to the species' range at the southern extent of the genus's distribution in the upper Paraguay basin. No synonyms are currently recognised.
Morphology
Creagrutus meridionalis is a moderately elongate, laterally compressed characin reaching approximately 4.5 in standard length. Like most members of the genus, it presents the typical unspecialised characin body plan: a fusiform profile, a complete lateral line, a small adipose fin posterior to the dorsal, and a forked caudal fin. Coloration in life is silvery, with a darker dorsal surface and a more or less distinct horizontal dark stripe running along the flank, a pattern common to many species of Creagrutus and shared with various other small river characins of the upper Paraguay drainage.
Distinguishing Creagrutus meridionalis from congeners in the field is difficult without examination of meristic and dental characters; the genus is taxonomically complex and several species may co-occur in the same drainage system. The 2001 description by Vari and Harold provides the diagnostic characters, principally based on tooth form and fin-ray counts, that separate C. meridionalis from related species in adjacent basins. Detailed morphological data for wild-caught material from the upper Paraguay are sparse in the hobbyist literature.
Habitat
Creagrutus meridionalis occupies the upper Paraguay River basin, with records from Mato Grosso state in Brazil and from eastern tributaries draining into the Paraguay River in Paraguay. FishBase characterises it as a freshwater, benthopelagic, tropical species, indicating it tends to occupy or forage near the substrate in the lower water column of rivers and streams rather than being strictly a surface or midwater fish.
Field observations from the Mato Grosso region, including accounts from the Sepotuba watershed and its surroundings, describe shallow, clear-water habitats along the margins of the upper Paraguay drainage where Creagrutus and other small characins form shoals in the shallow parts of the river system. Water in these upper Paraguay headwaters tends to be warm and relatively clear outside flood periods, with a seasonally variable flow regime tied to the pronounced wet and dry seasons of the central Brazilian plateau. The species is recorded as occurring with other characins typical of the upper Paraguay ichthyofauna.
Feeding
Members of the genus Creagrutus are generally regarded as opportunistic carnivores or omnivores that take small invertebrates — aquatic insects and their larvae, small crustaceans, and other invertebrates — along with some plant or detrital material. C. meridionalis almost certainly follows a similar diet given its benthopelagic habits and the feeding ecology typical of small upper Paraguay characins, though specific dietary data for this species are not available in the published literature.
In the aquarium, analogous small Creagrutus and related characins accept good-quality small flake and micro-pellets, as well as frozen and live foods such as Daphnia, baby brine shrimp, and bloodworm. A varied diet supports good colour and condition. As with most small characins, the species will feed at multiple levels of the water column and benefits from foods small enough to be handled easily.
Mating
No detailed observational data on the mating behaviour of Creagrutus meridionalis are available in the literature. Based on the reproductive biology typical of small non-guarding characins in the family Characidae, the species is expected to be an egg-scattering open spawner in which sexes come together briefly to scatter adhesive or semi-adhesive eggs among fine-leaved aquatic vegetation or over the substrate, after which no parental care is provided.
In the wild, spawning in upper Paraguay drainage characins is often linked to seasonal cues — rising water temperatures and changing water levels associated with the onset of the rainy season — though the precise cues triggering spawning in C. meridionalis have not been documented. Males of Creagrutus species may show minor differences in body depth or fin development during the breeding season, but dimorphism data specific to this species are not recorded.
Breeding
Creagrutus meridionalis has not been bred under aquarium conditions to any documented degree, and formal breeding protocols are not available. By analogy with other small egg-scattering characins, reproduction would likely involve the scattering of small adhesive eggs among fine-leaved plants or over a soft substrate, with no parental care and a willingness of the adults to consume their own eggs if not separated from them after spawning.
Incubation periods for small tropical South American characins under aquarium conditions typically range from one to three days at temperatures in the mid-to-upper 20s Celsius, with free-swimming fry requiring very fine initial foods such as paramecia or commercial liquid fry preparations before graduating to newly hatched brine shrimp nauplii. Should successful breeding be achieved, providing a separate, heavily planted or mop-equipped rearing container and removing the adults after spawning would be the standard approach.
In the aquarium
Creagrutus meridionalis is rarely encountered in the ornamental fish trade and is not an established aquarium species. Hobbyist literature mentions it primarily as a component of the upper Paraguay fauna rather than as a species sought out for keeping. Where small Creagrutus do appear in the trade, they are typically kept in the same way as other small peaceful characins: in groups of six or more in a well-filtered, softly lit aquarium with plants, some surface cover, and open swimming space, alongside similarly sized peaceful tankmates.
Water conditions replicating the warm, relatively clear, tropical rivers of the upper Paraguay basin — temperatures in the mid-to-upper 20s Celsius, near-neutral to slightly acidic pH, and low to moderate hardness — would be appropriate starting points. The species' benthopelagic tendencies suggest it would spend time in the lower water column, so a tank with a fine substrate and some bottom cover would be suitable. Because taxonomic identification of Creagrutus in the trade is unreliable, fish sold as this species should be treated with some caution regarding provenance.
Conservation
Creagrutus meridionalis has not been formally assessed by the IUCN Red List as a distinct taxon; FishBase records a Least Concern status, reflecting the species' presence across a reasonably broad section of the upper Paraguay basin in both Brazil and Paraguay, and the relative lack of evidence of major population decline. The upper Paraguay River and its tributaries, however, are subject to pressures including agricultural expansion, pasture conversion, and altered hydrology affecting the cerrado and pantanal transition zone through which much of the drainage flows.
The species occurs within the catchment of the Pantanal, one of the world's largest tropical wetlands and a system with some degree of international conservation attention. While C. meridionalis itself is not a focus of conservation effort, the health of the broader upper Paraguay ichthyofauna — which includes numerous small characins with similarly limited distributional data — is tied to the maintenance of natural hydrological regimes and riparian vegetation in the region. More distributional survey work and a formal IUCN assessment would clarify the conservation status of this understudied species.