Taxonomy & naming
Moenkhausia lopesi was described by Heraldo A. Britski and Marta de Silimon in 2001. The type locality is in the state of Mato Grosso, Brazil, within the upper Paraguay River drainage. Eschmeyer's Catalog of Fishes (CAS) records the valid name as Moenkhausia lopesi Britski & de Silimon, 2001; described in Moenkhausia, the species remains there and the authority is used without parentheses.
The genus Moenkhausia is one of the largest and most species-rich in the family Characidae, encompassing over 80 species of small to medium-sized tetras distributed across tropical South America. The group is defined by a fully scaled caudal fin — a feature that separates it from several allied genera — though it has historically been something of a wastebasket genus and its limits are the subject of ongoing systematic revision. M. lopesi belongs to the interior Brazilian fauna rather than to the Amazonian lowland radiation more familiar in the trade.
Morphology
Like most members of its genus, Moenkhausia lopesi is a laterally compressed, moderately deep-bodied characid with the typical tetra outline: a forked caudal fin, an adipose fin between the dorsal and the tail, and a small body scaled to the caudal fin base in the manner that defines Moenkhausia. The species is small, as expected from a Mato Grosso stream fish in this genus.
Detailed meristic data from the original description are not available in the sources consulted, so precise counts of lateral-line scales, fin rays, and vertebrae cannot be confirmed here. In overall habitus, the species is consistent with the small, silver-bodied Moenkhausia of central Brazilian drainages — compressed flanks, a moderately sized eye, and an unremarkable but characteristic tetra profile. Sexual differences, if any, are undocumented in the accessible literature.
Habitat
Moenkhausia lopesi is endemic to the Queimá-Pé River subbasin near Tangará da Serra in Mato Grosso, Brazil, within the upper Paraguay River drainage. A 2016 ecological study confirmed the species' presence in this system, which lies in Mato Grosso's cerrado-dominated interior — a landscape of seasonal savanna, gallery forests, and small to medium rivers subject to pronounced dry and wet seasons.
Cerrado stream systems in this region typically feature clear to lightly tannin-stained water, sandy or rocky substrate, and moderate gradients. Water temperatures in the Mato Grosso cerrado vary seasonally; rivers here tend toward warm, slightly acidic to near-neutral conditions, though local variation is significant. The species' restricted range in a single subbasin makes it vulnerable to any degradation of those specific watercourses.
Feeding
No published dietary analysis specific to Moenkhausia lopesi is available in the sources consulted. By analogy with other small Moenkhausia from similar cerrado stream habitats, the species is likely an opportunistic omnivore, taking small invertebrates — aquatic insect larvae, microcrustaceans, and terrestrial insects falling onto the surface — along with plant matter, algae, and detritus where available.
In an aquarium context, Moenkhausia species of similar size and habitat are undemanding feeders that accept high-quality flake foods, small pellets, and frozen or live invertebrates such as Daphnia and brine shrimp. A varied diet maintains condition; live and frozen foods are beneficial for keeping small characids in good health.
Mating
No specific observations on courtship behaviour in Moenkhausia lopesi have been published in the sources available. In common with the broad pattern for the genus and for small characids generally, the species is inferred to be an egg scatterer without parental investment: males likely pursue females during spawning drives, and eggs are broadcast among fine-leaved plants or deposited over substrate in open water.
Sexual dimorphism has not been documented for this species in the accessible literature. In closely related Moenkhausia, females typically become visibly fuller in the abdomen when ripe, and males may display marginally brighter colouration during the spawning period, though elaborate courtship structures are absent.
Breeding
Captive breeding of Moenkhausia lopesi has not been documented in the hobby literature consulted. Based on the reproductive mode standard for the genus — open egg scattering with adhesive or semi-adhesive eggs deposited among fine-leaved vegetation or over substrate — the species would be expected to spawn readily under warm, well-conditioned aquarium conditions without parental care.
For Moenkhausia generally, eggs hatch within 24 to 48 hours at tropical temperatures, and the tiny, yolk-absorbing larvae begin free swimming within a few days. First foods are infusoria or very fine commercial fry preparations, transitioning to newly hatched brine shrimp. Adults will consume their own eggs and offer no subsequent care; isolation of spawning adults or removal of eggs to a separate vessel is standard practice.
In the aquarium
Moenkhausia lopesi is essentially unknown in the aquarium hobby. No commercial collection from its Mato Grosso range is documented, and the species does not appear in trade or in standard husbandry references. It is occasionally encountered in specialist collections in Brazil but has not entered the international ornamental fish market to any meaningful degree.
Should the species become available, husbandry expectations would follow those for other small Moenkhausia from cerrado drainages: a modestly sized aquarium of 23.5 in or more for a school of six or more individuals, slightly warm water in the mid-20s Celsius, near-neutral to mildly acidic pH, good filtration, and a diet combining prepared and live or frozen foods. Its small size and presumed peaceful disposition would make it suitable for a biotope setup replicating the gallery-forest streams of central Brazil.
Conservation
Moenkhausia lopesi was assessed for the IUCN Red List in 2018 and is listed as Least Concern; the assessment was published in 2022 by the Instituto Chico Mendes de Conservação da Biodiversidade. The species is endemic to the Queimá-Pé River subbasin in Mato Grosso, a limited distribution that inherently raises conservation concern. The cerrado biome, which surrounds this drainage, is one of the most threatened biodiversity hotspots in South America: large-scale agricultural conversion, particularly soy and cattle ranching, has dramatically reduced native cerrado vegetation cover in Mato Grosso, with attendant effects on stream integrity through sedimentation, altered hydrology, and reduced riparian cover. A fish confined to a single subbasin in this landscape has little buffer against local habitat loss.