Taxonomy & naming
Moenkhausia cosmops was described by Francisco de Assis Melo Lima, Heraldo Antonio Britski, and Antonio Machado in 2007. The type locality is the córrego Vinte e Cinco de Maio, in the Rio Papagaio drainage near Sapezal, Mato Grosso, Brazil. The species is placed in the large characid genus Moenkhausia, which currently contains more than 80 species across tropical South America. Eschmeyer's Catalog of Fishes records the valid name as Moenkhausia cosmops Lima, Britski & Machado, 2007.
The species is nested within the M. oligolepis/M. sanctaefilomenae complex — a cluster of reticulated, schooling tetras that share a vertically elongate humeral blotch and a dark caudal peduncle spot. Moenkhausia cosmops can be separated from all congeners by its unique combination of blue-green irides and the conspicuous red stripe running above the mouth. Analyses of external morphology and coloration place it as the sister lineage of that broader complex rather than as a mere variant within it.
Morphology
Moenkhausia cosmops is a small, laterally compressed tetra reaching approximately 2 in standard length at maximum. Body shape is typical of the genus: moderately deep, with a blunt snout and a large eye. The lateral line is complete, and scale counts fall within the ranges seen across the M. oligolepis complex.
The species shares with close relatives a reticulated body pattern produced by dark scale margins, a vertically elongate dark humeral blotch, a prominent dark spot at the base of the caudal peduncle, and — in many individuals — a reddish or orange eye. What distinguishes M. cosmops is the blue-green cast to the iris, stronger than any red component, and a conspicuous red or red-orange horizontal stripe immediately above the lips. This lip stripe is the basis for the common name Lipstick Tetra and is consistent across adults of both sexes. As in many Moenkhausia, overall colour intensity depends on tank conditions, angle of light, and reproductive state.
Habitat
In the wild, Moenkhausia cosmops occurs in the Rio Paraguai and Rio Tapajós basins of Mato Grosso state, Brazil, with known localities including the Rio Juba, Rio Juruena, Rio Papagaio, and the type-locality stream. It inhabits clearwater, moderately fast-flowing streams with sandy bottoms and marginal macrophyte growth — habitats typical of the Cerrado-fringe watercourses of central Brazil.
FishBase records water temperatures of 75–82 °F for its range. The clearwater, sandy-substrate streams it prefers are relatively oligotrophic, with good water clarity and moderate current. The species forms large schools in the wild, with aggregations exceeding 50 individuals reported. This schooling behaviour is characteristic of the M. oligolepis complex as a whole and likely functions in predator dilution in the open-stream microhabitats the fish favour.
Feeding
Moenkhausia cosmops is an omnivore, as is typical of the genus and of schooling characids in Cerrado streams more broadly. The natural diet is presumed to include small invertebrates, microcrustaceans, insects, and plant material grazed from surfaces and the water column.
In the aquarium it accepts a wide variety of prepared and live foods without difficulty. Flake and micro-pellets form a practical staple; regular additions of frozen or live brine shrimp, bloodworms, daphnia, and mosquito larvae improve condition, support colour, and bring fish into breeding readiness. As with most schooling characids, small meals offered several times a day are preferable to a single large feeding.
Mating
Moenkhausia cosmops is an egg-scattering open spawner with no parental care — the reproductive strategy shared by virtually all members of the genus. Males pursue females actively and courtship consists of close-following behaviour; there is no reported territory defence or complex display. Spawning readiness in both sexes is promoted by conditioning with live and frozen foods and, in captivity, by slight temperature elevation and the addition of soft, slightly acidic water.
Females are typically plumper and more rounded in the ventral profile than males when gravid, which is usually the most reliable sex differentiation available in captivity. As in other members of the complex, spawning activity in a well-conditioned group is likely to occur in early morning hours, with eggs scattered among fine-leaved plants or spawning mops.
Breeding
Breeding Moenkhausia cosmops follows the standard Moenkhausia protocol. Conditioned adults should be moved to a separate spawning tank furnished with fine-leaved plants or synthetic spawning mops; the adults must be removed promptly after spawning, as they will consume both eggs and newly hatched fry. Females produce several hundred eggs per spawning, scattered and fertilised among the plants.
The eggs are light-sensitive and are best incubated in subdued or absent light. At typical breeding temperatures, hatching occurs within 24–48 hours, and fry become free-swimming 3–4 days after hatching. First foods should be very fine: infusoria, commercial liquid fry foods, or rotifers, transitioning to newly hatched brine shrimp as the fry grow. Growth is relatively rapid once feeding is established, and juveniles develop the adult lip stripe as body colouration intensifies.
In the aquarium
The Lipstick Tetra is an active schooling species that must be kept in groups; a minimum of five individuals is recommended, with larger schools of eight or more producing more natural behaviour and significantly better display. Aquadiction recommends a tank of at least 40 US gal with a frontage of 39.5 in or more to give the school adequate swimming space.
Water conditions should reflect the clearwater stream origins of the species: 75–82 °F, soft to moderately hard, and neutral to slightly acidic. A planted aquarium with open central swimming space, moderate filtration flow, and subdued or natural lighting shows the blue-green eyes and red lip stripe to best advantage. The species is peaceful toward tankmates of comparable size and does well with other small tetras, pencilfish, corydoras catfish, and dwarf cichlids. Its distinctive facial markings and active schooling behaviour make it an appealing display fish that is rarely seen in the trade relative to its visual quality.
Conservation
Moenkhausia cosmops is assessed as Least Concern on the IUCN Red List, based on an assessment completed on 7 November 2018 (assessment version 2022.2). The species has a relatively broad distribution across two major river basins in Mato Grosso and no acute threats to its populations were identified at the time of assessment.
The clearwater streams of the Cerrado biome that the species inhabits are, however, subject to broader regional pressures including agricultural conversion, siltation from deforestation, and water extraction — threats that affect many endemics of this biome. The Least Concern designation reflects current population data rather than an absence of regional environmental concern, and the long-term status of Cerrado stream fauna warrants continued monitoring.