Taxonomy & naming
Moenkhausia comma was described by Carl H. Eigenmann in 1908 from material deposited at the Museum of Comparative Zoology, where the type specimen (MCZ:Ich:20972) is held. The species belongs to Moenkhausia, a large and heterogeneous genus of Amazonian characids that remains the subject of ongoing revision; Eschmeyer's Catalog of Fishes (Catalog of Fishes, CAS) carries the valid name as Moenkhausia comma Eigenmann, 1908 without parentheses, indicating the species has not been moved from the genus to which it was originally assigned.
Moenkhausia is placed within Characidae in the traditional sense, a sprawling family that accounts for a substantial portion of the freshwater fish diversity of South America. Within the genus, M. comma is distinguished by the comma-like marking that gave it its epithet — a character shared to varying degrees by a few related species, complicating field identification and contributing to taxonomic uncertainty that still attends parts of the group. No subspecies are recognised.
Morphology
Moenkhausia comma is a moderately elongate, laterally compressed characid reaching a maximum recorded standard length of 3 in, making it a medium-sized member of its genus. The body is typically silvery with a degree of translucency in life, and the species carries the complete lateral line and adipose fin characteristic of the family. The eye is large relative to body size, as is common in tetras.
The common name and specific epithet both refer to a dark comma- or hook-shaped mark — typically located at or near the caudal peduncle base or along the posterior flank — which serves as the species' most readily noted field mark. Fin rays and scale counts follow standard Moenkhausia ranges; detailed meristic data were established in the original description and in subsequent revisions of the genus. Sexual dimorphism is subtle in most Moenkhausia: females are typically deeper-bodied when gravid, and males may be marginally slimmer, but the two sexes are otherwise difficult to distinguish on appearance alone.
Habitat
Moenkhausia comma inhabits the Amazon basin, where it occupies freshwater habitats with a benthopelagic lifestyle — moving through the water column from near the bottom to mid-water depending on activity and feeding. It tolerates a moderate range of physicochemical conditions: FishBase records temperatures of 73–82 °F (73–82 °F), pH 6.0–7.5, and hardness up to 18 dH for the species.
Like many Amazonian characids, M. comma is likely found in a range of river and stream types across the basin, from the main channel edges to tributary streams and flooded marginal habitats, seeking areas with cover provided by submerged vegetation, roots, or leaf litter. The Amazon basin spans multiple countries and a vast range of habitat types, and the species' broad distribution within it aligns with its IUCN Least Concern assessment, which reflects no documented major range contraction or population threat.
Feeding
Moenkhausia comma is an omnivore in the wild, as is typical of the genus. Small invertebrates — insect larvae, microcrustaceans, and similar items — form the core of the diet, supplemented by plant material, algae, and organic detritus encountered while foraging in the water column and near the substrate.
In the aquarium M. comma is undemanding. Good-quality dried foods — flake and small pellets — are accepted readily, and the diet is improved and colour maintained by regular supplementation with small live or frozen foods such as Daphnia, brine shrimp, and bloodworm. As an active, schooling tetra it benefits from frequent small meals rather than single large ones.
Mating
Moenkhausia comma is a typical egg-scattering characid. Males court females with chasing and displaying behaviours, and spawning occurs in open water or among fine-leaved vegetation, with no mate bond formed and no territory defended. Adhesive or semi-adhesive eggs are scattered among plants or over the substrate.
As in most Moenkhausia, males are likely slimmer-bodied and may display more active courtship behaviour, while gravid females become noticeably fuller-bellied. Spawning activity tends to increase in the early morning, and in a well-conditioned group with adequate plant cover some spawning will occur without deliberate intervention.
Breeding
Breeding Moenkhausia comma follows the standard egg-scattering tetra protocol. A conditioned pair or small group is placed in a tank furnished with fine-leaved plants such as Java moss, or with synthetic spawning mops, over a mesh bottom or pebble layer that lets eggs fall out of reach of the adults. Water should be soft to moderately hard, slightly acidic to neutral, and warm — around 77–81 °F.
Eggs are broadcast among plant stems and hatch in approximately 24–36 hours at typical spawning temperatures. The fry become free-swimming after a further 3–4 days once the yolk sac is absorbed; reports from related Moenkhausia species follow the same timeline. Adults offer no parental care and will eat eggs and small fry if given the opportunity, so eggs or adults should be removed after spawning. First foods are infusoria or commercial liquid fry food, followed by freshly hatched brine shrimp and finely powdered dry food as the fry grow.
In the aquarium
Moenkhausia comma is a peaceful, active schooling fish suited to a community aquarium of medium-sized, non-aggressive tankmates. A group of at least six individuals is recommended to allow natural schooling behaviour and reduce individual stress; larger groups are better still, and produce the natural formation swimming that characterises a healthy tetra school.
Water conditions should reflect the species' Amazonian origins: a temperature of 73–82 °F (73–82 °F), pH 6.0–7.5, and moderate hardness (to around 18 dH) will keep the fish in good health. A planted aquarium with open central swimming space, some gentle current from the filter return, and subdued or variable lighting suits the species. Good filtration and regular partial water changes are standard requirements. Suitable tankmates include other similarly sized, peaceful characids, rasboras, smaller catfishes, and comparable community species. M. comma is not commonly encountered in the trade relative to some of its congeners, but captive-bred stock is preferable when available.
Conservation
Moenkhausia comma is assessed as Least Concern (LC) on the IUCN Red List, based on an assessment dated March 2021. The species inhabits the Amazon basin — one of the world's largest river systems — and its distribution across that broad drainage means it faces no range-wide threat sufficient to qualify for a more precarious category under current data.
Generalised threats to Amazonian fish diversity, including deforestation-driven catchment degradation, sedimentation, pollution, and water abstraction, are ongoing across much of the basin and affect many species. At present, however, M. comma is not considered to face population-level risk from these pressures, and no specific conservation actions are directed at it. Maintaining healthy Amazonian riverine habitats broadly benefits this and thousands of co-occurring species.