Taxonomy & naming
Xenurobrycon macropus was described by George S. Myers and Alípio de Miranda Ribeiro in 1945. It is placed in the genus Xenurobrycon within the family Characidae, the large and diverse neotropical characin family. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) recognises the name as valid with the authority Myers & Miranda Ribeiro, 1945.
The genus Xenurobrycon comprises a small number of miniature characins from South America. The specific epithet macropus derives from Greek roots meaning 'large-footed' or 'large-limbed', though its precise application to morphological features of this species has not been widely discussed in accessible literature. The group belongs to the broader radiation of small-bodied, schooling characins that fill the middle and upper water column across South American river systems.
Morphology
Xenurobrycon macropus is a very small, slender characin with a maximum recorded standard length of approximately 1 in, placing it among the tiniest members of its family. The body is elongate and laterally compressed, with the semi-transparent quality typical of miniature characins in dimly lit, blackwater or clearwater habitats. The precise details of fin ray counts, scale rows, and other meristic characters have not been widely published in accessible sources.
As with other members of Xenurobrycon, the species likely shows the reduced pigmentation and delicate proportions associated with very small Characidae. Detailed morphological description beyond size data is sparse in the available literature, and the species has not been the subject of a recent morphological revision.
Habitat
Xenurobrycon macropus is a South American species within the family Characidae, but precise locality data and habitat descriptions are not well documented in the accessible literature. The species is attributed to river drainages in South America consistent with the distribution of the genus Xenurobrycon, which spans portions of the Amazon and associated drainages.
Given its tiny size and the ecology of related miniature characins, it is probable that the species inhabits shallow, slow-moving or standing waters with abundant vegetation or leaf litter, where small body size is an advantage in navigating dense cover. Water parameters for the wild habitat are not recorded in the available sources.
Feeding
Detailed dietary data for Xenurobrycon macropus are not available in the published literature accessible for this account. By analogy with other diminutive Characidae of similar size and body form, the species is likely a micro-predator or omnivore feeding on tiny invertebrates, zooplankton, and organic particles in the water column.
In captivity, very small tetras of this size class are typically fed infusoria, micro-worms, newly hatched brine shrimp nauplii, and finely powdered prepared foods — foods small enough for a fish whose adult length does not exceed 1 in.
Mating
No specific courtship or mating behaviour has been documented for Xenurobrycon macropus in the available literature. As a member of the Characidae, the species is expected to follow the typical characin reproductive pattern of egg-scattering with no parental care, depositing adhesive or semi-adhesive eggs among fine-leaved aquatic plants or over the substrate.
Males of miniature characin species frequently display lateral colour intensification and fin-spreading to court females, but whether Xenurobrycon macropus shows such behaviour is undocumented.
Breeding
Breeding records for Xenurobrycon macropus in captivity are absent from the accessible hobby and scientific literature. Based on the reproductive biology of related small characins, spawning likely involves the scattering of small, adhesive eggs among vegetation or spawning mops, with no subsequent parental care. Adults of egg-scattering characins will typically consume their own eggs if left together after spawning.
If ever bred in captivity, the very small adult size implies that fry would be extremely tiny at hatching, requiring infusoria or similar microscopic first foods. The incubation and larval development timeline is undescribed.
In the aquarium
Xenurobrycon macropus is rarely encountered in the aquarium trade and is essentially unknown in hobby circles. Its maximum size of around 1 in means it would require a species tank or a very carefully considered community with only the smallest and most peaceful tankmates, as it would be outcompeted or predated upon by most fish of normal aquarium size.
Water conditions for a species of South American Characidae origin would typically call for soft, slightly acidic to neutral water in the mid-20s Celsius, but specific requirements for this species are unrecorded. Anyone keeping or attempting to breed this fish would be working largely without a published template, and any detailed husbandry observations would constitute a genuine contribution to the hobby literature.
Conservation
Xenurobrycon macropus has not been assessed by the IUCN Red List at the time of writing, and is therefore considered Not Evaluated (NE) under the IUCN Categories and Criteria. FishBase records the species under Least Concern status based on available range data, but this does not represent a formal IUCN assessment.
The lack of assessment reflects the broader gap in conservation attention given to very small, rarely collected characins from South America. Without detailed distributional data and population surveys, the species' threat status cannot be formally determined. Its extremely small body size and likely dependence on intact freshwater habitats make it potentially vulnerable to the riverine habitat degradation affecting much of the Amazon basin, though this remains speculative without field data.