Taxonomy & naming
Hyphessobrycon tenuis was described by the French ichthyologist Jacques Géry in 1964. The valid name as recognised in Eschmeyer's Catalog of Fishes is Hyphessobrycon tenuis Géry, 1964. The species belongs to the large, polyphyletic genus Hyphessobrycon within the family Characidae, the characids — one of the most speciose freshwater fish families in the Neotropics.
The specific epithet tenuis is Latin for thin, slender, or fine, and was applied in reference to the species' notably slender, elongate body and narrow caudal peduncle — features that set it apart from many of the deeper-bodied species in the genus. In Spanish-speaking parts of its range it is sometimes called mojarita, a general term applied to small tetras and characins across South America. Hyphessobrycon is a diverse and taxonomically complex genus; the relationships of H. tenuis within the genus have not been the subject of extensive modern phylogenetic work.
Morphology
Hyphessobrycon tenuis is a small fish, reaching a maximum recorded standard length of approximately 1 in. The body is notably slender and elongate compared to the typical Hyphessobrycon form, with a narrow caudal peduncle — the features that inspired the species name. Like all Hyphessobrycon it carries the basic characid body plan: a laterally compressed, fusiform profile, a single dorsal fin, an adipose fin, and a forked caudal fin.
Detailed colour and meristic information in the available literature is sparse. As is typical for small Hyphessobrycon, the species is likely silvery to pale in overall ground colour with translucent fins, potentially carrying a dark lateral stripe or spot at the caudal base as seen in many congeners, but published descriptions beyond the slender body form are limited. Data on fin-ray and scale counts are not captured in accessible secondary sources reviewed here.
Habitat
Hyphessobrycon tenuis is a South American freshwater species. Available records place it within the freshwater systems of tropical South America, consistent with the distribution of the broader Hyphessobrycon assemblage in lowland river basins. The species inhabits freshwater habitats typical of small characins: slow-moving or still, often vegetated waters in river margins, floodplain pools, or forest streams.
Precise locality data and habitat characterisation beyond the South American tropical freshwater assignment are not captured in the sources available for this article. The species is considered a tropical freshwater fish, and water-parameter requirements can be broadly inferred from its South American origin, but detailed field records of temperature, pH, or substrate preferences specific to H. tenuis have not been documented in the sources reviewed here.
Feeding
As a small characin, Hyphessobrycon tenuis is presumed to be an omnivore in the manner typical of the genus, feeding on small aquatic invertebrates, microcrustaceans, insect larvae, and plant or algal matter in the wild. This generalisation reflects the feeding ecology well established for small Hyphessobrycon species across South America.
Specific dietary studies on H. tenuis are not available in the literature reviewed. In captivity, small tetras of this kind generally accept high-quality micro-pellets, finely crushed flake, and small live or frozen foods such as Artemia nauplii, micro-worm, or Cyclops, which provide nutrition appropriate to the species' size.
Mating
Hyphessobrycon tenuis, in common with other members of the genus, is expected to be an egg-scattering open spawner with no parental care — the reproductive strategy characteristic of Hyphessobrycon and the broader characid tribe. Males typically display to females by spreading fins and intensifying colour, herding receptive females into fine-leaved vegetation or open water where eggs are released and fertilised.
No specific observations of mating behaviour in H. tenuis have been published in the accessible literature. The general pattern in the genus involves non-pair bonding, with spawning occurring among dense plant growth or spawning mops where the slightly adhesive eggs settle. Given the species' very small size, spawning groups are likely small and breeding events subtle.
Breeding
Detailed breeding accounts for Hyphessobrycon tenuis are not available in the sources reviewed. Based on the biology of small Hyphessobrycon generally, the species deposits small, adhesive or semi-adhesive eggs among fine-leaved plants or fibrous spawning media. The eggs receive no parental care, and adults will consume eggs if not removed or separated.
Incubation and fry-rearing details specific to H. tenuis are undocumented in accessible sources. Breeders working with comparably sized small Hyphessobrycon typically raise fry on infusoria or commercial liquid fry food as first foods before transitioning to newly hatched brine shrimp nauplii as the fry grow.
In the aquarium
Hyphessobrycon tenuis is not a commonly traded species and is rarely encountered in mainstream aquarium retail. Its appeal for the specialist lies in its slender body form — an unusual silhouette among the often deep-bodied Hyphessobrycon — and in the modest challenge of keeping a small, delicate South American tetra well. At 1 in adult size, it is among the smaller members of the genus and requires appropriately scaled tankmates and furnishings.
As with small tetras generally, a densely planted tank with gentle filtration, soft to moderately soft and slightly acidic to neutral water chemistry, and a dark substrate that helps the fish feel secure would suit the species well. A group of at least six to eight individuals is recommended to support natural schooling behaviour and reduce stress. Live or frozen small foods are likely important for long-term condition. Reliable captive-husbandry information specific to H. tenuis is sparse in the hobby literature, so keepers should treat it as a somewhat delicate micro-tetra requiring careful attention to water quality.
Conservation
Hyphessobrycon tenuis is assessed as Least Concern (LC) on the IUCN Red List, with the assessment completed on 24 April 2014. The assessment notes the species as common within its range, with no identified major widespread threats at the time of evaluation.
As a small freshwater fish confined to South American river systems, it shares the broader vulnerability of Neotropical freshwater biodiversity to habitat degradation, deforestation, and water-quality change. However, based on the current IUCN assessment, the species does not face the acute population pressure that warrants a threatened category. Ongoing monitoring of South American freshwater ecosystems remains important for all characid species, particularly smaller and less visible ones that may not receive targeted study.