Taxonomy & naming
Hyphessobrycon melasemeion was described by the American ichthyologist Henry Weed Fowler in 1945, based on material from Brazil. The valid name, as recorded in Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong), is Hyphessobrycon melasemeion Fowler, 1945; the species remains placed in Hyphessobrycon, the largest genus in the family Characidae and one of the most species-rich in the entire order Characiformes.
The genus Hyphessobrycon encompasses several hundred small tetras ranging across Central and South America, and its limits have been the subject of ongoing revision. H. melasemeion belongs to the South American radiation centred on the Amazon basin, with its type locality in the Tapajós drainage of Brazil. No synonyms are recorded for the species in the primary nomenclatural literature, and no subspecies are recognised.
Morphology
Hyphessobrycon melasemeion is a very small tetra, reported at approximately 0.5 in in standard length — placing it at the extreme small end of the genus. Like other Hyphessobrycon, it has the typical characid body plan: a compressed, fusiform profile, a forked caudal fin, an adipose fin, and a complete lateral line. Detailed meristic and morphometric data for this species are sparse in the accessible literature given the rarity of published accounts.
The specific epithet melasemeion (from Greek melas, black, and semeion, mark or sign) implies the presence of a black marking or signal mark, likely a shoulder or humeral spot, which is a recurring feature across Hyphessobrycon. Beyond this inference from the name, precise colour pattern data are not well documented in sources currently available.
Habitat
The species is recorded from the Tapajós drainage in Brazil, one of the major clear-water tributaries of the Amazon River in the state of Pará. The Tapajós system is characterised by clear, warm, soft, and slightly acidic water draining off ancient Precambrian shield geology, with low conductivity and typically low to moderate pH.
Hyphessobrycon melasemeion inhabits forest streams within this drainage, sheltering among fine-leaved vegetation, leaf litter, and marginal cover typical of Amazonian igarapé stream habitat. Water temperatures in these streams typically range from approximately 73–81 °F, with soft, slightly acidic chemistry reflecting the low-mineral bedrock of the Tapajós watershed.
Feeding
Like the vast majority of small Hyphessobrycon, H. melasemeion is presumed to be a micropredator and opportunistic omnivore in the wild, feeding primarily on small invertebrates including micro-crustaceans, insect larvae, and other zooplankton encountered in the water column and at the surface of forest streams.
In aquarium conditions, species of comparable size from similar habitats accept fine-grade prepared foods, micro pellets, and small frozen or live foods such as baby brine shrimp, micro worms, and Cyclops nauplii. Given the very small adult size of approximately 0.5 in SL, food particle size is a practical constraint in captive care.
Mating
Hyphessobrycon melasemeion is expected to follow the typical Hyphessobrycon reproductive strategy: open-water egg-scattering among fine-leaved plants or over substrate, with no pair bond and no parental care. Males likely display with extended fins and intensified colouration to court females, with spawning occurring as a brief chase through vegetation.
No specific spawning observations for this species have been published in accessible sources. By analogy with closely related small tetras from similar Amazonian habitats, courtship is brief and opportunistic, triggered by good feeding condition, warm temperatures, and soft, slightly acidic water chemistry.
Breeding
Breeding details specific to Hyphessobrycon melasemeion are not documented in the sources currently available, owing to the species' obscurity in both the scientific literature and the aquarium hobby. The breeding mode inferred for the species is egg-scattering with adhesive or semi-adhesive eggs deposited among fine-leaved aquatic plants, consistent with the wider Hyphessobrycon pattern.
For related small Amazonian Hyphessobrycon bred in captivity, typical conditions involve soft, slightly acidic, warm water around 77–81 °F, a densely planted or mop-equipped spawning vessel, removal of adults after spawning to prevent egg predation, and very small first foods for the fry. Given the adult size of approximately 0.5 in, fry would be exceptionally small and would require the finest available first foods such as infusoria or rotifers.
In the aquarium
Hyphessobrycon melasemeion is essentially absent from the mainstream aquarium trade, and there are no established husbandry accounts for it in hobbyist literature. Its extreme small size — around 0.5 in SL — makes it a candidate for nano and planted pico-tank setups, but it would require careful selection of tankmates of compatible size and temperament, and food particle sizes appropriate to a fish that small.
Where a small Amazonian soft-water tetra of this type is kept, soft, slightly acidic water (pH approximately 6.0–7.0, low hardness) at 73–81 °F with gentle filtration and abundant fine-leaved plant cover would replicate its native forest-stream conditions. As with many obscure Hyphessobrycon, sourcing would likely require specialist breeders or importers dealing in rare South American characids.
Conservation
Hyphessobrycon melasemeion has not been formally assessed for the IUCN Red List, and its conservation status is therefore listed as Not Evaluated (NE). The Tapajós drainage, its known range, is a major Brazilian river system that has faced increased pressure from hydroelectric development, deforestation, and sedimentation over recent decades — pressures that affect the forest-stream habitats typical of small endemic Hyphessobrycon.
The species' obscurity in the hobby and scientific literature means that population trends and precise range limits are unknown. Formal assessment would require targeted survey work in the Tapajós to clarify its distribution and abundance. Until such data exist, no confident conservation judgement can be made.