Taxonomy & naming
Hyphessobrycon sebastiani was described by Carlos A. García-Alzate, César Román-Valencia, and Donald C. Taphorn in 2010, from material collected at Patecucho Creek in the San Juan River drainage on the Pacific versant of Colombia. The species belongs to the Hyphessobrycon heterorhabdus group, a cluster of small characids sharing certain pigmentation and meristic characters. Eschmeyer's Catalog of Fishes records the valid name as Hyphessobrycon sebastiani García-Alzate, Román-Valencia & Taphorn, 2010.
The species is placed in Hyphessobrycon Durbin, 1908, one of the largest and most diverse genera in Characidae, encompassing hundreds of small South and Central American tetras. Relationships within the heterorhabdus group and within Hyphessobrycon as a whole remain incompletely resolved; molecular work has fragmented the genus into multiple lineages, though no formal revision affecting this species has been published as of the time of writing. Colombian freshwater fish checklists confirm H. sebastiani as a valid species with a restricted Pacific-versant distribution.
Morphology
Hyphessobrycon sebastiani reaches a maximum standard length of 2 in, typical of the smaller members of the genus. Its meristic profile includes 11 dorsal soft rays and a vertebral count of 33, values that fall within the normal range for the heterorhabdus group. The species carries two distinct humeral spots — pigmented marks just behind the operculum — and lacks the caudal blotch found in some related species; this combination is a key diagnostic character used to distinguish it from close relatives.
Beyond these pigmentation details, the coloration and body form are consistent with the genus generally: a fusiform, laterally compressed body, a small mouth, and a colour pattern that likely includes iridescent flecks and lateral stripe elements characteristic of the heterorhabdus group, though published photographic documentation of live individuals is sparse in the available literature.
Habitat
The type locality is Patecucho Creek, a tributary of the San Juan River drainage on the Pacific versant of Colombia. The San Juan is one of the major Pacific-draining river systems of western Colombia, flowing through regions of high rainfall and dense lowland and foothill forest. Water chemistry data associated with the species' habitat records a pH of approximately 7.0 (mean 7.35), suggesting near-neutral, moderately soft to moderate conditions typical of Colombian Pacific-versant streams.
These streams are characterised by clear water, moderate current, and substrates of sand, gravel, and leaf litter beneath gallery forest cover. Hyphessobrycon sebastiani is a benthopelagic species, occupying the lower and mid-water column among submerged vegetation and woody debris. The Pacific versant of Colombia supports high fish diversity, but survey effort is lower than on the Atlantic (Orinoco/Amazon) side, which partly explains why so little is known of this species' broader distribution.
Feeding
FishBase data for the species indicates that the diet consists mainly of insects, consistent with the broader feeding ecology of small Hyphessobrycon in similar clear-water stream habitats. Like other members of the genus, H. sebastiani is likely an opportunistic microinvertivore, taking terrestrial and aquatic insects, small crustaceans, and other invertebrates of appropriate size, supplemented by organic particles and algae.
No detailed gut-content study specific to this species is available in the literature surveyed. In the aquarium, small Hyphessobrycon generally adapt readily to prepared foods — finely crushed flake, micro-pellets, and frozen or live small invertebrates such as Daphnia and Artemia nauplii — and there is no reason to expect H. sebastiani to differ substantially from this pattern.
Mating
No published observations of courtship behaviour exist for Hyphessobrycon sebastiani specifically. By analogy with the wider genus, the species is expected to be an egg-scatterer with no parental care, spawning among fine-leaved aquatic plants or submerged debris. Males in spawning condition in related species typically display lateral colour intensification and pursue females through vegetation, with spawning occurring in brief bursts.
The near-neutral to slightly acidic, soft-to-moderate water chemistry of the San Juan drainage suggests that conditioning and spawning would be promoted by similar conditions in captivity — slightly acid to neutral, warm water, live or frozen food to bring fish into condition, and fine-leaved plants or spawning mops as a substrate.
Breeding
No captive breeding records for Hyphessobrycon sebastiani appear in the available literature. As with other small Hyphessobrycon, the reproductive mode is inferred to be typical of the genus: egg-scattering over fine-leaved plants with adhesive or semi-adhesive eggs, no parental care, and adults that will consume their own eggs if not removed after spawning. Incubation duration and fry development details are not on record.
The species is essentially absent from the aquarium trade as of this writing, so hobbyist experience with breeding has not accumulated. Any attempt would logically follow the standard protocol for small Colombian tetras: a well-conditioned pair or small group in a bare or lightly planted tank with soft, warm, slightly acid water, live or frozen foods to trigger spawning, and prompt removal of adults after egg deposition.
In the aquarium
Hyphessobrycon sebastiani is not currently available in the ornamental fish trade and remains essentially unknown in the hobby. The species' small size (to 2 in SL), near-neutral water requirements, and presumably peaceful disposition suggest it would behave as a typical small tetra in a community setting — best kept as a group in a planted aquarium with soft to moderately hard, near-neutral water and temperatures in the range typical of Colombian lowland streams.
Given the lack of any established trade supply or hobbyist literature, practical care guidance is extrapolated from related Pacific-versant Colombian Hyphessobrycon rather than direct observation. Should the species become available, it would be suited to a quiet community with similarly sized peaceful fishes, subdued lighting, a dark substrate, and a varied diet of small prepared and live foods. Data on the species in captivity remain sparse.
Conservation
Hyphessobrycon sebastiani is assessed as Data Deficient (DD) on the IUCN Red List, with the most recent assessment dated 9 October 2014. The Data Deficient designation reflects insufficient information on the species' range, population size, and trends to make a reliable threat assessment — not necessarily an indication that the species is secure. The known distribution is restricted to the San Juan River drainage on the Pacific versant of Colombia, a region that, while currently under lower human pressure than some Atlantic-versant systems, faces ongoing deforestation, agricultural expansion, and artisanal mining.
Collecting broader distributional data and monitoring population status in the Patecucho Creek system and adjacent drainages would be prerequisites for a more informative assessment. The species' apparent habitat specificity and restricted known range argue for treating it with precaution. It has no established captive population in the hobby or in zoo/aquarium institutions, so in situ habitat protection represents the primary conservation concern.