Taxonomy & naming
Hyphessobrycon jackrobertsi was formally described by Axel Zarske in 2014, resolving the name of a fish that had moved through the hobby for at least five decades under the trade designation Hyphessobrycon robertsi — a name that was never validly published and thus a nomen nudum in the ichthyological literature. Zarske's description established the species within the large, diverse genus Hyphessobrycon, one of the biggest genera in the family Characidae, a family of several thousand species distributed across Central and South America.
The valid name is governed by Eschmeyer's Catalog of Fishes as the primary authority for current fish nomenclature. The genus Hyphessobrycon is characterised broadly by small size, presence of a Weberian apparatus (as in all characins), and typically a dark lateral stripe or humeral blotch, though variation within the genus is substantial and its boundaries remain subject to ongoing phylogenetic revision.
Morphology
H. jackrobertsi is a small tetra, with FishBase recording a maximum standard length of 1.5 in; most individuals in the aquarium are in the 1–1.5 in range. The body is laterally compressed and moderately deep in the characteristic tetra fashion. The most immediately striking feature is the male's dorsal fin: it is greatly elongated and falcate — sickle-shaped — accounting for both common names in English (Sickle Tetra) and German (Sichelsalmler, meaning 'sickle salmon-let').
The overall coloration is typical of softwater Hyphessobrycon species: a pale to silvery body with darker lateral markings, reddish highlights on the fins in good condition, and the elongated dorsal fin providing a clear visual distinction between the sexes. Females lack the extended dorsal and have a deeper, more rounded body profile, particularly when gravid. The species' striking fin morphology makes it easy to identify even among other small tetras.
Habitat
The species is recorded from the Río Pastaza basin in Peru — a major tributary of the upper Amazon system draining the eastern slopes of the Andes. In this region, lowland and foothill streams carry the soft, slightly acidic blackwater conditions typical of Amazonian white-sand and organic-substrate environments. Hobbyist and trade sources consistently describe the species as inhabiting areas with strong aquatic and marginal vegetation, decaying organic matter on the substrate, and sandy soils — conditions that produce tannin-stained, low-conductivity water.
Under brighter light the fish tends to seek cover among dense planting; it becomes more active and confident under diffuse or subdued lighting. This preference for shaded, vegetated microhabitats is relevant both to its ecology in the wild and to its behaviour in the aquarium.
Feeding
In the wild, H. jackrobertsi is an omnivore and micropredator, feeding on small invertebrates, insect larvae, and organic detritus in the manner of most small Hyphessobrycon species. It is not selective.
In the aquarium it accepts a wide variety of foods without difficulty. Good-quality flake food and micro-pellets form a practical staple, but the fish thrives and displays best when the diet is supplemented with frozen and live foods — Cyclops, Daphnia, mosquito larvae, brine shrimp, and bloodworm are all taken readily. Trade sources note that vegetable matter is not a significant part of its diet; frozen and meaty foods are preferred. Small, frequent feedings suit the fish's size and active metabolism.
Mating
Like other members of Hyphessobrycon, H. jackrobertsi is an egg-scattering open spawner. Males display their elongated, sickle-shaped dorsal fins prominently in courtship, using the exaggerated fin structure as a visual signal to both rival males and receptive females. The fish are peaceful among themselves but males compete visually, so keeping a group of several individuals brings out the most active display behaviour.
Spawning is initiated when a male courts a female into fine-leaved vegetation or among spawning mops, where eggs are scattered and left without parental attention. The pair do not form lasting bonds, and multiple females may spawn with the same male over a spawning period.
Breeding
Trade and hobbyist sources characterise H. jackrobertsi as an egg-scatterer that spawns in the manner typical of the genus. Adhesive eggs are deposited among fine-leaved plants or spawning mops, and the adults provide no parental care; they will eat their own eggs if not separated. A separate, dimly lit spawning tank with clumps of Java moss or synthetic mops, soft and slightly acidic water, and a conditioned breeding pair or trio produces the best results.
Eggs hatch within a few days at breeding temperatures in the mid-to-upper 20s Celsius. The fry are small and require infusoria or commercial liquid fry food as first foods before graduating to newly hatched brine shrimp and finely ground flake. The species has been maintained in captivity and bred in Europe since at least the 1950s or 1960s — trade records from Aquarium Glaser indicate it was present in the hobby under the trade name H. robertsi from around 1958 — so a stock of husbandry knowledge has accumulated even without a formal species description until 2014.
In the aquarium
H. jackrobertsi is a peaceful, active schooling fish well-suited to community aquaria with other small, non-aggressive species. It should be kept in groups of six or more to display natural schooling behaviour and to encourage the males to display their sickle-shaped dorsal fins competitively. A densely planted aquarium with some open swimming space, subdued or diffuse lighting, and a dark substrate shows the species to best advantage and replicates the shaded vegetated habitats it favours.
Water chemistry should lean toward the soft, slightly acidic conditions of its native blackwater habitat: temperatures of 72–82 °F (72–82 °F) and pH in the range of 5.5–7.2 are consistently reported across trade and hobbyist sources, with softer water and lower pH preferred for breeding. The species is robust and forgiving of a wider range in general keeping. Good tankmates include other small peaceful tetras, rasboras, small corydoras, and similarly sized non-aggressive community fish. It is not suitable with fin-nipping species, which would quickly damage the males' elongated dorsal fins.
Conservation
Hyphessobrycon jackrobertsi has not been formally assessed by the IUCN Red List as of this writing; all sources note it as Not Evaluated (NE). The species is distributed in the Río Pastaza basin of Peru, a region that remains relatively intact compared to more heavily deforested parts of the Amazon basin, though habitat degradation and collection pressure are ongoing concerns across Amazonian lowlands generally.
The species' long history in the aquarium trade — under trade names before it was formally described — means that captive populations are established, and most specimens available to hobbyists are likely captive-bred. This reduces pressure on wild populations. Given its small size and the broad range of its home basin, no immediate population collapse is indicated by available data, but no formal assessment has been conducted.