Taxonomy & naming
Hyphessobrycon citrus was described by Marinho & Dagosta in 2024 as part of a revision that also provided a redescription of the sympatric H. cachimbensis from the same Serra do Cachimbo locality. The original description appeared in Neotropical Ichthyology (volume 22, issue 2, article e230127; DOI 10.1590/1982-0224-2023-0127), published by the Sociedade Brasileira de Ictiologia on SciELO. The type locality is the headwaters of the rio Braço Norte, a tributary of the rio Teles Pires within the upper rio Tapajós basin, on the Serra do Cachimbo plateau, Pará, Brazil.
Within Hyphessobrycon — one of the largest and most species-rich genera in the Characidae, with over 150 recognised species — H. citrus belongs to a cluster of small, colourful tetras centred on the highlands of central Brazil. Diagnostic characters that distinguish it from congeners include the bright yellow body coloration, vivid red eye, a well-defined dark midlateral stripe running the length of the flank, and an anal-fin ray count of 14–17, with the anal-fin base notably convex in males. The species is most similar to H. cachimbensis, from which it differs in coloration and in details of fin morphology. Because the genus Hyphessobrycon has not been revised comprehensively, its monophyly and internal relationships remain incompletely resolved; the species is placed here in Characidae following current Catalog of Fishes usage.
Morphology
Hyphessobrycon citrus reaches a maximum recorded standard length of 2 in, placing it squarely in the small-tetra size range. The body form is the typical compressed, fusiform tetra shape — moderately deep, with a rounded snout and a forked tail — that characterises most Hyphessobrycon species.
The coloration is the species' most immediately arresting feature. A bright, saturated yellow covers much of the body, reminiscent of citrus-fruit rind — the quality that inspired the name. A bold, dark midlateral stripe runs horizontally along the flank from behind the operculum to the base of the caudal peduncle, contrasting sharply with the yellow ground colour. The eye is vivid red. The fins may show additional pigmentation, and in males the anal-fin base is convex rather than straight, a useful morphological character for sex determination in hand. Details of fin-ray counts and body proportions beyond those listed in the original description have not been widely reported in the secondary literature.
Habitat
The species is endemic to the Serra do Cachimbo, a sandstone plateau in southern Pará, Brazil, where it inhabits the headwaters of the rio Braço Norte, a tributary of the rio Teles Pires and thus part of the upper rio Tapajós drainage. The Serra do Cachimbo sits near the watershed divide between the Tapajós and Xingu river systems, a topographic position that isolates its headwater streams and generates the very high rates of freshwater endemism documented throughout the plateau.
Headwater streams of this type in the cerrado-forest transition of the central Brazilian highlands are typically shallow, clear to lightly tea-stained, and low in dissolved minerals. Precise water-chemistry records for the H. citrus collection site are not available in the published sources consulted, but conditions typical of similar upland tributaries in the Tapajós headwaters include soft, slightly acidic to neutral water, warm temperatures year-round, and substrates of sand or fine gravel with leaf litter and submerged root systems. The restricted watershed means the species has no known populations outside this single drainage system.
Feeding
No published data on the natural diet of Hyphessobrycon citrus is available in the sources consulted. By analogy with other small Hyphessobrycon species studied in the wild, the species is likely an opportunistic micropredator and omnivore, foraging in the mid-water and near the surface for small aquatic invertebrates — insect larvae, microcrustaceans, and zooplankton — as well as incidentally consuming algae and fine plant material.
In the aquarium, small Hyphessobrycon species typically accept a wide range of prepared foods including micro-pellets and fine flake, as well as small live and frozen foods such as newly hatched brine shrimp, Daphnia, and cyclops. A varied diet is advisable to support condition and colour. Because the species is newly described and not yet widely established in the trade, detailed captive-feeding observations are limited.
Mating
No specific observations on the mating behaviour of Hyphessobrycon citrus have been published. The genus as a whole comprises open-water egg-scatterers that spawn among fine-leaved vegetation or over the substrate without forming pair bonds or establishing territories in any lasting sense. Males of many Hyphessobrycon species can be distinguished from females by a convex anal-fin base — a character explicitly recorded for H. citrus in the original description — and by somewhat more intense coloration when in breeding condition.
Typical courtship in the genus involves brief, active chases as males pursue females through vegetation, culminating in side-by-side quivering spawning runs during which eggs and sperm are released simultaneously. No parental care is provided, and adults will consume eggs given the opportunity. Seasonal cues such as temperature drops, rainfall simulation, and longer photoperiods are often used by aquarists to condition Hyphessobrycon for spawning, reflecting the seasonality of natural rainfall regimes in the species' Brazilian highland range.
Breeding
Specific captive breeding records for Hyphessobrycon citrus are not available in the literature consulted; the species was described only in 2024 and its availability in the hobby remains limited. General breeding protocols for small Hyphessobrycon species provide a reasonable guide. Breeding is most reliably achieved in a dedicated spawning tank with soft, slightly acidic, warm water and bunches of fine-leaved plants such as Java moss or synthetic spawning mops to catch the adhesive eggs.
Egg-scattering Hyphessobrycon species typically produce small, adhesive eggs that adhere to plant material after release. Adults must be removed promptly after spawning to prevent egg predation. Eggs hatch within one to three days at typical breeding temperatures, and the fry are free-swimming a few days later. First foods should be very fine: infusoria or commercial liquid fry preparations, followed by newly hatched brine shrimp as the fry grow. Because the species is newly described and not widely held in captivity, husbandry information will accumulate as it enters the hobby more broadly.
In the aquarium
Hyphessobrycon citrus is not yet established in the mainstream aquarium trade as of 2024–2026, having been formally described only in 2024. Its brilliant yellow coloration, red eye, and compact size of under 2 in SL make it potentially attractive to specialist collectors of rare South American tetras, and it may become available through specialist breeders or import as interest in Serra do Cachimbo fauna grows.
Based on its ecology and on care requirements typical for small Hyphessobrycon species from upland Brazilian headwaters, it is likely best maintained in soft, slightly acidic to neutral, warm water (approximately 75–82 °F), in a well-filtered, moderately planted aquarium. A group of six or more individuals is advisable to observe natural schooling behaviour and reduce individual stress. Tankmates should be small, peaceful species of comparable size. Because the natural habitat is a restricted upland watershed, sudden changes in water chemistry or temperature should be avoided. The species' care requirements in captivity remain to be fully documented.
Conservation
Hyphessobrycon citrus has not been assessed by the IUCN Red List as of the time of writing; as a species described in 2024, it falls into the common post-description gap before formal conservation evaluation is conducted. FishBase lists the species as Not Evaluated.
The biology of H. citrus raises immediate conservation concern regardless of formal status. The species is known only from the headwaters of the rio Braço Norte on the Serra do Cachimbo plateau — a single restricted watershed with no known additional populations. Serra do Cachimbo is also the site of the Cachimbo Air Force Base, and the plateau's sandstone forests and headwater streams are subject to pressures from land-use change in the surrounding region. Headwater endemics with no population buffer are inherently vulnerable: any sustained degradation of the type locality could affect the species significantly. The region's high overall endemism makes it a conservation priority for Brazilian freshwater biodiversity, and formal IUCN assessment of H. citrus should be a near-term goal as the species becomes better known.