Taxonomy & naming
Hyphessobrycon kayabi was formally described by Teixeira, Lima & Zuanon in a paper published in Copeia in 2013 (volume 2013, issue 4, pages 612–621). The valid name in Eschmeyer's Catalog of Fishes is Hyphessobrycon kayabi Teixeira, Lima & Zuanon, 2014, with authority parentheses absent since the species was placed directly in Hyphessobrycon at description. The species belongs to the large and taxonomically complex genus Hyphessobrycon, which encompasses several hundred small characids distributed across South and Central America.
Hyphessobrycon is a member of the family Characidae in the broad modern sense. The genus is notoriously difficult to delimit on morphological grounds alone, and phylogenetic work has repeatedly shown it to be non-monophyletic, with various subgroups revised or split over time. H. kayabi joins a guild of small, macrophyte-associated Hyphessobrycon known from clear-water habitats in central Brazil, though its precise affinities within the genus await comprehensive molecular study.
Morphology
Hyphessobrycon kayabi is a diminutive tetra, with a recorded maximum standard length of 1 in — placing it at the smaller end of the genus. FishBase records 10–12 dorsal soft rays and 21–24 anal soft rays, proportions consistent with other small Hyphessobrycon. The body is laterally compressed and fusiform in the typical characid manner.
Detailed colouration data from the original description is not captured in the available sources; the fish's striking visual appeal at the type locality would be in keeping with the clear, spring-fed water of Lagoa Azul, where light penetration is high and the crystal medium would showcase any lateral stripe or flank iridescence. Published imagery from the type locality suggests a small, modestly patterned tetra without extreme ornamentation — functional for a fish whose primary niche involves browsing periphyton from macrophyte surfaces rather than open-water schooling.
Habitat
The type locality of H. kayabi is Lagoa Azul, a terra-firme lake near the Rio Teles Pires in Mato Grosso State, Brazil. This lake is fed by springs and holds crystal-clear water with well-developed subaquatic macrophytes — the kind of high-transparency, low-colour habitat that differs markedly from the blackwater and whitewater rivers more often associated with Hyphessobrycon. The species is placed within the Rio Tapajós drainage, one of the great clear-water tributaries of the Amazon.
The macrophyte beds that characterise Lagoa Azul are central to the species' ecology: H. kayabi grazes periphyton from the leaf surfaces of submerged plants and takes small invertebrates associated with that vegetation. FishBase lists the species as benthopelagic and freshwater tropical, with a trophic level of 2.9 reflecting its mixed plant-and-invertebrate diet. High resilience (population doubling time under 15 months) may reflect the productive, spring-fed conditions of its habitat.
Feeding
In the wild, H. kayabi consumes a diet dominated by vegetal matter — macrophyte fragments and algae — supplemented by small invertebrates including testate amoebae, Cladocera, aquatic Hemiptera, and Chironomidae larvae. FishBase characterises its foraging behaviour as browsing periphyton from submerged macrophyte leaves, suggesting it moves deliberately through vegetation rather than cruising open water for suspended particles.
In captivity, small characids with similar ecological profiles generally accept high-quality micro-pellets, finely crushed flake, and live or frozen small invertebrates such as Cyclops, Daphnia, and newly hatched brine shrimp. A vegetable component — spirulina-enriched foods or blanched plant matter — would reflect the significant algal fraction of its natural diet. Frequent small meals are preferable to single large feedings for fish of this size.
Mating
No dedicated mating observations of H. kayabi in the wild or in captivity have been published in the available sources. By analogy with the genus and family, Hyphessobrycon species are egg-scattering open spawners: males court females with fin-display behaviour, and spawning takes place over or among fine-leaved aquatic plants or other fibrous substrate. Adhesive or semi-adhesive eggs are scattered among the vegetation with no site preparation.
For a macrophyte-associated species like H. kayabi, spawning among the dense subaquatic plant beds of its spring-fed habitat would be the expected context in the wild. Both sexes likely reach sexual maturity within their first year, as is typical for small, high-resilience characids.
Breeding
Captive breeding of H. kayabi has not been documented in the hobbyist literature captured by available sources. The species is rarely encountered in the aquarium trade. Given its ecological setting — crystal-clear, well-oxygenated, spring-fed water — a breeding setup emulating those conditions would be the logical starting point: soft to moderately hard, neutral to slightly acidic water of high clarity, with clumps of fine-leaved plants or synthetic spawning mops as egg deposition sites.
As with essentially all Hyphessobrycon, no parental care is expected: adults should be removed after spawning to prevent egg predation. At the small adult size of under 1 in, the fry would be correspondingly tiny and would require infusoria or commercial micro-fry foods as first feeding before graduating to larger live foods. Water quality and temperature stability are likely more critical than specific chemistry targets for such a spring-adapted species.
In the aquarium
Hyphessobrycon kayabi is not an established aquarium fish and is essentially absent from the hobby. It has not entered significant commercial breeding lines. An aquarist who did obtain specimens would be working with a small, spring-water-adapted tetra best suited to a biotope or species aquarium that mimics the crystal-clear, macrophyte-rich conditions of Lagoa Azul: high water clarity, moderate temperature, neutral to slightly acidic pH, and dense planting with fine-leaved species.
At under 1 in, it belongs with similarly sized, peaceful tankmates if kept in a community setting — other small Hyphessobrycon or comparably gentle micro-tetras. A species aquarium with a fine sand substrate, abundant macrophyte planting, and careful attention to water quality would do most justice to its ecology and make any future captive-breeding work more likely to succeed.
Conservation
Hyphessobrycon kayabi is assessed as Least Concern (LC) on the IUCN Red List, with the assessment completed on 7 November 2018 by Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio). No threats were documented in the assessment at that time.
The species' restricted known range — a single spring-fed terra-firme lake and its immediate environs in the Teles Pires watershed — means that localised habitat changes could have disproportionate effects on the population. The Teles Pires basin has experienced significant hydroelectric development, with multiple dams constructed and others planned; the long-term implications for spring-fed terra-firme lakes such as Lagoa Azul remain worth monitoring, even if the species currently meets no quantitative threshold for a threatened category.