Taxonomy & naming
Danio dichromatus was formally described by Ralf Britz and Sven O. Kullander in 2024, making it one of the more recently erected species in the genus Danio. The type locality is the Melaung Chaung, a stream in Palaw Township, Tanintharyi Region, southern Myanmar (approximately 12°45'26"N, 98°44'22"E). The Catalog of Fishes (Eschmeyer, CAS) is the governing authority for its valid name.
The species had circulated in the aquarium hobby under the informal trade names 'yellow hikari danio' and 'blue hikari danio' before its formal description — names that already hinted at the dichromatic coloration that eventually became the basis of its scientific epithet. Both trade names referred to the same species, with the colour difference reflecting the sexual dimorphism rather than distinct varieties.
Danio dichromatus belongs to the true danios, a group of small, slim, horizontally patterned or spotted schooling cyprinids within the family Danionidae. The boundaries of the genus Danio have shifted over decades — some authors have historically split off smaller species into Brachydanio — but the Catalog of Fishes retains dichromatus and its relatives in Danio.
Morphology
As its specific name indicates, Danio dichromatus is notably dichromatic: the two sexes differ markedly in coloration, which is unusual within a genus where sexual differences are typically limited to body depth and condition. The trade names 'yellow hikari' and 'blue hikari' capture this dimorphism, with one sex showing yellowish-gold iridescent tones and the other a bluish iridescence.
Beyond coloration, the species shares the typical danio body plan — a slender, laterally compressed form with a slightly upturned mouth, small barbels, and a horizontal emphasis to the patterning. Precise size data from the original description are not available in current secondary sources, but the species is consistent with the small-danio size range typical of its congeners in Myanmar and adjacent drainages.
Habitat
Danio dichromatus is known from the Tanintharyi, Dawei, and Lenya river drainages in the Tanintharyi Region of southernmost Myanmar, a long, narrow peninsula bordering the Andaman Sea. The type locality, Melaung Chaung in Palaw Township, is a freshwater stream within this region.
The river systems of Tanintharyi drain forested hills and coastal lowlands, and small cyprinids in such drainages typically occupy clear to moderately turbid streams with substrates of gravel, sand, and leaf litter, often beneath riparian forest canopy. Water conditions in analogous southern Myanmar stream habitats tend toward soft, slightly acidic to neutral parameters, reflecting the predominantly granitic and sedimentary catchments. Specific temperature, pH, and hardness data for the type locality are not yet available in the scientific literature accessible at time of writing.
Feeding
Like its congeners, Danio dichromatus is presumed to be an omnivorous micropredator in the wild. Danios from similar stream habitats feed on small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, and worms — taken from the surface and water column, supplemented by algae, plant fragments, and organic detritus that drift through the current.
In the aquarium, hikari danios adapt readily to standard small-fish foods: quality flake, micro-pellets, and small live or frozen items such as daphnia, cyclops, and newly hatched brine shrimp. Offering a varied diet maintains colour intensity and condition, which is especially relevant for a species whose appeal rests partly on its vivid sexual dichromism.
Mating
Danio dichromatus is an egg-scattering, non-guarding spawner, as is characteristic of the genus. There is no pair bond, no nest, and no parental investment beyond the act of spawning. Courtship follows the pattern typical of danios: a driving male pursues a gravid female through vegetation or open water, pressing alongside her as eggs and milt are released simultaneously.
The marked sexual dichromatism in this species — the very feature encoded in its name — suggests that colour plays a role in mate recognition or male competition, though the details of courtship signalling have not been studied formally. Spawning readiness in females is apparent from the noticeably distended abdomen that gravid individuals develop.
Breeding
Breeding Danio dichromatus in captivity follows the standard approach for small stream danios. A shallow spawning tank with a mesh bottom or a dense layer of fine-leaved plants prevents the adults from eating the eggs immediately after scattering them. Conditioning the pair or group on live and frozen foods over several days before introducing them to the spawning setup typically accelerates spawning.
Eggs are adhesive and fall among plants or onto the substrate. Adults should be removed after spawning is complete. Eggs hatch in approximately two to three days at tropical temperatures and larvae become free-swimming within a few more days, at which point they accept infusoria, green water, or commercial fry foods before transitioning to newly hatched brine shrimp. No parental care is given at any stage.
In the aquarium
Danio dichromatus entered the hobby as an undescribed 'hikari danio' before its formal description in 2024, and its vivid dichromatic coloration made it sought-after among enthusiasts of small SE Asian cyprinids. It is a shoaling species that should be kept in groups of at least six individuals; the interplay of male and female coloration is most visible in a properly sized group in a mature, well-planted aquarium.
A longer tank with open midwater swimming space, areas of fine-leaved planting, and gentle filtration approximating soft, slightly acidic stream water suits the species well. Because it originates from a restricted range in forested Myanmar drainages, water quality should be maintained carefully. Peaceful, similarly sized tankmates from comparable habitats — small rasboras, other slim danios, microdevario — make appropriate companions. Given its restricted natural range and the pressures on Tanintharyi forests, acquiring captive-bred stock from reputable sources is strongly preferable to wild-caught imports.
Conservation
Danio dichromatus has not yet been formally assessed by the IUCN Red List, which is unsurprising given that it was only formally described in 2024. Its conservation status is therefore Not Evaluated (NE) at present.
The species' known distribution is restricted to three river drainages in the Tanintharyi Region of southern Myanmar — a peninsula that, while still relatively forested compared to other parts of mainland Southeast Asia, faces growing pressure from logging, agricultural expansion, and infrastructure development. Small-stream cyprinids with limited ranges are inherently vulnerable to habitat modification, and the restricted range of D. dichromatus places it in the category of species that warrant monitoring as survey data accumulate. Formal assessment through IUCN would be a valuable next step as knowledge of its distribution and population status develops.