Taxonomy & naming
Microdevario microphthalmus was described by Jiang, Chen and Yang in 2008, originally placed in the genus Microrasbora as Microrasbora microphthalma. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) now records the valid combination as Microdevario microphthalmus (Jiang, Chen & Yang, 2008), the parenthetical authority reflecting the subsequent generic transfer out of Microrasbora.
The genus Microdevario was erected to receive a cluster of very small Southeast Asian cyprinids previously scattered across Microrasbora and Danio. Molecular analyses and morphological revision confirmed that the Microdevario species form a distinct lineage from the true rasboras and danios, though they remain members of the broader danionin group. The species is known only from its type locality and has accumulated very little post-description literature, so information on intraspecific variation and full distribution remains limited.
Morphology
As its name suggests — microphthalmus from the Greek for 'small-eyed' — the species has notably small eyes relative to head size, a feature that distinguishes it from several of its congeners. It is an extremely diminutive fish, with a maximum standard length of approximately 1 in; total length reportedly reaches 1.5–2 in, though figures for the largest specimens are tentative given the sparse material examined.
The body form is typical of the genus: slender and laterally compressed, with a small, slightly upturned mouth suited to surface and mid-water feeding. Colouration in life has not been extensively documented in the aquarium literature; the original description notes the reduced eye as the most immediately diagnostic external character, alongside standard meristic counts used to separate it from congeners such as Microdevario nanus and M. kubotai.
Habitat
The type locality is a small tributary of the Nanwan River, itself a tributary of the upper Irrawaddy, located approximately 1 mi from Simenkan Village in Longchuan County, Yunnan, China, at an elevation of 946 metres (coordinates 24°16'32.5"N, 97°54'06.6"E). This is montane, fast-flowing hill-stream habitat in the upper Irrawaddy catchment — quite different from the soft, warm, tannin-stained blackwater peat swamps associated with some Microdevario species further south in mainland Southeast Asia.
At nearly 3117 ft elevation the water is likely clear, cool by tropical standards, and well-oxygenated. The broader Irrawaddy upper drainage supports a mix of hill-stream specialists, and the species is presumably a benthic or near-benthic fish of rocky riffles or cobble-bottomed runs. Specific water-chemistry measurements from the type locality have not been published, but the montane hill-stream context suggests harder, more neutral water than the acidic lowland systems favoured by some congeners.
Feeding
No dedicated feeding study exists for Microdevario microphthalmus. By inference from its morphology and the biology of closely related Microdevario species, it is a micropredator and opportunistic omnivore, taking small invertebrates — aquatic insect larvae, microcrustaceans and zooplankton — along with algae and fine organic matter.
Its small, slightly upturned mouth positions it as a mid-water to surface feeder, typical of danionin cyprinids generally. In captivity, congeners such as M. kubotai accept finely crushed flake and micropellets alongside live or frozen foods; the same diet can be expected to sustain M. microphthalmus, though the species is rarely seen in the hobby and husbandry data are sparse.
Mating
Microdevario microphthalmus has not been bred in documented captive conditions, and no field observations of courtship or spawning have been published. Like all known members of Microdevario and the broader danionin assemblage, it is assumed to be an egg-scattering, non-guarding spawner.
In related micro-danios, a male in breeding condition pursues a gravid female through vegetation or over substrate, the pair pressing together as eggs and milt are released simultaneously. No pair bond forms, and neither parent guards eggs or fry. Spawning is likely tied to seasonal conditions in its montane habitat — perhaps to rainfall and rising water temperature in the warmer months.
Breeding
No confirmed captive breeding of Microdevario microphthalmus has been recorded in the aquarium or scientific literature. The species is virtually absent from the hobby, having been collected only at its type locality with very limited material reaching the trade.
Based on congener biology, the breeding mode would be egg-scattering over fine substrate, moss or plants, with adhesive or weakly adhesive eggs that receive no parental care. Adults would eat eggs and fry given access. Were the species to become available, the approach used for M. kubotai — a small, densely planted tank with soft acidic water, careful conditioning on live foods, and separation of adults after spawning — would serve as a reasonable starting template, adjusted for the cooler, possibly harder water of its montane origin.
In the aquarium
Microdevario microphthalmus is extremely rarely encountered in the aquarium trade and must be considered a specialist or research fish rather than a standard hobby species. No husbandry accounts exist in the mainstream literature. At under 1 in standard length it would require a small, stable, well-filtered tank, peaceful nano or micro-fish tankmates, and very fine foods.
The montane hill-stream origin suggests somewhat cooler and potentially harder, more neutral water than the soft-acid conditions recommended for lowland Microdevario species. Until further collection data or captive experience is published, the specific requirements of this fish remain poorly defined. Any aquarist fortunate enough to obtain specimens would be contributing genuinely useful documentation to an otherwise data-sparse species.
Conservation
Microdevario microphthalmus has not been formally assessed by the IUCN Red List. It is known only from a single small tributary of the upper Irrawaddy in Yunnan, China — a geographically restricted range that would likely qualify it for a threatened or Data Deficient category were a formal assessment conducted.
The upper Irrawaddy drainage in Yunnan lies in a region facing growing pressures from agricultural expansion, deforestation on steep slopes, and hydropower development. A species restricted to a single, small montane stream at an area so narrowly defined is inherently vulnerable to any localised habitat disturbance. The very limited collection material and near-total absence from the aquarium trade mean the population status of this fish is unknown; targeted field surveys at and near the type locality would be needed to establish its conservation standing.