Taxonomy & naming
Microdevario nanus was originally described as Microrasbora nana by Maurice Kottelat and Jens-Werner Witte in 1999, from material collected in the Sittaung river basin of Myanmar — specifically from Kyauk Tan Chaung stream, roughly 2 mi from the town of Daik U in the Irrawaddy Delta. The epithet nanus (Latin: dwarf) was applied in reference to the species' exceptionally small adult size.
In 2009, Fang and colleagues erected the genus Microdevario following molecular phylogenetic analyses demonstrating that the group of tiny, laterally compressed fishes previously placed in Microrasbora formed a distinct clade within the family Danionidae, separate from the true rasboras and from the broader Devario/Danio cluster. The current valid combination is therefore Microdevario nanus (Kottelat & Witte, 1999), with the parenthetical authority reflecting the change of genus. The Catalog of Fishes (Eschmeyer, CAS) treats Microdevario as a valid genus within Danionidae, subfamily Danioninae.
In older literature and in some aquarium trade references the species still appears under its former name Microrasbora nana; these should be treated as synonyms. It has no widely accepted common name in the English-language hobby.
Morphology
Microdevario nanus is one of the smallest cyprinids recorded, reaching a standard length of only 0.5 in at full adult size. The body is slender and laterally compressed, strongly translucent, and the thin musculature makes the vertebral column visible under good lighting. The mouth is small and slightly upturned, consistent with a fish that picks invertebrates from the water column and surface film.
Colouration is subtle: the body is pale gold to silvery with a thin horizontal lateral stripe or iridescent lateral sheen running along the mid-flank. The fins are generally colourless to faintly yellowish. At this size, individual fish can be easy to overlook, but a shoal moving together through a planted tank creates an appealing shimmer. No elaborate finnage or striking polymorphism has been recorded.
Habitat
The type locality of Microdevario nanus is Kyauk Tan Chaung stream in the Sittaung river basin, with the species' range extending across the Irrawaddy Delta system of Myanmar — including the lower Sittaung and Bago river tributaries. FishBase notes it may have a wider distribution within the delta than the type locality alone suggests.
It inhabits sluggish, heavily vegetated freshwater bodies: rice paddies, seasonal swamps, weedy ponds, roadside ditches and slow backwaters. These are warm, shallow habitats with dense submerged vegetation, organic substrates and generally low water movement. Recorded water chemistry indicates warm temperatures of 68–81 °F, pH from mildly acidic to neutral (6.0–7.5) and moderate hardness (18–215 ppm), suggesting tolerance of a fairly broad range of soft-to-slightly-hard conditions typical of lowland delta environments.
Feeding
Microdevario nanus is a micropredator and omnivore. In nature its diet is composed of small invertebrates — zooplankton, small crustaceans, insect larvae — together with algal material and organic detritus filtered from the water column. The upturned mouth positions it well for feeding near the surface and in the mid-water zone rather than at the substrate.
In the aquarium it requires suitably small food items. Micro-pellets, finely crushed flake and powdered foods are accepted, but the species thrives with regular offerings of live and frozen zooplankton-sized invertebrates: Daphnia, baby brine shrimp (Artemia nauplii) and micro-worms. Given its tiny mouth gape, standard-sized dry foods may be too large for consistent ingestion, so particle size is an important consideration.
Mating
Like all danionin fishes, Microdevario nanus is an egg-scattering, non-guarding spawner. There is no pair bond, no nest construction and no territory held in connection with reproduction. Courtship consists of an energetic pursuit in which a conditioned male drives a gravid female through dense vegetation or mossy substrate, the two fish briefly pressing together as eggs and milt are released simultaneously.
Spawning may be stimulated in captivity by small water changes with slightly cooler fresh water, increased feeding with live foods and the provision of fine-leaved plants or moss. Because adults will consume their own eggs, the eggs must be protected or adults removed promptly if intentional breeding is the goal.
Breeding
In the aquarium, Microdevario nanus can be induced to spawn in a small, heavily planted breeding vessel. A layer of fine-leaved plants such as Java moss or spawning mops provides cover for scattered adhesive eggs and reduces adult predation. Breeders typically condition a small group of fish on live or frozen invertebrates, then transfer them to the breeding tank, where spawning usually occurs among the plants in the morning hours.
Incubation lasts approximately 72 hours at typical tropical temperatures. The larvae are very small on hatching and require appropriately tiny first foods — infusoria, commercial fry powder or rotifers — before graduating to Artemia nauplii. Growth is slow relative to larger danios, reflecting the species' diminutive adult size. Adults should be removed after spawning to prevent egg predation.
In the aquarium
Microdevario nanus is a nano-fish best suited to small, dedicated planted aquaria rather than community tanks with larger species, which may out-compete or intimidate it at feeding time. A group of ten or more individuals is recommended: like all shoaling danionins it is bolder and feeds more confidently in numbers, and the collective shimmer of a healthy shoal is the chief visual appeal of the fish.
Water conditions should reflect the species' lowland delta origins: temperatures of 68–81 °F, a pH of 6.0–7.5 and soft to moderately hard water. Dense fine-leaved planting, gentle filtration and subdued lighting bring out the fish's natural behaviour and reduce stress. Suitable tankmates are other nano-scale species of similar temperament — small rasboras, pygmy corydoras and diminutive livebearers — and caution is warranted with fish large enough to view this tiny cyprinid as prey.
Conservation
Microdevario nanus is assessed as Least Concern (LC) on the IUCN Red List. The species' distribution in the Irrawaddy Delta lowlands is not narrowly restricted to a single water body, and FishBase notes high resilience with a minimum population doubling time of less than 15 months — a characteristic of small, fast-maturing, prolific species.
Nonetheless, the lowland floodplain habitats it occupies — rice paddies, seasonal swamps and ditches in the Irrawaddy Delta — are subject to intensifying agricultural development, pesticide application, water extraction and habitat modification across Myanmar's delta region. The species' dependence on dense aquatic vegetation in sluggish, shallow waters makes it sensitive to the draining or chemical degradation of those microhabitats. Collection pressure for the nano-fish aquarium trade is a minor additional factor. No specific conservation measures are currently in place for this species.