Taxonomy & naming
Sundadanio margarition was described by Conway, Kottelat & Tan in 2011, as part of a broad taxonomic review of the Southeast Asian miniature cyprinid genus Sundadanio — a genus established to accommodate a cluster of tiny, blackwater-associated danionin cyprinids from the peat swamps and blackwater streams of Borneo and Sumatra. The review (published via Pfeil Verlag and available through ResearchGate) diagnosed seven new species within the genus alongside S. axelrodi, the type species.
The genus name Sundadanio references the Sunda region — the biogeographic landmass encompassing Sumatra, Borneo, Java and the Malay Peninsula — combined with danio, the well-known genus of small cyprinids to which these fishes are allied. The species epithet margarition derives from the Latin or Greek for 'pearl' or 'little pearl,' an allusion to the lustrous, iridescent markings of this tiny fish. The Catalog of Fishes (Eschmeyer) is the authority for the current valid name Sundadanio margarition Conway, Kottelat & Tan, 2011.
Sundadanio belongs to the tribe Danionini and is closely related to other miniature cyprinid genera of Sundaland. It should not be confused with Microrasbora, Boraras, or the celestial pearl danio (now Danio margaritatus) — the latter's epithet margaritatus ('adorned with pearls') is similar but refers to a completely different species from the Inle/Hopong plateau of Myanmar.
Morphology
Sundadanio margarition is among the smallest fishes in its already miniature genus, recorded at a maximum standard length of approximately 0.5–1 in (SL) on FishBase. Like all Sundadanio, it is a laterally compressed, delicate fish with a notably small mouth and the reduced scaling texture typical of cyprinids adapted to low-conductivity blackwater environments.
The body is translucent in life, with iridescent markings that give the species its pearlescent epithet — small, lustrous patches or spots that catch light against a background of coppery or golden transparency. The fins are small and rounded; there is no striking sexual dichromatism documented in the literature, though females are typically rounder-bodied when gravid, as in other micro danios. The combination of extreme small size, translucency and iridescent markings is characteristic of the genus as a whole and distinguishes Sundadanio from the slightly larger and more robustly built members of Rasbora or Devario with which it might superficially be grouped.
Habitat
Sundadanio margarition is restricted to the coastal peat swamp forests of Sarawak, in the Malaysian portion of Borneo, within the Rajang and Sarawak River drainages. Peat swamp forest streams are among the most chemically extreme freshwater habitats on Earth: water percolates slowly through deep accumulations of partially decomposed plant matter, emerging dark brown with dissolved humic and fulvic acids, with pH values commonly below 4.0 and conductivity close to zero. These are warm, oxygen-stratified, deeply shaded environments, and the fish communities that inhabit them are correspondingly specialised.
The broader genus review (Conway, Kottelat & Tan 2011) confirms that Sundadanio species are characteristic inhabitants of peat swamp and blackwater streams across Borneo and Sumatra, with S. margarition specifically recorded from the Sarawak coastal peat swamp zone. Temperatures in such lowland equatorial habitats remain broadly stable, typically in the range 75–82 °F year-round, with little seasonal fluctuation. The fish lives among submerged roots, leaf litter and tangles of aquatic vegetation in water that carries almost no dissolved minerals.
Feeding
Like other members of the genus and subfamily, Sundadanio margarition is presumed to be a micropredator and micro-omnivore, feeding on the tiny invertebrates — small crustaceans, aquatic insect larvae, microorganisms and zooplankton — that inhabit peat swamp water and associated leaf litter. Its small, upward-facing mouth is adapted for feeding in the upper portions of the water column and at the surface film.
In the aquarium, Sundadanio are known to accept very fine live and frozen foods: micro-worms, baby brine shrimp nauplii, daphnia, cyclops and suitably small prepared foods. The extremely small mouth and body mean that particle size must be carefully matched; standard community flakes may be too large. Feeding frequency and variety are important to maintain condition in so diminutive a fish.
Mating
Sundadanio margarition is an egg-scattering, non-guarding spawner — the reproductive strategy shared by danios and the vast majority of small cyprinids. There is no pair bond, no nest construction, no bubble-nest, and no mouthbrooding. A reproductively active male pursues a gravid female through vegetation and root tangles, and spawning occurs as both fish release eggs and milt into the water column or among fine-leaved plants and substrate.
The adhesive or weakly adhesive eggs settle among vegetation and detritus, where the adults give them no further attention and will consume them if encountered. In the peat swamp environment, the acidic, humic-stained water may play a role in egg conditioning, as it does in related Sundanic species where soft, acidic water is strongly associated with spawning success.
Breeding
Captive breeding of Sundadanio species, including S. margarition, demands careful replication of the extreme blackwater conditions of their native peat swamps. This means very soft water (near-zero hardness), strongly acidic pH — often 4.5–6.0 — and tannin-rich, amber-coloured water achieved with leaf litter (Indian almond leaves or oak leaves) or peat filtration. Temperature should be maintained in the equatorial range of 75–81 °F. A heavily planted or moss-rich tank with subdued lighting suits the natural habitat.
Spawning proceeds as for other micro danios: eggs are scattered among plants or moss and are weakly adhesive. Because the parents will eat eggs and newly hatched fry, a separate breeding vessel with a fine mesh or dense moss layer is recommended to protect the spawn. Fry are extremely small at hatching and require the finest live foods — infusoria, rotifers and freshly hatched brine shrimp nauplii — before graduating to slightly larger items. Growth is slow relative to larger danio species, and conditioning the adults on high-quality live food before and during attempts significantly improves spawning frequency and fry survival.
In the aquarium
Sundadanio margarition is a challenging species for the specialist aquarist rather than a general community fish. Its requirements for extreme soft, acid, tannin-stained water exclude most standard community tankmates; the ideal setting is a dedicated nano blackwater aquarium replicating the peat swamp conditions of Sarawak. A small, densely planted or moss-lined tank of 5.5–10 US gal with slow filtration through peat or over a deep leaf-litter bed, very subdued lighting, and zero mineral hardness provides the best environment.
Compatible tankmates are limited to other Sundanic blackwater specialists with identical chemistry needs — other Sundadanio species, Boraras, small Pangio loaches or Sphaerichthys gouramis where water chemistry overlaps. Because of its tiny size and retiring nature, it should never be combined with species that might outcompete it for food or treat it as prey. Sundadanio margarition is rarely exported for the aquarium trade and commands specialist interest; its availability is sporadic and it should be considered a fish for experienced keepers who can reliably maintain blackwater conditions.
Conservation
Sundadanio margarition is assessed as Near Threatened (NT) on the IUCN Red List, evaluated in January 2019. The assessment cites habitat degradation as the primary threat: the coastal peat swamp forests of Sarawak, which constitute essentially the entire known range of this species, have been subject to extensive conversion for oil palm plantation and other agriculture, drainage for development, logging, fire, and peat oxidation. The Rajang and Sarawak River drainages where this fish is endemic have seen substantial peat swamp loss over recent decades.
A second pressure noted by the IUCN is collection for the ornamental fish trade, which, while not the dominant threat, adds incremental pressure to a species with a restricted, endemic range. The specialised chemistry of intact peat swamp water — which cannot simply be replicated by adding acid to tap water without the full suite of humic compounds — means that habitat degradation effectively eliminates suitable conditions without replacement. Given continuing pressure on Borneo's remaining lowland peat swamp forests, the conservation trajectory for narrow-range endemic species like S. margarition is a matter of ongoing concern.