Taxonomy & naming
Sundadanio echinus was formally described by Kevin W. Conway, Maurice Kottelat and Tan Heok Hui in 2011. The Catalog of Fishes (Eschmeyer, CAS) recognises the valid combination as Sundadanio echinus Conway, Kottelat & Tan, 2011. Before the genus Sundadanio was established, this fish was known under the earlier combination Rasbora echinus, placing it among a sprawling assemblage of small Asian cyprinids that has since been progressively dismembered. The erection of Sundadanio recognised a clade of extremely small, peat-swamp-associated micro-cyprinids endemic to the Sunda shelf whose anatomy and ecology set them apart from both true Rasbora and the broader danio assemblage.
The species epithet echinus is Latin for 'hedgehog' and refers to the prominent, spiny tubercles on the lower jaw — an unusual tuberculation for fish of this size and, together with other osteological details, one of the characters that helped diagnose the species when the 2011 description was prepared. The genus name Sundadanio combines 'Sunda' (the Sunda shelf biogeographic region of western Indonesia and Malaysia) with 'danio,' reflecting both geography and systematic affinity.
Morphology
Sundadanio echinus is one of the world's tiniest fish, with a maximum recorded standard length of 1 in — placing it at the extreme lower end of vertebrate body size. The body plan is slender and laterally compressed with a small, slightly upturned mouth suited to surface and mid-water feeding. The most distinctive external feature is the coarse, keratinised spiny tubercles that stud the lower jaw, giving the snout a rough texture visible under magnification and evoking the epithet echinus.
Colouration in life is delicate: the flanks typically carry fine horizontal stripes or a series of spots and iridescent flecks against a translucent or pale background, with the lateral coloration varying subtly between the Bornean and Sumatran populations. The finnage is small and rounded. As with many micro-cyprinids of the peat swamps, precise colour descriptions from living specimens are the most reliable guide, and preserved material loses the iridescence quickly.
Habitat
This species is native to Borneo — principally West Kalimantan (Indonesian Borneo) and western Sarawak (Malaysian Borneo) — and western Sumatra, where it inhabits lowland peat swamps and slow-moving blackwater streams draining the Sunda shelf peat deposits. These habitats are characterised by water of extraordinary chemistry: temperatures are warm and stable (roughly 75–82 °F), the water is soft and often mineralised only by organic acids, pH values can fall well below 5 in the deepest peat swamp blackwater, and visibility is low due to dissolved tannins and humic acids leached from decomposing vegetation.
The surrounding canopy of peat-swamp forest shades the water, keeps temperatures moderate and supplies a constant rain of terrestrial invertebrates and plant matter. Aquatic vegetation is sparse in the most acidic sites; submerged roots, leaf litter and waterlogged wood provide the principal structural complexity. Sundadanio echinus is almost always collected in or around such structures, where it shelters and forages in small groups.
Feeding
Like other micro-cyprinids of comparable size, Sundadanio echinus is an opportunistic micropredator and micro-omnivore, feeding on the small invertebrates, zooplankton, insect larvae and organic detritus that characterise the food web of peat-swamp blackwater. Its upturned mouth and small gape constrain prey size: the diet is dominated by organisms no larger than small chironomid larvae, copepods and cladocerans.
In the aquarium the species must be offered appropriately sized live or frozen foods — micro-worms, infusoria, Walter worms, baby brine shrimp and similar items — because its mouth is simply too small to handle standard community flakes. Quality micro-pellets can supplement the diet once fish are settled, but live foods remain essential for conditioning and long-term health in captivity.
Mating
Sundadanio echinus is an egg-scattering, non-guarding spawner in the broad danio tradition. Spawning behaviour has not been extensively documented in the wild, but aquarium observations and the biology of closely related Sundadanio species indicate a pattern of active male pursuit of gravid females through fine-leaved vegetation, roots or moss, with eggs and milt released together during brief lateral presses.
There is no pair bond and no nest. Males in breeding condition may develop slightly more intense colouration and become persistently active in pursuit of ripe females, but the social dynamic is diffuse within a group. The soft, acidic, warm conditions of the peat swamp likely serve as seasonal spawning triggers, though in the stable year-round climate of the Sunda lowlands the triggers are less dramatic than the monsoon-driven cycles seen in continental Asian species.
Breeding
Breeding Sundadanio echinus in captivity is a rewarding but demanding undertaking reserved for experienced micro-fish keepers. Replicating the soft, hyper-acidic chemistry of the natural habitat is the critical first step: water with pH 4.5–5.5, near-zero hardness (conductivity below 50 µS/cm), temperature around 77–81 °F and heavy tannin staining from catappa leaves or peat filtration represents the target environment. Spawning is most likely over a dense mat of Java moss or fine-leaved plants.
Eggs are adhesive and scattered among the plants or on the substrate; the adults will consume them if not separated promptly. Given the fish's minute size, all fry-rearing foods must be correspondingly tiny — infusoria cultures are the standard first food, graduating to micro-worms and nauplii as the fry grow. Because of this species' small wild range and Vulnerable conservation status, successful captive propagation by dedicated hobbyists carries genuine conservation value.
In the aquarium
Sundadanio echinus is a specialist fish that rewards the keeper willing to meet its precise requirements. The overriding priority is water chemistry: this is not a fish that will tolerate neutral or alkaline tap water long-term. A mature, heavily tannin-stained blackwater setup with pH around 5–6 and very low hardness is the baseline; many keepers use RO water buffered with catappa extract or peat. Temperature should remain in the mid-twenties Celsius.
Because of its minute size, tankmates must be chosen with great care — only fish of similar stature and placid temperament are suitable, such as other Sundadanio species, small Boraras, or dwarf rasboras from comparable blackwater habitats. A species or near-species tank is often the safest arrangement. A group of at least eight to ten individuals helps the fish behave naturally and reduces stress. A planted tank with root tangles, leaf litter and a dark substrate showcases the species' iridescent lateral markings and enables the natural foraging behaviour that keeps these fish in condition.
Conservation
Sundadanio echinus is assessed as Vulnerable (VU) on the IUCN Red List (2019 assessment). The classification reflects the species' restricted natural range — confined to the peat-swamp habitats of West Kalimantan and western Sarawak on Borneo plus western Sumatra — and the severe, ongoing degradation of that habitat. The lowland peat swamps of the Sunda shelf are among the most carbon-dense and biologically distinctive ecosystems on Earth, and they have been lost at catastrophic rates to oil-palm agriculture, logging, drainage schemes and peat fires over recent decades.
For a species with such a small range and such specific habitat requirements, the continued contraction of peat-swamp forest presents an existential threat. Collection for the aquarium trade is a minor secondary factor compared to habitat destruction, though responsible trade practices and captive breeding can at least reduce extraction pressure. Conservation of Sundadanio echinus is ultimately inseparable from the broader challenge of protecting what remains of Sundaland's lowland peat-swamp forest.