Taxonomy & naming
Rasbora dies was described by Maurice Kottelat in 2007, with the original description published in Ichthyological Exploration of Freshwaters, volume 18 (pages 301–305). The Catalog of Fishes (Eschmeyer, CAS) records the valid name as Rasbora dies Kottelat, 2007. The species belongs to the family Danionidae, subfamily Danioninae — the same subfamily that encompasses danios and a large swath of the small Asian cyprinids broadly called rasboras.
The genus Rasbora has historically been a large holding group, and over the past two decades numerous species formerly treated as rasboras have been moved into segregated genera: Boraras, Trigonopoma, Brevibora, Kottelatia and Rasboroides were carved out on the basis of morphological and molecular evidence. Rasbora dies, however, has not been subject to such a transfer; the Catalog of Fishes retains it in Rasbora. No subspecies or synonyms are currently recognised.
Morphology
Rasbora dies is a small, slender cyprinid in the general body plan typical of the genus — laterally compressed, with a pointed snout and an upturned or terminal mouth suited to surface and mid-water feeding. The defining diagnostic characters, as established in Kottelat's original description, are a reticulated pattern on the flank and a small, diamond-shaped spot at the base of the caudal fin. A complete lateral line distinguishes it from some congeners in which the lateral line is incomplete or absent.
FishBase records a trophic level of 3.1 (±0.4 se), consistent with an insectivore or generalised micropredator, and the species is assigned high resilience, with a minimum population doubling time estimated at less than 15 months. Published size data are sparse; no maximum standard length has been widely reported, and like most Borneo-endemic Rasbora it is presumed to be a small fish, likely in the 1–2 in range.
Habitat
Rasbora dies is endemic to eastern Borneo (Indonesia) and its natural habitat belongs to the lowland freshwater systems of Sundaland — the island-arc region that includes Borneo, Sumatra and the Malay Peninsula, much of which formed a single landmass during glacial sea-level low-stands. Borneo's eastern lowlands are drained by rivers that flow through or originate in peat-swamp forests, and the blackwater streams associated with these systems are characterised by soft, strongly acidic water stained amber-brown with dissolved humic and fulvic acids from decomposing organic matter.
Habitat data specific to Rasbora dies are sparse, but Borneo-endemic rasboras from comparable lowland ecoregions typically inhabit shallow, slow-flowing or still water with leaf litter, submerged roots and overhanging riparian vegetation. Water temperatures in such environments generally range from around 72–82 °F, with pH values spanning roughly 4.0 to 7.0, hardness very low, and dissolved oxygen sometimes depressed. Specific collection-locality data for this species remain unpublished beyond the broad attribution to eastern Borneo.
Feeding
No specific field diet studies have been published for Rasbora dies. Its trophic level of 3.1, assigned by FishBase, places it among omnivores and generalised micropredators — consistent with the feeding ecology of most small Rasbora species, which take a mix of small aquatic and terrestrial invertebrates, zooplankton, algae and organic detritus.
In the aquarium, fish of this type are reliably accommodated on a varied diet of fine dried foods — quality micro-pellets and flakes — supplemented with small live or frozen items such as daphnia, mosquito larvae and micro-worms, which support condition and encourage natural foraging behaviour.
Mating
No breeding observations from the wild have been published for Rasbora dies. Like all Rasbora and the broader danio-rasbora assemblage, it is presumed to be an egg-scattering, non-guarding spawner. In the typical rasbora reproductive mode, a ripe male pursues a gravid female through vegetation or over substrate; eggs and milt are released in a brief close-contact spawning embrace, then left entirely unattended.
There is no pair bond, no nest construction, and no parental care whatsoever. Adults will eat their own eggs if given the opportunity, which in the wild is partly mitigated by the cover and complexity of leaf-litter and plant root habitats.
Breeding
Rasbora dies has not been bred in captivity to any documented degree in the available literature. Based on what is known of closely related Borneo-endemic rasboras, the breeding approach would involve soft, acidic water (pH 4.5–6.5) with low hardness and a temperature around 77–79 °F, with fine-leaved plants or Java moss to receive the scattered adhesive eggs.
Egg-scattering rasboras typically produce small clutches of adhesive eggs that lodge among vegetation or settle to the substrate. Parents show no care and should be removed after spawning. Fry are small at hatching and initially require infusoria-grade foods.
In the aquarium
Rasbora dies is essentially unknown in the aquarium trade. It has not been regularly imported or commercially propagated, and no dedicated husbandry accounts exist. Any specimens that have been kept would logically be housed under the conditions suited to other Borneo blackwater rasboras: a heavily planted or botanicals-rich aquarium with soft, acidic water (pH 4.5–6.5), low hardness, subdued lighting, and a dark substrate to bring out the reticulated flank pattern.
As a presumed small shoaling cyprinid from still or slow-flowing blackwater, it would benefit from the company of its own kind and the security of a dense planting scheme. Water movement should be gentle; strong filtration flow is not appropriate for fish from peat-swamp habitats. The diagnostic caudal-spot and reticulated pattern make it a visually distinctive species, and it is likely that any aquarium population would attract the interest of specialist keepers once the species becomes available through targeted export or captive breeding.
Conservation
Rasbora dies is assessed as Data Deficient (DD) on the IUCN Red List (assessed 1 January 2019). The Data Deficient designation reflects the near-absence of survey data on its distribution, abundance and population trends — not necessarily that the species is not at risk, but that the information required to make an assessment does not currently exist.
Eastern Borneo faces substantial and ongoing land-use pressure: peat-swamp forests have been drained, burned and converted to oil-palm plantation at high rates across Kalimantan, and the blackwater streams and peat-swamp drainage systems that endemic freshwater fish depend on are among the most threatened freshwater habitats in Southeast Asia. A species with a restricted, lowland Bornean range and a dependence on intact blackwater habitat is logically exposed to those pressures, even if population data to confirm decline are lacking. Targeted surveys to establish the species' range and population status would be needed before a more informative IUCN category can be assigned.