Taxonomy & naming
Rasbora lacrimula was described by Renny Kurnia Hadiaty and Maurice Kottelat in 2009, published in Ichthyological Exploration of Freshwaters (volume 20, issue 2, pages 105–109). The type locality is the Ulu Belayan, at coordinates 0°16'57.4"N, 116°18'07.8"E, within the Mahakam River drainage of Kalimantan Timur (East Kalimantan), Indonesian Borneo. The Catalog of Fishes (Eschmeyer, CAS) is the authority for the valid name; the combination Rasbora lacrimula remains current.
The genus Rasbora has historically served as a catch-all for small to medium Southeast Asian cyprinids, and over recent decades several genera have been carved out of it — among them Boraras, Trigonopoma, Brevibora, Kottelatia and Rasboroides — leaving a more circumscribed but still large core genus. Rasbora lacrimula sits within the remaining Rasbora sensu stricto and has not been reassigned. It is closely allied to other small Bornean rasboras and represents the kind of range-restricted, morphologically subtle species that Kottelat and colleagues have routinely uncovered through systematic revision of Sundaland's freshwater fauna.
The specific epithet lacrimula is Latin for 'little tear', almost certainly an allusion to a small tear-like marking on the body, consistent with the pattern-based epithets common across Rasbora.
Morphology
Rasbora lacrimula is a small cyprinid with a maximum standard length in the range of 1 in, placing it among the smaller members of its genus. FishBase records a total length of approximately 1.5 in. The body is slender and laterally compressed, typical of the shoaling rasboras, with a slightly upturned mouth suited to feeding at or near the water surface.
In life the fish displays a reddish to cherry-red flush across the body, the colouration that inspired the vernacular name Red Cherry Rasbora among hobbyists. Detailed meristic and morphometric data are set out in the original description by Hadiaty and Kottelat (2009). Sexual dimorphism follows the pattern typical of small rasboras: females tend to be deeper-bodied and visibly rounder when gravid; males are slimmer and typically more intensely coloured.
Habitat
The species is endemic to the Ulu Belayan, a tributary within the lower Mahakam River drainage of East Kalimantan, Indonesian Borneo. The type locality lies within what was at the time of collection an oil palm plantation, and specimens were taken from gutters and small streams associated with agricultural land — narrow, slow-flowing channels over sandy, clay and muddy substrates with marginal vegetation.
FishBase describes the species as also inhabiting clear, shallow forest streams typically 2–3 metres wide and about 1 metre deep over sandy or muddy substrates, as well as calm, tannin-rich blackwater pools beneath a dense forest canopy. These two microhabitats — the open agricultural ditch and the shaded blackwater pool — share slow flow and low-mineral, acidic water chemistry. Seriously Fish gives the recommended aquarium range of pH 4.0–7.5 and temperature 72–79 °F (72–78 °F), with a hardness ceiling of about 12°dGH, reflecting the soft, low-conductivity waters of Sundaland's interior drainage.
Feeding
Rasbora lacrimula is a micropredator in the wild, feeding on small invertebrates such as zooplankton, tiny aquatic insects and other minute animal prey available at and near the water surface or in the water column. The upturned mouth is consistent with surface and near-surface feeding.
In the aquarium, Seriously Fish recommends small live and frozen foods — Daphnia and Artemia nauplii are particularly suitable — supplemented with high-quality micro-pellets and fine flake food. The species' small adult size means it requires appropriately sized particles and benefits from the enrichment that varied live and frozen invertebrates provide.
Mating
Like all Rasbora species examined to date, R. lacrimula is presumed to be an egg-scattering, non-guarding spawner with no pair bond and no parental care beyond the act of spawning itself. In the wild, spawning in small Bornean rasboras is typically triggered by seasonal conditions — rising water levels or rainfall — and takes place among fine-leaved aquatic vegetation, submerged roots or over sandy and muddy substrates.
Courtship involves a male pursuing a gravid female closely through vegetation, pressing against her flanks as they release eggs and milt together. Adhesive eggs sink or attach loosely to plants and substrate, after which the adults take no further interest in the spawn and will consume eggs they encounter.
Breeding
Rasbora lacrimula has not been widely bred in captivity and detailed hobbyist breeding reports are sparse. By analogy with other small Bornean rasboras in the genus, a soft, warm, acidic spawning environment is the most likely requirement: pH toward the lower end of the species' tolerance (around 5.0–6.5), temperature around 75–79 °F, very soft water with low mineral content, and a substrate of fine-leaved plants, Java moss or similar material to catch the scattered adhesive eggs.
Adults should be removed after spawning to prevent egg predation. Fry are expected to be small and to require infusoria or very fine zooplankton as a first food, progressing to Artemia nauplii as they grow. Given the species' sensitivity to water quality noted by hobbyists, maintaining pristine, stable parameters through the breeding attempt is essential.
In the aquarium
Rasbora lacrimula is a peaceful, active shoaling fish well suited to a quiet species or community aquarium with small, non-aggressive companions. Seriously Fish recommends a minimum group of eight to ten individuals; smaller numbers leave the fish nervous and the shoaling behaviour that makes the species rewarding to watch is not expressed properly. Good companions include other small Southeast Asian cyprinids, small loaches and Corydoras of similar temperament.
The tank should replicate the species' Bornean blackwater origins: soft, acidic water (pH 4.0–7.0 in the aquarium), temperature 72–79 °F, and ideally some tannin-staining from driftwood or leaf litter to provide both chemistry and visual security. Fine-leaved plants or floating vegetation are useful. The species is noted to be sensitive to deteriorating water quality, so good filtration and regular partial water changes are non-negotiable. Its small adult size of around 1 in standard length means it is vulnerable alongside larger or boisterous tankmates.
Conservation
Rasbora lacrimula is assessed as Data Deficient (DD) on the IUCN Red List, reflecting the limited distributional and population data available for a species known from a single river system in East Kalimantan. The type locality in the Ulu Belayan lies within an oil palm plantation, and the original collectors noted it from agricultural drainage channels as well as natural streams — a finding that may reflect the species' ability to persist in modified habitats, or simply that natural habitat in the area has already been largely replaced.
Kalimantan's interior lowland streams face severe pressure from oil palm and timber expansion, peat drainage, erosion and water-quality degradation. A species with as restricted a known range as R. lacrimula is inherently vulnerable to localised habitat loss, and the Data Deficient listing should not be read as reassurance: it signals that assessment was impossible rather than that the species is secure. Systematic field surveys of the Ulu Belayan and adjacent Mahakam tributaries are needed to establish whether populations are stable and whether the range extends beyond the single type locality.