Taxonomy & naming
Dawkinsia assimilis was first described by the British surgeon and naturalist Thomas C. Jerdon in 1849, and for most of its taxonomic history it was known under the combination Puntius assimilis — the old catch-all genus Puntius having absorbed an enormous diversity of South and Southeast Asian barbs. It also appears in the literature as Systomus assimilis, an earlier umbrella combination used for larger-bodied Indian barbs.
The genus Dawkinsia was erected by Pethiyagoda, Meegaskumbura and Maduwage in their landmark 2012 revision of the South Asian Puntius complex, which broke that unwieldy assemblage into several more natural genera based on morphological and phylogenetic evidence. The genus was named in honour of the evolutionary biologist Richard Dawkins. Under the Catalog of Fishes (Eschmeyer, the authority for valid names) the current valid combination is Dawkinsia assimilis (Jerdon, 1849), the parenthetical authority marking that the species has been moved from Jerdon's original genus. A neotype has been designated from the Nethravati River, Karnataka (specimen BNHS FWF 1010, 3 in SL), anchoring the name to material from the type locality.
Morphology
The mascara barb is a moderately deep-bodied, laterally compressed cyprinid reaching roughly 2–2.5 in standard length in typical aquarium specimens, though the neotype from the Nethravati River measured 3 in SL, indicating the species can attain somewhat larger sizes in the wild. The body is silvery to brassy with a pattern of dark markings that give the species its common name — the name "mascara" likely alludes to bold dark pigmentation around the eye or along the body.
Sexual dimorphism follows the typical barb pattern: females become noticeably deeper-bodied and fuller in the belly when gravid, while males are slimmer and may show more intense colouration during the breeding season. The species shares the general Dawkinsia body plan — a moderately pointed snout, a moderately large eye, and the single dorsal fin positioned above the pelvic fins that is characteristic of the family Cyprinidae.
Habitat
Dawkinsia assimilis is confined to the rivers of the southern Western Ghats, with documented records from the Nethravati and Chalakudy rivers in Kerala and Karnataka, and the Kallada river system; it has also been noted from the area of the Kanjirappally waterfall. These are rivers that rise in the forested Western Ghats escarpment and flow westward to the Arabian Sea, dropping steeply through hill and foothill terrain before reaching the coastal lowlands of Kerala.
The species inhabits cool, fast-running, well-oxygenated water typical of the hill-stream zone — rocky and gravelly substrates, clear or lightly turbid water with moderate to high current. Water chemistry across the range of Seriously Fish records points to slightly acidic to neutral conditions, pH approximately 6.5–7.2, and temperatures between 18 and 82 °F, with the cooler end of that range reflecting the altitude of many collection sites. Like most Western Ghats endemics it is associated with intact forest catchments; deforestation and upstream impoundments alter the hydrology and temperature regime on which it depends.
Feeding
Dawkinsia assimilis is an omnivore in the wild, taking small aquatic invertebrates — insect larvae, worms, and crustaceans — along with algae, diatoms, and plant matter picked from the rocky substrate and the water column. Fast-flowing hill-stream habitats typically support a productive invertebrate fauna, and the species' relatively slender build and active disposition suit a fish that forages continuously over rocks and riffles.
In aquaria it accepts standard prepared foods readily — quality sinking pellets and flakes, supplemented with live and frozen items such as bloodworm, daphnia, and brine shrimp. Some vegetable component (spirulina flakes, blanched greens) is a useful addition to approximate the plant matter and algae in its natural diet. Regular feeding with varied foods supports good condition and bright colouration.
Mating
Like all barbs, Dawkinsia assimilis is an egg-scattering, non-guarding spawner. There is no nest, no bubble-nest, and no parental bond beyond the brief act of spawning itself. When fish are in condition, a driving male pursues a gravid female through fine-leaved plants or over the rocky substrate, pressing alongside her as eggs and milt are released together in short bursts.
The eggs are adhesive and settle among plant leaves, roots, gravel interstices, or onto the bed. No territory is established and no guard is maintained. Males may compete intensely for access to receptive females, with colour and activity heightened during courtship, but once spawning is complete all adults disperse and the eggs are left entirely to their own fate — or to be eaten by the spawning fish themselves.
Breeding
In the aquarium, Dawkinsia assimilis can be bred following the standard approach for Western Ghats hill-stream barbs. A separate, shallow spawning tank is set up with slightly cooler, soft to moderately hard, near-neutral water — reflecting the cool hill-stream conditions of the native range. Fine-leaved plants or a spawning mop provide a substrate for the adhesive eggs to lodge in, and a grid or mesh covering the bottom reduces predation by the adults once spawning is complete.
Conditioned adults — fed for several days on live and frozen foods — are introduced in the evening, and spawning typically occurs in the morning. A female may scatter several hundred small adhesive eggs in a session. Adults are removed promptly after spawning, as they will consume the eggs. Hatching takes approximately 24–36 hours, and the larvae become free-swimming a few days later, initially taking infusoria or commercially available fry foods before graduating to newly hatched brine shrimp and finely crushed flake.
In the aquarium
Dawkinsia assimilis is an active, sociable barb that should be kept as a shoal of at least six individuals to express natural behaviour and reduce stress. A roomy aquarium with moderate to brisk water movement — simulating the hill-stream current of the Western Ghats — suits the species well, with a sandy or fine-gravel substrate, smooth rocks, and some planting. Good filtration and high oxygen levels are important, as the species comes from fast, well-aerated water.
Water parameters should aim for the cool to mid-temperature range (approximately 68–79 °F), slightly acidic to neutral pH (6.5–7.2), and soft to moderately hard conditions. Peaceful, similarly sized active fishes make the best tankmates. The species is best not combined with very slow, long-finned fishes or with species demanding consistently warm tropical temperatures, as these conflict with the cooler conditions the mascara barb prefers. It is not the most commonly available Western Ghats barb in the trade, and some collection pressure has been noted.
Conservation
Dawkinsia assimilis is assessed as Endangered (EN) on the IUCN Red List. The species is a narrow-range endemic of the southern Western Ghats — one of the world's most significant freshwater biodiversity hotspots and simultaneously one of the most threatened. Its documented range spans only a handful of river systems in Kerala and Karnataka, and several of those rivers have been altered by upstream impoundments that fragment habitat, raise water temperatures, and change the flow regimes on which hill-stream cyprinids depend.
Collection for the regional aquarium pet trade adds to the pressure, and this interaction between trade collection and wild-population decline in small-range Western Ghats endemics has been the subject of academic concern. Forest clearance in the catchments reduces riparian shade and accelerates siltation, degrading the rocky, clear-water microhabitats the species requires. Conservation of the mascara barb is closely tied to the broader protection of the Western Ghats forest and river systems — a challenge that intersects with competing demands for water, hydroelectric power, and agricultural land across the region.