Taxonomy & naming
Devario laoensis was originally described from Ban Nam Khueng in north-western Laos and placed under the older combination Parabarilius laoensis. The species is also known from the synonym Danio myersi, reflecting the unsettled generic boundaries that characterised danionin taxonomy through much of the twentieth century. Under the current Catalog of Fishes (Eschmeyer et al., CAS), the valid name is Devario laoensis (Pellegrin & Fang, 1940), with authority parenthesised because the species has moved out of its original genus.
The genus Devario, the 'giant danios' of the trade, contains the larger-bodied members of the tribe Danionini — fishes often exceeding 2 in that share the same egg-scattering breeding strategy and shoaling behaviour as the smaller Danio species but occupy a subtly different ecological niche as more active open-water swimmers of fast hill streams. Devario is an Asian genus distributed from the Western Ghats of India through mainland Southeast Asia to southern China.
Morphology
Devario laoensis reaches a maximum total length of around 3 in, with aquarium specimens typically settling at 2–2.5 in standard length. It is a slim, fusiform fish with a slightly upturned mouth suited to surface and mid-water feeding, and the elongate, laterally compressed body plan typical of fast-water Devario.
The species is identified by its lateral colour pattern: a dark stripe that originates as a relatively thin line behind the head, then thickens and intensifies to a deep, prominent band as it approaches the caudal peduncle, where it terminates at the base of the tail. This anteroposterior gradient of stripe width and darkness distinguishes it from all other Devario species known at the time of its description. Sexual dimorphism is moderate; females are deeper-bodied and rounder in the belly, particularly when gravid, and are somewhat larger and less vividly marked than males.
Habitat
In the wild, Devario laoensis inhabits rapidly flowing hill streams and small rivers in the upper Mekong system, with confirmed localities in north-western Laos (the type locality) and the Chiang Rai and Chiang Mai provinces of northern Thailand. The habitat is characterised by clear, well-oxygenated water, gravel and boulder substrates, and moderate to dense marginal and riparian vegetation.
Water conditions in these Indochinese montane drainages are soft to moderately hard and near-neutral in pH (approximately 6.0–7.5), with temperatures ranging from around 73 °F to 79 °F in the warmer months. The catchments receive heavy monsoon rainfall between May and October, producing seasonal spate conditions and elevated turbidity, before dropping to clearer, cooler flows in the dry season. The species is benthopelagic in habit, holding in the current over gravel beds and schooling actively in open water.
Feeding
Devario laoensis is an opportunistic omnivore, feeding in the wild primarily on insects and small aquatic invertebrates taken from the surface film and the water column. The upturned mouth is well suited to intercepting terrestrial insects and emergent midges that form a significant proportion of the diet in fast-running streams, particularly during the monsoon months when terrestrial productivity is high.
In the aquarium the species is unfussy and accepts both dry and live or frozen foods readily. Good-quality flakes and small pellets form a practical staple, supplemented with live or frozen daphnia, bloodworm and brine shrimp to maintain colour and condition. Feeding at the surface and in the upper water column, it can be an energetic competitor at mealtimes and should be kept with equally active tankmates to ensure equitable food distribution.
Mating
Devario laoensis is an egg-scattering, non-guarding spawner. No pair bond is formed and no nest or territory is established. In the wild, spawning is likely tied to the onset of the monsoon rains, when rising water levels, increased flow and a flush of new invertebrate food prime fish to spawn in shallow, vegetated margins.
Courtship follows the pattern typical of danionin cyprinids: a ripe male pursues a gravid female through the water column, pressing close alongside her among plants or over the substrate. The pair briefly press together and release eggs and milt simultaneously. There is no contact beyond the spawning act itself, and the adults take no further interest in the eggs.
Breeding
Like all Devario, D. laoensis scatters adhesive eggs over fine-leaved plants, moss, gravel or other substrate, and gives no parental care. Adults will consume their own eggs and fry given the opportunity, so a separate breeding tank with a layer of fine-leaved plants such as Java moss, or a mesh bottom above a layer of marbles, is the standard approach to protect the spawn.
A small temperature rise to the upper end of the species' range and the addition of fresh, slightly cooler water can trigger spawning in a conditioned group. Eggs typically hatch within two to three days at 75–77 °F, and larvae become free-swimming a few days after that, taking infusoria or commercial fry foods initially before graduating to newly hatched brine shrimp nauplii. Growth is reasonably rapid, and the species reaches sexual maturity within a few months under good conditions.
In the aquarium
Devario laoensis is an active, mid-to-upper water schooling fish that performs best in a longer tank with good water flow and open swimming space. A group of at least six individuals is recommended — this is a fish that schools naturally in fast-moving streams and becomes nervous or prone to pestering tankmates when kept in small numbers. Ideally, the tank should replicate the species' native hill-stream environment: a strong current from a powerhead or spray bar, a substrate of smooth gravel or rounded river stone, and some driftwood and fine-leaved plants along the margins.
Water parameters should reflect the montane Mekong origin: temperatures of 73–79 °F, a pH of 6.0–7.5 and soft to moderately hard water (roughly 18–180 ppm as GH). The species will not thrive in warm, stagnant or oxygen-poor conditions. Compatible tankmates include other fast-water Devario and larger Danio, hill-stream loaches, and other active cyprinids that can keep pace with it. It is not aggressive but its speed and vigour make it a poor match for slow, delicate, or long-finned companions.
Conservation
Devario laoensis is assessed as Least Concern (LC) on the IUCN Red List (2011 assessment). Its range spans suitable hill-stream habitat across the upper Mekong basin in Laos and northern Thailand, and there is no evidence of significant population decline across this range at present.
Nonetheless, the montane stream habitats of the upper Mekong are not without pressure. Deforestation in the catchments of northern Laos and Thailand increases silt loading and reduces water quality in the clear, fast-flowing streams the species requires. Hydroelectric development on Mekong tributaries can alter flow regimes and fragment habitat. The species is not known to be heavily collected for the ornamental trade. Given the relatively limited collection data from this part of the Mekong system, ongoing monitoring of upland stream health in the region is warranted.