Taxonomy & naming
Rasbora trilineata was described by Franz Steindachner in 1870. The Catalog of Fishes (Eschmeyer, the authority for valid names) records the species in the genus Rasbora, and the type locality is Johore (Pengulon Patie) on the Malay Peninsula — the lectotype held at the Natural History Museum Vienna (NMW 51459:9) was formally designated by Kottelat in 2013.
The genus Rasbora has been substantially revised over the past three decades. Several genera — including Boraras, Trigonopoma, Brevibora, Kottelatia, Rasboroides and Trigonostigma — were carved out of the old, broadly circumscribed Rasbora by Kottelat, Witte and others from 1999 onward. Rasbora trilineata itself has remained in Rasbora throughout these revisions and is not considered a candidate for generic transfer. The epithet trilineata (Latin: 'three-lined') refers to the three dark stripes that characterise this species: the mid-lateral stripe, the stripe along the anal fin base and, less prominently, a dorsal component to the lateral banding.
Morphology
The scissortail rasbora is a large rasbora, with adults commonly reaching 4–5 in total length in the wild; FishBase records a maximum of 5 in TL, and captive specimens typically reach around 4 in under normal aquarium conditions. The body is elongate and laterally compressed, with a terminal mouth, a rounded snout and a deeply forked caudal fin whose lobes taper to fine tips.
Coloration is understated but precise: the dorsal surface is olive-grey to darkish green, the flanks are bright silver, and a well-defined black stripe runs from behind the head to the fork of the tail along the mid-lateral line. A second dark stripe follows the base of the anal fin. The caudal fin lobes each carry a dark blotch near their base, and these markings flash conspicuously when the fish snaps its tail — the source of the 'scissortail' common name. The remainder of the fins are largely hyaline. Sexual dimorphism is modest: females are noticeably deeper-bodied when gravid.
Habitat
Rasbora trilineata occupies a wide swathe of lowland freshwater habitats across mainland Southeast Asia and the Sunda shelf. Its range spans the Chao Phraya and lower Mekong systems in Thailand, Laos and Cambodia, extends south through Peninsular Malaysia, and continues across the Strait of Malacca into Sumatra and Borneo. The Mekong records include the lower basin in Laos, Thailand and Cambodia.
In the wild it inhabits forest streams, pool margins, peat-swamp fringes and other shaded, slow to moderately flowing lowland waterways. Such habitats are often tea-stained by leaf litter and dissolved organic matter and may be notably soft and acidic, particularly in Sundaland peat-swamp settings. FishBase reports pH 6.0–8.0 and hardness around 5–12 dH, with water temperatures of 73–77 °F; the Seriously Fish profile notes a typical aquarium range of 72–77 °F and indicates the species tolerates pH 5.0–8.0, with 6.5–7.5 considered ideal. It is benthopelagic and strongly schooling in the wild, forming large shoals in mid-water.
Feeding
Rasbora trilineata is an insectivore and opportunistic invertebrate feeder. In the wild it takes aquatic and terrestrial insects, small invertebrates and organic matter from the water column and surface. Its active, schooling behaviour suggests a fish that feeds throughout the day across a broad depth range in open water.
In the aquarium it is undemanding and readily takes high-quality flake and small granule foods. Condition and colour are maintained by regular offerings of live or frozen invertebrates — daphnia, artemia, bloodworm, mosquito larvae and similar items — which also encourage natural foraging behaviour. Its size means it should not be housed with very small fishes or offered food particles too large for its relatively modest mouth.
Mating
Like all Rasbora, the scissortail is an egg-scattering, non-guarding spawner. There is no nest construction, no bubble-nest, no mouthbrooding and no guarding of eggs or larvae. Spawning is triggered by a combination of warmth, water movement and the presence of fine-leaved plants or other substrate onto which eggs can adhere lightly. In the wild, breeding probably coincides with seasonal wet-season flooding that brings fish into new, heavily vegetated shallow margins.
Courtship is a vigorous, male-driven pursuit: a conditioned male follows a gravid female closely through the water column, and the pair press together fleetingly as eggs and milt are released into vegetation or open water above the substrate. The adults then disperse; there is no pair bond, no recognition of offspring and no further investment beyond the spawning act itself.
Breeding
Captive breeding of Rasbora trilineata has been documented in detail. A study published via PubMed Central (PMC) recorded eggs produced per female at up to 343, with a mean fertilisation rate of approximately 80.3%; eggs hatched within 18 hours at 82–86 °F, producing larvae of around 0.5 in at hatch that reached roughly 1 in total length by 40 days post-hatch — the size at which juveniles are typically exported for the aquarium trade. Breeding temperatures recorded in that study were 77–86 °F at pH 6.0–6.5.
In the aquarium, conditioning with live and frozen foods brings females into spawning condition readily. A separate, well-planted spawning tank or one furnished with fine-leaved plants and moss is recommended; a gentle rise in temperature and a partial water change with slightly softer water often triggers spawning. Because the adults scatter adhesive eggs freely and will consume them given the chance, parents should be removed once spawning is complete. Eggs hatch in 18–48 hours at typical spawning temperatures, and larvae are free-swimming within a few days. First foods are infusoria and vinegar eels, progressing rapidly to newly hatched brine shrimp nauplii.
In the aquarium
The scissortail rasbora is a peaceful, active and visually rewarding schooling fish best suited to medium to large community aquaria. Its eventual size — approaching 5 in in optimal conditions — means it needs a longer tank than most rasboras, with generous open swimming space in the mid-water. A school of six or more is the minimum; larger groups produce the full shoaling effect and the characteristic tail-flicking synchrony from which the common name derives.
The species is tolerant of a broad water-chemistry range (pH 5.0–8.0, soft to moderately hard) and temperatures of 72–77 °F, making it compatible with a wide range of community fish from Southeast Asian and broadly tropical biotopes. Tankmates should be of comparable size; very small fish are not at risk as food, but may be outcompeted at feeding time. Strong planting along the sides and back provides cover and reduces aggression, while open mid-tank space allows the natural mid-water schooling behaviour to express itself. The species is not a specialist feeder or particularly sensitive to standard water quality, and adapts readily to aquarium life.
Conservation
Rasbora trilineata is assessed as Least Concern (LC) by available conservation criteria, consistent across multiple monitoring sources. The species has a large range across mainland Southeast Asia and the Sunda shelf, is present across multiple major river basins, and is not known to face significant range-wide population declines attributable to a single pressure.
Nevertheless, the PMC breeding study notes that wild populations show evidence of decline in some areas associated with over-collection for the ornamental fish trade and with habitat degradation. The lowland forest streams and peat-swamp margins it inhabits in Sundaland are under ongoing pressure from drainage for agriculture and plantation development, sedimentation, and water extraction across Malaysia, Sumatra and Borneo. Widespread captive breeding for the trade reduces — but does not eliminate — direct collection pressure on wild stocks.