Taxonomy & naming
Hemigrammus unilineatus was described by Theodore Gill in 1858. The species has had a stable taxonomic history within Hemigrammus, and the valid name is recorded by Eschmeyer's Catalog of Fishes as Hemigrammus unilineatus (Gill, 1858); the parenthetical authority indicates that the species was originally placed in a different genus before being transferred to Hemigrammus.
Hemigrammus is one of the largest tetra genera in the family Characidae, encompassing dozens of small, laterally compressed schooling fishes distributed across tropical South America. H. unilineatus is a member of this broadly distributed lowland radiation and is not considered to represent any monotypic or particularly isolated lineage within the genus — it is part of a species-rich assemblage of small characins that share broadly similar ecology and body plans across the Amazon and Orinoco drainages.
Morphology
The featherfin tetra is a small, slender, laterally compressed characin of typical Hemigrammus form. The body is translucent to silvery, with a thin, dark lateral stripe running from behind the head to the base of the caudal fin, from which the specific epithet unilineatus (Latin: 'one-lined') is derived. The caudal fin shows the dark-tipped patterning seen in many congeners.
The most distinctive feature in males is the extended dorsal fin, whose anterior rays are drawn out into a feathery point — the characteristic that gives the fish its common name. This elongation is less pronounced or absent in females. Like most Hemigrammus, the species carries a small adipose fin between the dorsal and caudal fins, and the body outline is typical of the genus: deepest at mid-body, tapering to a narrow caudal peduncle.
Habitat
Hemigrammus unilineatus inhabits slow-moving rivers, tributaries, oxbow lakes, and drainage ditches across its range in northern South America and the island of Trinidad. Water conditions range from clear to murky, and the species tolerates suspended sediment. It is a lowland fish associated with the shallow, vegetation-fringed margins of still and sluggish water bodies where cover is available.
The reported water parameters for wild populations span a broad range: temperatures of 73–82 °F, pH 5.5–7.5, and hardness 1–20°dH. This tolerance of soft, acidic water at one end of the spectrum and near-neutral, moderately hard water at the other reflects the variety of lowland South American water chemistry across its range, from blackwater tributaries to more mineralized systems.
Feeding
In the wild, featherfin tetras feed primarily on small invertebrates — aquatic insect larvae, tiny crustaceans, and other zooplankton available in the mid- and upper water column of their slow-water habitat. As with most Hemigrammus, the diet is opportunistic, and the fish will take surface-fallen prey as well as benthic items.
In captivity the species is undemanding: it accepts high-quality dried foods including flakes and small granules, and benefits from regular offerings of live or frozen foods such as Daphnia, brine shrimp nauplii, microworms, and bloodworm. A varied diet supports condition and helps maintain the natural vitality of the fish in the aquarium.
Mating
Hemigrammus unilineatus is an egg-scattering open spawner with no parental care, following the pattern typical of the genus. Courtship is brief, with males pursuing females among fine-leaved vegetation or spawning mops. The adhesive eggs are deposited among plant material where they cling until hatching.
Breeding is triggered by soft, acidic water and conditioning on live or frozen foods. Spawning groups or pairs placed in a separate, dimly lit breeding tank with fine-leaved plants or synthetic mops will usually spawn within a few days of conditioning, typically in the morning hours. The adults show no parental instinct and will consume their own eggs if given the opportunity.
Breeding
Breeding featherfin tetras follows the standard small-tetra protocol. A separate breeding tank with very soft, acidic water (pH 5.5–6.5, general hardness 1–5°dH) and a temperature at the warmer end of the species' range promotes spawning. Fine-leaved plants such as Java moss, or synthetic spawning mops, provide the egg-deposition substrate. A thin layer of mesh over the bottom can help protect eggs from being eaten before the adults are removed.
Eggs hatch within approximately 24–36 hours, and the larvae become free-swimming after a further 3–4 days once the yolk sac is absorbed. First foods must be very small: infusoria or commercial liquid fry food in the earliest days, followed by microworms and brine shrimp nauplii as the fry grow. Removing adults after spawning is essential, as they will readily eat both eggs and larvae. Growth to juvenile size is relatively rapid under good feeding conditions.
In the aquarium
The featherfin tetra is a very peaceful species that causes no problems with comparable tankmates. It should be kept in groups of at least six, and preferably ten or more — in smaller numbers the fish become timid and spend less time in open water. A group of a dozen or more will school actively and display the males' extended dorsal fins to best advantage. Suitable tankmates include other small tetras, pencilfish, dwarf cichlids such as Apistogramma, and Corydoras catfish.
Water conditions should be soft to moderately hard and slightly acidic to neutral (pH 5.5–7.5, 1–20°dH), with a temperature of 73–82 °F. The species is not demanding about water chemistry within this range, but it is a fish of slow, sheltered water and does not appreciate strong currents. A planted tank with areas of subdued lighting and open swimming space suits it well. The featherfin tetra is notably uncommon in the hobby despite its abundance in nature — most specimens available are accidental imports rather than targeted catches, and tank-bred stock is rarely available commercially.
Conservation
Hemigrammus unilineatus has not been formally assessed by the IUCN Red List, and its status is listed as Not Evaluated (NE). The species is reported to be abundant across its natural range in northern South America and Trinidad, and there is no current indication of significant population decline. Its broad tolerance of water conditions and its occurrence across a wide geographic area, including disturbed habitats such as drainage ditches, suggest that it is not under immediate conservation pressure.
The species' scarcity in the aquarium trade is a commercial rather than a conservation issue — it enters the hobby primarily through incidental collection alongside more sought-after species, not because wild populations are limited. Its abundance in the wild and relatively broad habitat tolerance distinguish it from the more narrowly endemic or sensitive Hemigrammus species that warrant closer monitoring.