Taxonomy & naming
Farlowella gracilis was formally described by Charles Tate Regan in 1904 in the Transactions of the Zoological Society of London (volume 17, part 3, number 1, page 303, plate 20, figure 3). The species was later re-described and reviewed within the genus by Retzer and Page (1997) in their systematic revision of Farlowella, which consolidated a number of synonyms across the genus. Two earlier combinations — Acestra gracilis and Farlowella boliviana — are now treated as synonyms of F. gracilis under the Catalog of Fishes (Eschmeyer, CAS).
The type locality is the Río Caquetá (Cauca Valley, southern Colombia), and the species' distribution extends downstream into the Japurá basin in Brazil, where the Caquetá crosses the border under that name. The genus Farlowella belongs to the subfamily Loricariinae, the whiptail and twig catfishes — a predominantly Neotropical clade characterised by extreme body elongation and close association with woody substrates.
The genus name commemorates Dr. William Gilson Farlow (1844–1919), a Harvard algologist whose research on algae earned him this tribute from taxonomists cataloguing the algae-grazing loricariids. The epithet gracilis is Latin for 'slender' or 'graceful', pointing directly to the fish's attenuated build.
Morphology
Farlowella gracilis reaches a maximum total length of approximately 10 in (FishBase; a standard-length measurement of 7 in is also on record for preserved specimens, indicating proportions consistent with a long, pointed caudal fin extending the TL significantly). The body is one of the most elongate in the family: a cylindrical, twig-like tube tapering to a fine rostrum anteriorly and a slender caudal peduncle posteriorly. The head profile is almost perfectly round in cross-section at the snout tip, broadening only slightly toward the interorbital region.
Armour consists of overlapping bony scutes covering most of the body. Ground colouration is typically tan to mid-brown, broken by two dark lateral stripes running from the rostrum tip, through or just above the eye, and extending toward the tail base — a pattern common across the genus but variable in intensity. The dorsal fin is positioned near the body's midpoint. The rostrum (fleshy snout extension) is pronounced even by Farlowella standards, accounting for the 'longnose' common name.
The mouth is small and ventral, equipped with slender teeth suited to scraping thin biofilm from smooth surfaces. Sexual dimorphism in Farlowella follows the twig-catfish pattern: males develop small odontodes (bristle-like denticles) along the sides of the snout, which become more prominent in mature, breeding-condition fish. Females are typically broader across the abdomen when gravid.
Habitat
Farlowella gracilis is known from the Caquetá River basin of Colombia, with records extending into the corresponding Japurá drainage of northwestern Brazil — both representing the upper and mid-Amazon catchment. The Caquetá rises in the Colombian Andes and descends through transitional terrain onto the Amazonian lowland plain, carrying soft, slightly acidic water typical of Andean-origin Amazonian rivers.
FishBase records a pH range of 6.0–7.0 and a hardness range of 3–8 dH for the species, with temperatures of 72–79 °F — parameters consistent with the soft, mildly acidic conditions of mid-elevation Colombian Amazon tributaries. The genus as a whole is strongly associated with submerged woody debris: twig catfishes press against branches, rootlets, and leaf-litter accumulations, relying on camouflage and stillness rather than speed for predator avoidance. Current levels vary across the basin, but shallow, structured microhabitats with abundant organic material appear to be the preferred setting.
The genus is also noted to possess facultative air-breathing capability (FishBase reference 126274), allowing Farlowella to supplement gill respiration in warm or poorly oxygenated reaches — an adaptation shared with many loricariids occupying seasonally variable environments.
Feeding
Like all Farlowella, F. gracilis is primarily a periphyton and biofilm grazer. The small ventral mouth and fine teeth are adapted to scraping thin films of algae, bacteria, diatoms, and organic detritus from smooth, hard surfaces — submerged twigs, smooth bark, broad leaves, and rocks. The diet is low in caloric density, and these fish feed almost continuously when conditions permit, moving slowly across surfaces and rasping with fine, repetitive strokes.
In the aquarium, established biofilm on glass, wood, and broad-leafed plants (Anubias, Amazon swords) provides an important supplemental food source alongside offered foods. Sinking algae wafers, spirulina-based pellets, and thin slices of blanched vegetables — courgette, cucumber, and spinach are commonly accepted — should be offered after lights out when the fish are more active. The digestive system is calibrated for frequent small meals rather than large, protein-rich items; high-protein foods are best kept to a minimum.
Mating
Courtship behaviour in Farlowella species is not extensively documented in the scientific literature, but aquarist accounts of related species provide a consistent picture. Males develop more prominent snout odontodes during the reproductive season and appear to use these structures in male-to-male disputes as well as in courtship contact with females. The male typically positions himself near a favoured spawning site — a flat leaf surface, a section of smooth wood, or the aquarium glass — and may nudge or shadow the female during the approach to spawning.
No PlanetCatfish breeding reports are on file for F. gracilis specifically. The broader Farlowella pattern, observed in several congeners, involves the male claiming a spawning site and engaging in side-by-side or parallel positioning with the female before eggs are deposited.
Breeding
Farlowella species are open-substrate spawners, depositing adhesive eggs on flat surfaces — most commonly a broad leaf, a length of smooth wood, or aquarium glass. Males take sole responsibility for guarding and tending the clutch: the male fans the eggs with his pectoral fins to maintain oxygen circulation, removes infertile or fungused eggs, and remains positioned over or beside the clutch through the incubation period.
No confirmed breeding accounts for F. gracilis appear in the available literature or in the PlanetCatfish records consulted. Congeners such as Farlowella acus have been bred in aquaria under soft, slightly acidic water conditions (pH 6.5–7.0, temperature 75–79 °F), with conditioning on high-quality biofilm and varied plant-based foods triggering spawning. Fry hatch after roughly five to seven days and are tiny, requiring green water or fine algae as first food. Farlowella fry are notably delicate, and mortality is high in the early weeks even under careful management.
In the aquarium
Farlowella gracilis is considered difficult to maintain long-term, a reputation it shares across most of the genus (FishBase reference 13371). The chief challenges are water quality, diet, and the fish's extreme sensitivity to stress. Very clean, well-oxygenated water is non-negotiable: the species does not tolerate elevated nitrate, dissolved organics, or temperature fluctuations well. Target parameters of pH 6.0–7.0, 72–79 °F, and soft water (3–8 dH) should be maintained with consistent partial water changes.
The aquarium should contain an abundance of plant material and smooth wood: dense stands of Echinodorus or Vallisneria, broad-leafed Anubias, and pieces of driftwood provide both feeding surfaces and resting sites. The fish spends most of its time motionless against a surface at an angle, and it will use plants as much as wood for perching. Open water and bright lighting without vegetative cover cause stress.
Compatible tankmates are limited to small, non-competitive species: small tetras, pencilfish, hatchetfish, Corydoras, and other gentle bottom dwellers are suitable. Fast-moving or nippy species (danios, tiger barbs, larger cichlids) are inappropriate, both because they outcompete the slow-feeding Farlowella for food and because persistent movement prevents the fish from resting and feeding normally. Water flow should be moderate and even — a spray bar or wavemaker set to gentle circulation rather than a single turbulent jet is preferable.
Conservation
The IUCN Red List assessed Farlowella gracilis as Data Deficient (DD) in October 2014, a status reflecting insufficient distributional and population data rather than documented absence of threats. The species' range within the Caquetá/Japurá basin spans both Colombian and Brazilian territory, but precise survey data on population size, habitat quality, and commercial collection pressure are lacking.
The Caquetá basin faces significant pressure from deforestation and agricultural expansion in Colombia, which increase sedimentation and alter the riparian structure on which twig catfishes depend. The species is occasionally encountered in the ornamental trade, but it does not appear to be heavily collected. Until systematic field surveys are completed, it is not possible to determine whether F. gracilis is genuinely rare or simply under-sampled. It is not listed on CITES appendices.