Taxonomy & naming
Farlowella hahni was described by Hermann Meinken in 1937, based on specimens originating from the Paraguay basin. The species epithet honours the collector or donor of the type material, a common practice in Meinken's descriptions of aquarium fish. The genus Farlowella Eigenmann & Eigenmann, 1889 belongs to the subfamily Loricariinae and the family Loricariidae, the armoured suckermouth catfishes. Farlowella as a whole is characterised by extreme body elongation, a pointed rostrum with a relatively small inferior mouth, and a covering of keeled lateral scutes that give the fish its distinctive stick-insect appearance. Retzer and Page's 1996 systematic revision of Farlowella provides the most comprehensive morphological treatment of the genus, and F. hahni is treated as a valid species within that framework. The Catalog of Fishes (Eschmeyer, CAS) lists F. hahni as a valid name with no junior synonyms currently recognised. Some historical trade literature has conflated this species with other southern Farlowella, but diagnostic characters — including rostral length ratios and scute counts — separate it from co-occurring forms.
Morphology
Farlowella hahni shares the family's hallmark extreme elongation: the body is slender, roughly cylindrical to slightly dorsoventrally depressed, and tapers gradually toward the caudal peduncle. The rostrum is pointed and moderately long, though not as exaggeratedly attenuate as in F. acus. Lateral scutes form a continuous ridge along the body flanks and are well-developed and keeled. The mouth is small and inferior, with fine teeth adapted to scraping biofilm and soft algae. Overall coloration is a warm tan to dark brown with variable mottling and fine pale speckling; a narrow mid-lateral stripe of slightly paler or darker pigment is often visible. The caudal fin is forked, and the lower caudal-fin rays in adult males may be somewhat extended. Adult standard length typically falls in the range of 5.5–7 in, with females tending to be slightly more robust in the abdominal region when gravid. Mature males develop short odontodes on the operculum and along the pectoral-fin spines, providing a reliable sexing character in adult fish. The pectoral fins are fan-like and are used both for manoeuvreing and for fanning clutches during paternal care.
Habitat
Farlowella hahni is native to the Paraguay River basin and the lower reaches of the Paraná system in Brazil, Paraguay, and Argentina. This is a geographically and ecologically distinct range from the Amazonian and Orinoco species of the genus, with water conditions in the Paraná-Paraguay system differing significantly from those of tropical blackwater rivers. The species inhabits shallow, vegetated margins of slow-flowing and still-water environments — flooded meadows (pantanal margins), oxbow lakes, and densely planted backwaters where submerged stems, roots, and fallen wood provide abundant grazing surface and concealment. Water temperature in these habitats is seasonally variable, dipping to approximately 68–72 °F (68–72 °F) during the austral winter and rising to 82–86 °F (82–86 °F) in summer. pH is typically near-neutral to slightly alkaline (6.8–7.8), and hardness is moderate — somewhat higher than in Amazonian blackwater systems. The species likely undertakes local movements in response to seasonal flood pulses, exploiting inundated vegetation during high-water periods.
Feeding
Like its congeners, Farlowella hahni is a specialised algivore and biofilm grazer. Its inferior mouth with fine, comb-like teeth is adapted to removing periphyton, diatoms, and soft algae from submerged woody debris, plant stems, and leaf surfaces rather than to hard algae scraping. In the Paraguay basin, the species exploits the abundant growth of aufwuchs on submerged aquatic macrophytes and the bark of partially submerged trees. In the aquarium, F. hahni accepts a range of vegetable-based foods: sinking algae wafers, spirulina flake settled to the substrate, blanched courgette or cucumber, and boiled spinach. Occasional offerings of sinking pellets with a high spirulina or Chlorella content are beneficial. The species is not a strong feeder in bright, active environments; it should be fed in the evening or under subdued lighting. Unlike large, powerful loricariids, Farlowella species will not consume heavy algae growth on glass and cannot subsist on algae alone in a clean, well-lit aquarium — supplemental feeding is always required.
Mating
Courtship in Farlowella hahni is consistent with the pattern documented for the genus in captivity. The male initiates contact by closely following the female, pressing against her flank and nudging her with his snout. Once a female is receptive, the pair positions themselves on a flat, smooth surface — a broad leaf, a length of smooth driftwood, or a slate tile — where the male fertilises and the female deposits eggs. The male's role becomes dominant immediately after egg deposition: he positions himself over the clutch, fanning it with slow, rhythmic pectoral-fin movements to maintain water circulation and prevent fungal colonisation of eggs. Females do not participate in clutch care and are driven away by the guarding male. Breeding in the Paraguay-Paraná region is likely seasonal, timed with the transition from the cool dry season to warmer and slightly more nutrient-rich wet-season conditions. In aquaria, conditioning triggers often include slightly higher temperatures, richer feeding, and increased water-change frequency.
Breeding
Farlowella hahni is among the better-documented Farlowella in aquarium breeding literature, owing to its relative availability compared to strictly Amazonian species. Pairs or small groups (one male, two or more females) kept in well-planted tanks with ample driftwood and broad-leaved plants such as Anubias will spawn without elaborate intervention once properly conditioned. Triggers include gradually raising the temperature from a cooler holding period (72–73 °F / 72–73 °F) to 79–81 °F (79–81 °F), increasing feeding frequency, and performing more frequent partial water changes. Clutches of 40–100 adhesive eggs are deposited on a flat surface chosen by the male. Incubation lasts approximately 7–9 days at 77–79 °F (77–79 °F). The male guards the clutch throughout, and his fanning behaviour is easily observable. Newly hatched larvae are small, yolk-sac-bearing, and cling to nearby surfaces; they begin grazing on biofilm within 3–5 days of hatching. At this stage, powdered spirulina, microworms, or fine commercial fry powders applied near the clutch site support growth. Juveniles grow steadily and can reach 1.5–2 in within the first two to three months.
In the aquarium
Farlowella hahni is a rewarding species for hobbyists willing to meet its specific needs. A tank of at least 25 US gal with a generous footprint is recommended; a long, shallow tank is far preferable to a tall, narrow one. Decor should replicate a vegetated river margin: tangles of driftwood, twisted roots, and robust broad-leaved plants. Floating plants that diffuse surface light are appreciated, as the species is shy under bright illumination. Water temperature of 72–82 °F (72–82 °F) suits this species well; unlike strictly tropical Farlowella, it tolerates a seasonal temperature drop, which also aligns with its natural range. pH of 6.5–7.8 and moderate hardness (5–15 °GH) are acceptable. Gentle to moderate filtration with no strong, turbulent flow is ideal — a sponge filter or spray bar directed along the tank wall rather than across open water works well. F. hahni is strictly peaceful and must not be housed with fin-nipping or aggressive species; it coexists well with small tetras, pencilfish, corydoras, and dwarf cichlids. Careful attention to feeding is essential, as these fish can easily be outcompeted; sinking wafers placed near their resting spots in the evening are recommended. The species can be long-lived — five years or more in well-managed aquaria.
Conservation
Farlowella hahni has not been formally evaluated by the IUCN Red List, and its conservation status is Not Evaluated (NE). Its range in the Paraguay and lower Paraná basins encompasses some of the most biologically significant freshwater habitats in South America, including the Pantanal — the world's largest tropical wetland. The Pantanal and associated drainages face significant threats from agricultural expansion (particularly soybean farming and cattle ranching), wetland drainage, dam construction, and water extraction. Additionally, industrial and agricultural pollution entering the Paraguay and Paraná rivers from the expanding agricultural frontier of Brazil's Cerrado and Mato Grosso regions is a growing concern. Twig catfishes are dependent on intact riparian and floodplain vegetation; large-scale conversion of these habitats could affect local populations. The species is occasionally exported in the aquarium trade, but collection pressure is considered minor relative to habitat threats. Aquarium breeding, which has been achieved with some regularity, reduces dependence on wild-caught individuals. Systematic assessments of Farlowella population status in the Paraguay system are lacking, and formal surveys would be necessary before any meaningful conservation recommendation could be made.