Taxonomy & naming
Reganella depressa was originally described by Rudolf Kner in 1853 as Hemiodon depressus, published in the Denkschriften der Kaiserlichen Akademie der Wissenschaften in Wien (vol. 6, p. 91). The type locality is Marabitanas (0°57'N, 66°55'W), a locality on the upper Río Negro in the state of Amazonas, Brazil. The holotype held at the Naturhistorisches Museum Wien (NMW 9438) was designated as lectotype by Isbrücker & Nijssen (1974), who also transferred the species to the genus Reganella, which had been erected to honour the British ichthyologist Charles Tate Regan (1878–1943). The Catalog of Fishes (Eschmeyer, CAS; updated 10 June 2026) treats the valid combination as Reganella depressa (Kner, 1853), with Hemiodon depressus Kner, 1853 as basionym.
Reganella is a monotypic genus — it contains only this one species. This distinction matters because the genus name carries a high phylogenetic diversity index (PD50 = 1.0, per FishBase modelling), reflecting the absence of close surviving relatives at the same genus-level rank. The subfamily placement is Loricariinae. The name Reganella is a Latinised diminutive of Regan, honouring the British Museum ichthyologist who made significant contributions to South American freshwater fish systematics in the early twentieth century. The species carries no aquarium L-number, consistent with its rarity in the ornamental trade.
Morphology
Reganella depressa is a small, markedly flattened loricariid, with a maximum standard length of 4.5 in according to Ferraris (2003) as recorded in the IUCN assessment. The specific epithet depressus (Latin: flattened) directly describes the most conspicuous aspect of the body form — the head and anterior trunk are strongly depressed, giving a pancake-like silhouette in dorsal view that is more pronounced than in many other loricariin whiptails. The body tapers into a long, slender tail that bears a filament at its tip.
Scutes cover the dorsal and lateral body surfaces in the loricariid pattern. Ground colour tends towards pale to medium brown above, with darker blotching or banding along the flanks; the underside is pale. The ventral mouth is positioned for substrate-scraping. Teeth are small and suited to grazing biofilm from hard surfaces.
Sexual dimorphism follows the general pattern of Loricariinae: males of this group typically develop elongated odontodes on the pectoral spines and snout region during the breeding season, and show narrower bodies than gravid females. Specific published descriptions of sexual characters in R. depressa are limited, so detailed dimorphism data should not be inferred beyond what is established for the subfamily.
Habitat
Reganella depressa is endemic to Brazil and occurs in the basins of the Negro, Branco, Trombetas, Xingu, and Tapajós rivers — all within the Amazon drainage. The IUCN assessment (ICMBio 2022) records the species from the Brazilian states of Amazonas, Pará, and Roraima. The original type locality on the Río Negro places it firmly in blackwater territory: the Negro is one of the world's preeminent blackwater rivers, characterised by extremely low conductivity (often below 20 µS/cm), very low pH (typically 4.5–6.0), near-zero hardness, and high dissolved humic and fulvic acid content from decomposing vegetation in the surrounding várzea and terra firme forests.
The occurrence in the Tapajós and Trombetas systems, which include both clearwater and whitewater components depending on the tributary, suggests the species can tolerate a somewhat broader hydrochemical range than a strict blackwater specialist — though the overall emphasis remains on soft, warm, very low-mineral water typical of Amazonian lowland systems. Demersal in habit, the species is associated with permanent rivers and streams (IUCN habitat classification: Permanent Rivers/Streams/Creeks).
Its natural scarcity in surveys (described as 'naturally infrequent and low in abundance' in the IUCN assessment) suggests it occupies specific microhabitats rather than being broadly distributed within its basin range.
Feeding
No detailed published diet analyses specific to Reganella depressa are available in the accessible literature. Consistent with its loricariid morphology — suction-cup mouth, scraping teeth, strongly depressed body allowing close contact with substrate — the species is inferred to feed primarily on periphyton, algal biofilm, and fine detrital material scraped or sieved from hard and sandy substrates.
The blackwater and clearwater rivers it inhabits are generally low in suspended nutrients, with biofilm and benthic algae constituting the primary available food resources for substrate-scraping fishes. Given the typical resource scarcity of blackwater environments, the fish is likely an efficient grazer adapted to low-productivity conditions.
In the aquarium, this ecology points toward a diet of algae wafers and spirulina discs as a foundation, supplemented with blanched vegetables (courgette, cucumber) and sinking wafers with a high vegetable content. Meaty supplementation should be minimal. A mature tank with a natural algal bloom on rocks and glass provides additional nutritional benefit. The small body size (under 4.5 in) means food portions should be correspondingly modest.
Mating
Reproductive biology specific to Reganella depressa has not been described in the published scientific or hobbyist literature. Based on its placement within Loricariinae and the morphological features typical of the subfamily, the species is expected to follow a broadly similar reproductive pattern to related whiptail genera — involving territorial males, benthic egg deposition or substrate attachment, and some degree of parental care of eggs and early larvae.
The strongly flattened body may facilitate spawning on flat rock surfaces or broad leaves close to the substrate, as is seen in some other flattened loricariin genera. Given the extreme softness and acidity of its blackwater habitat, conditioning for captive reproduction would almost certainly require very soft, slightly acid water (pH 5.5–6.5, TDS under 100 µS/cm) to mimic the natural environment. Whether the genus uses an abdominal brooding strategy like some other loricariin genera, or deposits eggs on a fixed substrate, is not established in the literature and should not be assumed.
Breeding
No captive breeding accounts for Reganella depressa are recorded in the hobbyist literature, and the species essentially never appears in the ornamental trade. Its absence from aquarium stocks and the lack of an L-number mean that essentially no first-hand breeding data exist outside museum collections.
For aquarists acquiring wild-caught specimens (e.g. via specialist catfish importers dealing in Amazonian blackwater fish), breeding attempts would logically focus on replicating the physicochemical extremes of the Río Negro: very soft water (total hardness under 1°dH), pH 4.5–6.0, minimal conductivity, warm temperatures around 79–82 °F, tannin-staining from leaf litter and driftwood, dim lighting, and a sand substrate with flat rocks or smooth driftwood pieces as potential spawning surfaces.
Any captive breeding observations would constitute novel scientific data for this species. The high phylogenetic uniqueness of this monotypic genus makes such records especially valuable for understanding Loricariinae reproductive evolution.
In the aquarium
Reganella depressa is effectively absent from the aquarium hobby. It carries no L-number, does not appear in standard pleco or catfish import lists, and its occurrence in the wild is described as naturally infrequent. Any specimens available to hobbyists would be uncommon wild-caught imports from specialist dealers in Amazonian blackwater fish, or possibly collected during ichthyological expeditions.
Hobbyists who do obtain specimens should treat this as a blackwater specialist requiring extreme water conditions: very soft water (near-zero hardness), pH in the 4.5–6.5 range, temperatures around 79–82 °F, heavy tannin staining, and minimal water movement. The tank should be heavily matured to establish biofilm on flat rock surfaces and smooth driftwood. Tankmates, if any, must be species equally tolerant of very soft acidic water — cardinal tetras, pencilfish, and small hatchetfishes are all appropriate companions from the same blackwater biome.
The small maximum size (around 4.5 in SL) makes R. depressa manageable in a 15–20 US gal blackwater setup. Its natural rarity and the absence of captive-bred stock make careful husbandry especially important for anyone lucky enough to maintain it. Documentation of behaviour, feeding response, and especially any breeding attempts should be shared with the catfish-keeping community and with institutions such as PlanetCatfish.
Conservation
The IUCN Red List assessed Reganella depressa as Least Concern in 2022 (assessment date 7 November 2018; assessor: Instituto Chico Mendes de Conservação da Biodiversidade, ICMBio). The justification is that the species is endemic to Brazil, occurring across the Negro, Branco, Trombetas, Xingu, and Tapajós basins, and while it is naturally infrequent and of low abundance, no significant threats to its population have been identified.
No specific conservation measures are in place for the species, and it is not known to occur within any Brazilian protected area (Unidade de Conservação) based on available data. No ongoing research on the species was identified at the time of assessment.
The IUCN assessment was conducted as part of the Brazilian Red List process (Livro Vermelho da Fauna Brasileira Ameaçada de Extinção, ICMBio 2018). While the current assessment is Least Concern, the blackwater basins it inhabits face long-term threats from deforestation, gold mining (which generates mercury pollution throughout the Negro watershed), and climate-driven hydrological changes. Population monitoring remains essentially absent for this species.