Taxonomy & naming
Dekeyseria scaphirhynchus was originally described by Rudolf Kner in 1854 as Ancistrus scaphirhynchus, based on material from the Amazon basin. The species was subsequently moved to the genus Peckoltia before Lúcia Rapp Py-Daniel effected the combination Dekeyseria scaphirhynchus in 1985, providing a redescription that compared it directly to Dekeyseria amazonica and established the genus on firmer footing. Sonia Fisch-Muller's treatment of Loricariidae-Ancistrinae in the 2003 Checklist of Freshwater Fishes of South and Central America (Reis, Kullander & Ferraris) represents the most recent comprehensive taxonomic synthesis for the genus.
The genus Dekeyseria is placed within the tribe Ancistrini of subfamily Hypostominae. Early trade literature sometimes assigned striped Dekeyseria species to the segregate genus Zonancistrus, but this split has not been supported by subsequent work, and all species — including the striped forms — are retained in Dekeyseria. The Catalog of Fishes (Eschmeyer, CAS) recognises Dekeyseria scaphirhynchus (Kner, 1854) as the valid combination.
In the aquarium trade the species carries two L-numbers: L151, the original DATZ coding, and L216, assigned independently in Denmark (where it circulates as 'Pandekagesugemalle'). Both codes refer to the same taxon. PlanetCatfish lists the type locality as Manaus, Brazil, consistent with the broader Rio Negro basin provenance of the early museum material.
This fish was long listed as Dekeyseria scaphirhyncha; Eschmeyer's Catalog of Fishes places the species in Dekeyseria, and the current combination Dekeyseria scaphirhynchus is followed here.
Morphology
Adults reach approximately 8.3 in standard length (SL) — sources consistently give 7.87 in SL (PlanetCatfish) and 8.3 in SL (FishBase). This places D. scaphirhynchus toward the larger end of the Ancistrini, considerably larger than the dwarf loricariids but well short of the large Hypostomus and Pterygoplichthys species.
The common name 'flat-head pleco' captures the species' most visually distinctive feature: the head is notably broad and depressed when viewed from above, a proportional character that becomes more pronounced in larger adults relative to smaller specimens or related species. The body is encased in loricariid bony scutes. PlanetCatfish notes that D. scaphirhynchus is 'similar to Lasiancistrus but broader when viewed from above, especially in larger adults' — a useful field distinction.
Sexual dimorphism is pronounced and reliable. Males develop substantial odontodes on the thick leading ray of the pectoral fin and on the posterior body flanks — structures that are reduced or absent in females. Males also develop a notably broader head than females of equivalent length. These characters make adult sexing straightforward in well-conditioned fish.
Habitat
Dekeyseria scaphirhynchus is recorded from the Rio Negro basin of Brazil, including the lower Amazon and Rio Nhamundá. The Rio Negro is one of the world's great blackwater rivers — its tannin-stained, humic-acid-rich waters are highly acidic, extremely soft, and very warm. Field-derived water parameters for the species span pH 5.0–7.0, general hardness up to 16 dH, and temperature 72–79 °F, though the core of its blackwater range sits at the lower end of those pH and hardness figures.
Microhabitat preferences have not been formally documented, but the species' cave-breeding behaviour and the general ecology of Rio Negro loricariids suggests an association with submerged woody debris, root masses, and rocky crevices along river margins and flooded igapó forest edges. The Rio Negro regime is strongly seasonal, with major flood pulses inundating vast areas of forest and creating the igapó habitat that characterises much of the river's fish diversity.
Dekeyseria scaphirhynchus has been documented as a facultative air-breather (FishBase, Ref. 126274), a common loricariid adaptation to the low-oxygen conditions that can develop in warm, organically rich blackwater environments during certain seasons.
Feeding
No detailed dietary study has been published specifically for D. scaphirhynchus. The Ancistrini to which it belongs are predominantly algae and biofilm scrapers, with supplementary intake of aufwuchs invertebrates and detritus. In the blackwater Rio Negro the epilithic algal layer is typically thin compared with whitewater rivers, and loricariids in this system may rely more heavily on fine detritus and biofilm associated with submerged wood and leaf litter.
In the aquarium, D. scaphirhynchus accepts the standard loricariid diet: algae wafers, spirulina-based sinking foods, blanched vegetables (courgette, cucumber, peas), and occasional meaty items such as bloodworm. Driftwood in the tank is beneficial both as refuge and as a supplementary biofilm substrate, though the species is not a confirmed xylophage in the manner of Panaque.
Mating
Courtship has not been described in published literature for D. scaphirhynchus, but aquarium keepers working with the species report behaviour consistent with the broader Ancistrini pattern. Males in breeding condition become territorial around preferred cave sites, and the pronounced pectoral and body odontodes are likely important in male–male competition and in tactile signalling during courtship. The broader head of males is a consistent secondary sexual character that may also play a role in mate assessment.
The clear dimorphism in D. scaphirhynchus — odontode development and head width in males, broader body profile in gravid females — makes conditioning and pair identification more tractable for aquarists than in species with subtler dimorphism.
Breeding
Dekeyseria scaphirhynchus is described by PlanetCatfish as a 'narrow (height) cave breeder', indicating that it selects cavities with a particularly restricted vertical dimension — tight horizontal slots or narrow tubes rather than broad chambers. No registered breeding reports were on file in the PlanetCatfish database as of the time of writing, making it one of the less-bred members of its size class in the trade despite its relatively long presence under the L151 designation.
Based on general Ancistrini biology, spawning is expected to produce a clutch of adhesive eggs deposited on the interior walls of the chosen cavity, with the male remaining to guard and fan the eggs through incubation. Conditioning pairs on a varied, nutritious diet, providing appropriate narrow-profile caves (ceramic breeding tubes of restricted internal height are often recommended), and simulating seasonal water-quality variation — cooler temperatures and slightly elevated acidity during the 'dry' phase followed by warming and additional water changes — are standard approaches for stimulating spawning attempts in cave-breeding loricariids.
In the aquarium
Dekeyseria scaphirhynchus grows to approximately 8.3 in SL, which translates to a total length of around 9.4–10.2 in including the tail — a substantial fish requiring a tank of at least 53 US gal for a single adult, and larger for a breeding pair or small group. The blackwater origin imposes real water-chemistry demands that distinguish it from more forgiving plecos: pH should be maintained in the range 5.0–7.0, preferably toward the acidic end; hardness should be low to moderate (up to around 16 dH); and temperature 72–79 °F.
Soft, warm, acidic blackwater also implies good organic filtration and careful attention to nitrogen compounds — the Rio Negro's natural bacterial communities process waste differently from hard-water systems, and nitrate accumulation in a blackwater tank can be more stressful than the absolute levels might suggest. Frequent, moderate water changes with soft, slightly acidic replacement water are preferable to infrequent large changes.
Decoration should include ample driftwood and hiding places. As a cave breeder, the species appreciates narrow-profile hiding spots: slate caves, ceramic tubes, or hollowed wood. The facultative air-breathing capacity means the tank must allow surface access, which is standard practice for most loricariid setups.
Compatibility with other species is generally peaceful, though the substantial adult size means tankmates should be chosen accordingly. Pairing with smaller, delicate fish risks predation or competition. Other soft-water species — cardinal tetras, smaller Corydoras, angelfish — are appropriate companions at a community level.
Conservation
Dekeyseria scaphirhynchus is listed as Least Concern on the IUCN Red List (assessed 2020, per PlanetCatfish), reflecting a distribution across the large and relatively intact Rio Negro basin. The Rio Negro catchment, while subject to some localised pressure from gold mining, deforestation, and urban expansion around Manaus, retains vast areas of primary igapó and terra firme forest and remains one of the Amazon's better-protected major river systems.
The species' presence in the aquarium trade is moderate — imported under the L151 and L216 codes — but captive breeding has not been established at commercial scale. Wild-caught specimens are periodically available through specialist catfish suppliers. The broad, intact distribution and Least Concern status mean no conservation intervention is currently warranted for D. scaphirhynchus.