Taxonomy & naming
Spatuloricaria euacanthagenys was described by Isaäc Johannes Hubertus Isbrücker in 1979. The Catalog of Fishes (Eschmeyer, CAS) recognises the valid combination Spatuloricaria euacanthagenys (Isbrücker, 1979). It is one of the later-described species within a genus where eleven of the thirteen valid species were named before 1945.
The genus Spatuloricaria belongs to subfamily Loricariinae — the whiptail clade of Loricariidae — and is distributed widely from Panama south through South America. The genus name derives from Latin spatula (a broad blade or paddle) combined with lorica (cuirass, armour), alluding to the paddle-shaped or spoon-shaped teeth found in mature males of the genus, which distinguish them from the related Loricaria. These spatulate teeth are a key apomorphy of the genus.
The species is distributed in the Caquetá River basin of Colombia, a system that drains south and east from the Colombian Andes into the Amazon basin. S. euacanthagenys is therefore an Amazon-drainage species, unlike several Spatuloricaria that occur in Andean Pacific-slope drainages.
Morphology
FishBase records a maximum size of 20.5 in for Spatuloricaria euacanthagenys, though the entry does not specify whether this figure represents standard length (SL) or total length (TL). This figure warrants caution: most Spatuloricaria species reach 10–14 in, and 20.5 in would make S. euacanthagenys substantially the largest member of the genus. The cache figure is reported here as recorded, but whether it reflects an unusually large individual, a TL measurement of a fish whose SL would fall within the typical congener range, or a data error cannot be determined from sources available at the time of writing.
In body form, Spatuloricaria euacanthagenys conforms to the general whiptail pattern: strongly depressed dorsoventrally, elongated, and tapering to a long, filamentous caudal peduncle. The body is covered in keeled bony scutes. The species is named for its cheek odontodes — the epithet euacanthagenys (from Greek eū, well/very + akantha, thorn + génys, cheek) describes the dense, prominent odontode bristles on the sides of the head, which are particularly well-developed compared to congeners.
In mature males of Spatuloricaria, the front teeth become broadened and spatula-like — paddle-shaped rather than slender — a sexually dimorphic feature that gives the genus its name. Ground colour is reportedly cryptic: brownish-grey with darker blotching or mottling, providing camouflage among gravel, rocks, and wood debris.
Habitat
Spatuloricaria euacanthagenys is known from the Caquetá River basin of Colombia, a major Amazonian drainage rising in the Colombian Andes and flowing east through foothill and lowland rainforest before reaching the Amazon proper. The Caquetá system encompasses diverse habitats from steep Andean tributaries with fast current and rocky substrate to broader, slower lowland channels with sandy and silty substrate.
Genus-level ecology from S. caquetae — which co-occurs in the same basin — indicates a preference for areas with moderate to strong current, good oxygenation, and substrate of flat stones and gravel with crevices for shelter. Temperature in the Caquetá foothills and lowlands ranges approximately 75–82 °F. No specific water chemistry data for the type locality of S. euacanthagenys are available in the sources consulted.
Spatuloricaria species are benthopelagic, spending much of their time pressed to the substrate or tucked into gaps between stones and wood. The suction-disc mouth enables firm attachment to hard surfaces in current, and the fish is reported to move and investigate substrate objects (stones, driftwood) while foraging.
Feeding
Spatuloricaria euacanthagenys is presumed to be an omnivorous, non-selective benthic feeder consistent with other members of the genus. Spatuloricaria species in captivity accept algae wafers, sinking granules, frozen foods, and vegetable matter — a broad diet reflecting opportunistic grazing on biofilm, periphyton, detritus, and small invertebrates on substrate surfaces.
The robust suction mouth and close association with hard substrate suggest that rasping biofilm from stones and wood is a major feeding mode in the wild. No published dietary analysis specific to S. euacanthagenys is available.
Mating
Mating behaviour specific to Spatuloricaria euacanthagenys has not been documented in the literature. Genus-level observations indicate that males become territorial during the breeding period, particularly around flat stones, rocky crevices, or overhanging ledges that may serve as spawning and egg-guarding sites. Males display their odontode ornamentation — particularly the pronounced cheek bristles — to rivals and potential mates.
The spatulate teeth of mature Spatuloricaria males are thought to function in male–male competition and possibly in mate assessment, analogous to the odontode development seen across Loricariinae. Territorial skirmishes between males may involve contact along the pectoral spines and head margins.
Breeding
Spatuloricaria euacanthagenys is inferred to be a paternal egg-guarder on the basis of the known reproductive strategy for the genus: males guard eggs deposited on flat stones or in rocky crevices, fanning them until hatching. This pattern is consistent with the subfamily Loricariinae more broadly.
No species-specific breeding data — clutch size, incubation period, fry development — are available for S. euacanthagenys. Breeding in captivity for Spatuloricaria species is reported as very rare, and S. euacanthagenys is not established in the ornamental hobby as a recognised species. Any captive-breeding observations would represent a meaningful contribution to the literature on this genus.
In the aquarium
Spatuloricaria euacanthagenys is not established in the ornamental fish hobby. Given the uncertainty around its maximum size — reported at 20.5 in but possibly a TL figure that inflates the effective body length, or potentially reflecting an outlier individual — potential keepers should plan for a very large specimen. If the figure represents a typical adult, a tank of at least 78.5 in in length would be required; even if the effective body length is closer to that of typical congeners (10–14 in), a minimum of 59 in is advisable.
Water conditions consistent with the Caquetá foothills origin — temperature 75–82 °F, moderate to good oxygenation, and moderate current — are recommended. Substrate should include flat stones and sheltered crevices, with sections of driftwood. The species is expected to be cryptic and primarily crepuscular.
Spatuloricaria are reported to be territorial toward conspecifics in captivity; multiple specimens should only be attempted in very large aquaria with ample physical territory division. They are generally peaceful toward dissimilar species.
Conservation
The IUCN Red List assessed Spatuloricaria euacanthagenys as Data Deficient (DD) in October 2014. The Data Deficient designation reflects the limited distributional and population data available for this species rather than a determination that it is not threatened.
The Caquetá River basin in Colombia has experienced significant land-use pressure: deforestation for cattle ranching and agriculture, coca cultivation with associated chemical runoff, and ongoing conflict-linked disturbance have all affected watershed integrity across parts of the drainage. Large-bodied benthic loricariids dependent on specific substrate microhabitats may be disproportionately affected by sedimentation events from watershed disturbance, but without survey data on S. euacanthagenys populations this cannot be assessed. Targeted ichthyological surveys of the Caquetá basin would help clarify the species' range, abundance, and ecological requirements.