Taxonomy & naming
Guyanancistrus tenuis was formally described by Fisch-Muller, Mol, and Covain in 2018 in PLOS ONE (volume 13, issue 1, e0189789, DOI: 10.1371/journal.pone.0189789). The holotype — MZUSP 117148, measuring 3.5 in SL — and paratypes (MNHN 2002-3537 and MHNG 2745.067) were collected on 25 October 2002 by P. Keith and P. Gaucher from a small forest tributary of the Mapaoni River, upper Jari River Basin, Massif du Mitakara, French Guiana. The Catalog of Fishes (Eschmeyer, CAS) recognises this as a valid species in the genus Guyanancistrus, subfamily Hypostominae, family Loricariidae.
The genus Guyanancistrus reflects the Guianan biogeographic distribution of its constituent species: the name combines Guyana with Ancistrus, the large and familiar genus of Hypostominae bristlenose catfishes. No L-number has been assigned to G. tenuis, which remains entirely outside the ornamental trade.
Morphology
The maximum recorded standard length is 3.5 in SL, making G. tenuis a genuinely small loricariid. The species is notably slender — the epithet tenuis (Latin: thin, slender) alludes directly to the elongated body form that distinguishes it within the genus. The body is covered in the typical loricariid bony-scute armour. The fin formula includes 2 dorsal spines, 7 dorsal soft rays, and 6 anal soft rays.
FishBase characterises the body shape as elongated, consistent with the original description. Beyond these parameters and the holotype measurements, detailed morphological data — including colour pattern, plate counts, and odontode morphology as described in the original publication — are the principal diagnostic sources. No sexual dimorphism data have been published for this species.
Habitat
Guyanancistrus tenuis is known from a single locality: a small forest tributary of the Mapaoni River, upper Jari River Basin, in the Massif du Mitakara highlands of French Guiana. The collection site was characterised as rocky and shallow — 8–23.5 in depth — with medium to strong currents and some pools, in a forested mountain creek setting.
Despite subsequent survey effort in the region, no additional populations have been found; the species appears genuinely restricted to this single site. No water temperature, pH, or conductivity data from the collection locality have been published. The Mapaoni River drains toward the Jari River system, which eventually reaches the Amazon mainstream at the Pará–Amapá state border.
Feeding
No species-specific feeding data have been published for Guyanancistrus tenuis. Rocky, shallow mountain creeks with moderate to strong current in the Guiana Shield typically support epilithic periphyton communities — films of algae, diatoms, and associated organic matter on exposed stones — and this is the most likely foraging substrate for a small Hypostominae specialist in this microhabitat. No diet composition studies exist for this taxon.
Mating
No observations of mating behaviour exist for Guyanancistrus tenuis. The species has not been maintained in captivity, and no field observations of courtship or territorial behaviour have been published. Data sparse.
Breeding
No breeding data are available for Guyanancistrus tenuis. The species has not been bred in captivity, and no nest sites, clutch parameters, or parental behaviour have been documented from the wild. Data sparse.
In the aquarium
Guyanancistrus tenuis is not present in the ornamental aquarium hobby and has no L-number. No husbandry records exist. Its small size (under 4 in SL) and rocky mountain-creek habitat suggest, in principle, suitability for a cool, well-oxygenated, high-flow aquarium with stone substrate — but no tested aquarium parameters are available, and this species cannot realistically be sourced by hobbyists. It should be regarded as a wild, scientifically known taxon with no captive history.
Conservation
Guyanancistrus tenuis was assessed as Data Deficient (DD) by the IUCN Red List on 21 October 2020 (Version 2025-2). The Data Deficient designation reflects genuine uncertainty: population size, population trend, and the full extent of the species' range are all unknown. No commercial or aquarium collection pressure has been recorded.
The single known locality — a remote highland creek in the Massif du Mitakara — is geographically isolated and not currently subject to the large-scale industrial or agricultural pressures that threaten lowland Amazonian rivers. However, the extreme range restriction means that any localised habitat disturbance — a landslide, a pollution event, or changes to the hydrology of the Mapaoni tributary — could affect the entire known population. The IUCN noted limited knowledge of population status and threats as the basis for the DD assessment. Further surveys to clarify the species' range are needed before a more precise threat category can be assigned.