G. nassauensis is a rare species, highly endemic to the Nassau Mountains in Suriname, and is threatened with extinction by proposed or ongoing mining activities.

Guyanancistrus nassauensis is a small-sized species, the largest specimen observed having a standard length (SL) of 61.0 mm.

Dorsal profile gently convex from snout tip to dorsal-fin origin, usually more flattened posterior to orbit, slightly convex and sloped ventrally from dorsal-fin origin to adipose fin, then slightly concave to procurrent caudal-fin rays, and rising to caudal fin.

Evertible cheek plates with enlarged odontodes in highly variable number, from fewer than ten up to approximately 35 in some large specimens.

Pectoral-spine short, tip usually reaching the first quarter of pelvic spine, exceptionally extending up to the first third in large specimens (presumably males).

[5] Guyanancistrus nassauensis is morphologically discriminated from all congeners by a small adult size (largest specimen observed 61 mm SL; adult size likely reached around 40 mm SL), by a reduced number of anal-fin rays (4 branched rays vs 5, apart from exceptional specimens), and by a wide oval mouth with both large dentary and premaxillary tooth cups (in % of head length, respectively: 24.2–31.9, mean 27.6, vs 23.6 or less except in Guyanancistrus niger, and 25.4–31.4, mean 28.1, vs 24.5 or less).

[1] G. nassauensis is distinguished from G. longispinis and from G. niger by a much shorter pectoral-fin spine (in % of SL: 22.2–26.3, mean 24.4, vs 31.9–45.5, mean 40.2, and 33.3–48.0, mean 42.8, respectively), and by color pattern (body and fins uniformly brown or with indistinct medium-sized paler spots, vs brown-black with either small roundish yellow spots for G. longispinis, or white dots for G. niger).

It is further separated from all G. brevispinis group species by having, on average, the widest body, the deepest and longest head, the largest interbranchial distance, the shortest fins, and the highest number of teeth.

G. nassauensis was active both during the day and at night, and did not retreat in shelter, even for an individual that was transferred to a larger tank with numerous other fishes.

G. nassauensis mostly spent its time on the aquarium panes and on macrophytes, woody debris and rock, but not on the sandy substrate on the bottom.

[6] Guyanancistrus nassauensis is known solely from Paramaka Creek and some of its tributaries, Marowijne River Basin, in the Surinamese Nassau Mountains (an area of approximately 20x20 km2).

[5] In Paramaka Creek, G. nassauensis occurs syntopically with juvenile G. brevispinis and with Harttiella crassicauda, a second endemic species from the Nassau Mountains.

attached to it) and near the edge of the plateau in slightly deeper water (approximately 50 cm) stands of the emergent macrophyte Thurnia sphaerocephala occur.

[7] At an elevation of 277 m, the type locality is located in a northern branch of Paramaka Creek, a medium-sized and shallow stream (3–7 m. width; less than 50 cm depth) with pools and some riffle habitat, a rocky substrate, and bordered by terra firme rainforest.