Megalancistrus is a genus of large armored suckermouth catfish in family Loricariidae, part of the diverse South American pleco group. Species in the atlas are found across the Paraná-Paraguay river system, spanning Brazil, Argentina, Paraguay, and Uruguay, and they carry the classic pleco body plan: a flattened head, bony armor plates covering the body, and a ventral sucker-disc mouth used to graze algae and biofilm off rock and wood in slow to moderate current. What sets the genus apart is sheer size - atlas species range from 16.5–23 in, making Megalancistrus one of the largest pleco genera kept by hobbyists at all.
The genus has real standing in the pleco-keeping hobby. Megalancistrus barrae, the black spiny pleco, and M. parananus, traded under the L-number L113, both turn up in the specialist import trade, prized for their bold size and armored, spine-studded profile, though their adult dimensions mean they are only realistic for large, well-filtered aquariums with plenty of driftwood and rockwork to graze and hide among. Both atlas species carry a Least Concern conservation status, and the genus is not currently a conservation priority.