Ancistomus is a genus of suckermouth armored catfish in family Loricariidae, subfamily Hypostominae, the large group that holds most of the true plecos. The atlas's five species are all recorded from Brazil and run 4.5–8.5 in, sharing the family's signature body plan: a flattened, bony-plated body and a ventral sucker mouth used to graze algae and biofilm off submerged wood and rock in flowing Amazonian streams and rivers. Its boundaries with closely related genera such as Peckoltia have shifted across taxonomic revisions, which is typical for this diverse group of Hypostominae catfish.
Several Ancistomus species entered the hobby first as unidentified imports under the L-number system - Ancistomus feldbergae as L012, Ancistomus spilomma as L036, and Ancistomus snethlageae as L141 - before formal scientific description caught up, a common path for Amazonian plecos. They're kept much like other small-to-mid Hypostominae: driftwood and rockwork to graze on, good water movement and oxygenation, and a mostly vegetable-and-biofilm diet. Conservation status is mixed rather than reassuring across the five atlas species - three Least Concern, but one Near Threatened and one Endangered - so at least one member of this small genus is under genuine pressure.