Acnodon is a genus in family Serrasalmidae, the piranha and pacu family, found in the Amazon basin and the rivers of the Guiana Shield - the atlas records it from Brazil, French Guiana, Guyana, and Suriname. Atlas species range 5–8 in. Unlike their famous piranha relatives, Acnodon species sit on the herbivorous, pacu side of the family, with blunt, multicusped teeth suited to cropping plant material, fruit, and detritus rather than flesh.
This is a genus known mainly to specialist Serrasalmidae keepers and field ichthyologists rather than the general fishkeeping trade - none of the three atlas species (A. normani, A. oligacanthus, or Sena's pacu, A. senai) has much of an established aquarium presence, and their size and herbivorous appetite would demand a large, plant-tolerant setup rather than a typical community tank. Conservation status is reasonably favourable where known: two of the three atlas species are Least Concern, with the third Data Deficient.