Protocheirodon pi can be distinguished by possessing four synapomorphies proposed for the Cheirodontinae by Malabarba (1998): presence of a pseudotympanum, a hiatus in the body wall musculature lateral to at least a portion of the anterior chamber of the swimbladder; absence of dark humeral spot; possession of pedunculate dentition; and presence of only a single row of regularly arranged premaxillary teeth. It differs from all species of the Cheirodontinae and indeed members of the Characiformes by the unique form of the swimbladder which has a smaller rotund anterior chamber and the posteriorly chamber elaborated into two vertically elongate, side by side, slightly inclined chambers. These modifications resulted to a unique overall pi (?)-shaped structure (Ref. 109753).