The genus Pygocentrus is characterized by several anatomical features
Key To The Species of Pygocentrus
1a. Adipose fin in large individuals with rays. Branched dorsal-fin rays 15-18, usually 16;6-8, usually 7 neural spines anterior to first dorsal-fin pterygiophore; vertebrae 36-39, usually 38................................................P. piraya.
1b. Adipose fin without rays. Branched dorsal-fin rays 14-18, usually 15; 5-7, usually 6 neural spines anterior to the first ptrygiophore; vertebrae 35-38, usually 36................2
2a. Large dark humeral spot in specimens over 100 mm SL; few body spots above that size; 4-5, usually 4 supraneurals.................P. cariba.
2b. Humeral spot relatively small or lacking at all sizes; body spots may be numerous; 4-5, usually 5 supraneurals........................P. nattereri.
Eye color has little value since some P. nattereri have "clear" eyes.