Fishes are known for their strange teeth on both pharyngeal and oral jaws, but many fishes are edentulous. Edentulous animals use specialized gastrointestinal anatomy for mechanical and chemical breakdown of food. Birds and some fishes eat biomechanically demanding prey (e.g. plants) and dismantle tough foods through a grinding mill in the gut, the gizzard. This region of the gut is composed of thick, multi-layered muscle and is filled with mucus and grit. While all birds have this organ, our understanding of the morphology, function, and presence of gizzards in fishes remains unclear. Moreover, the differentiation between gizzards and stomachs in fishes has not been established. We use PTA staining, micro-CT scans, particle-size analysis, and histology to describe variation in gizzard morphology across several lineages to elaborate on potential functions. Food particles dissected from the anterior digestive tract (esophagus and gizzard) compared to food particles found in the intestine decrease substantially in species with gizzards. The size of food particles from species without gizzards remains consistent throughout the length of the gut. We used these morphological data to design a motorized gizzard to assess the roles of grit and mucus during food processing; we found that grit composed of both sand and small pebbles was most effective for breakdown of food. We show the importance of a grit-filled gizzard for pulverizing prey in fishes and use our morphological descriptions to better differentiate between a stomach and a gizzard among fishes.