Pacu fish (Family: Characiformes) possess a deceptively simple lower jaw joint that does little to reflect the complexity of the full feeding apparatus. Within red-bellied pacu, Piaractus brachypomus, the quadrato-mandibular joint (QMJ) restricts the lower jaw to a single open-close hinge motion. However, these fish consume a wide breadth of foods, posing differing mechanical challenges, which the jaws and teeth must overcome and process. Through the use of X-ray Reconstruction of Moving Morphology (XROMM), we found that P. brachypomus can recruit an additional degree of freedom for jaw motion that permits modulation of feeding behavior on the basis of food type. We have observed that the neurocranial hyomandibular joint (NCHY) is capable of permitting up to 1.8 degrees of suspensorial protraction through rotation of the hyomandibula about a medio-lateral axis at the NCHY joint. Additionally, we observed that while the magnitude of rotations at the QMJ showed no significant differences food to food, the rotations of the hyomandibula varied on the basis of food type with brittle foods requiring 0.5 ± 0.025 degrees hyomandibular rotation on average, and tough foods requiring 0.9 ± 0.05 degrees of rotation (mean ± s.e.; n=19 brittle, n=21 tough chewing cycles from 2 individuals). These rotations are small, but they are large enough to protract the lower jaw by 1-2 mm and we confirmed that they exceed our precision threshold (± 0.2 deg) for measurement of this rotation. We suggest that in protracting the suspensorium, and by extension the connected lower jaw, the pacu is capable of changing the occlusal positioning of the upper and lower tooth rows. In doing so the pacu would have the ability to change whether teeth were best positioned to slice through those tougher foods such as grasses while shearing apart more brittle foods like seeds; both of which have been repeatedly found in stomach contents.