The functions and mechanisms of the protrusible upper jaws of two species of cyprinid fish



Photographs have been taken of Idus and Gobio feeding. Idus, a primitive cyprinid, protrudes its premaxillae very little when it opens its mouth to take in food, but closes the mouth by further protrusion of the premaxillae. This enables it to close its mouth while keeping its buccal cavity fully expanded. Gobio, a more advanced cyprinid, protrudes its premaxillae strongly as it opens its mouth and is thereby able to apply the mouth closely to a surface from which it is taking food. The mechanisms of the movements are described.