What does the upper jaw do on a fish?

In lower jawed fishes, which comprises the majority of living fish species, the upper jaw is immovably attached to the skull. The lower jaw is the primary movable bone that opens and closes, allowing the fish to open its mouth.

Feeding in bony fishes.

For bony fishes, also known as teleosts, the upper jaw, also called the maxilla, plays a less prominent role in prey capture compared to the lower jaw. In many teleosts, the upper jaw forms the upper margin of the fish's mouth, while the lower jaw moves up and down. Some bony fishes have protrusible premaxillary bones, allowing them to extend the upper jaw forward, aiding in capturing and processing prey.