Are Both Jaws on a Fish Equally Movable?

Lower-Jaw Mobility

• Many insects and other simple life forms have pairs of jaws that both move, creating a pincer or scissor effect. Most higher forms of life, from humans to dogs to fish, have only one moving jaw. The other is generally molded into the skull, providing a sturdy surface for the moving jaw to push against. This design makes more efficient use of muscle than two moving jaws would, and the skull is also made sturdier by incorporating a substantial jawbone.

Variations in Fish Jaws

• Fish jaws follow a number of consistent patterns, determined by a fish's habitat and diet. In some, the upper "lip" is relatively short and the lower jaw meets it at a steep angle. These fish are designed for taking food from the surface of the water, such as floating flies. The jaws of many other fish come to a fairly symmetrical point, indicating that these fish are free-swimmers, approaching their prey in a straightforward manner. Bottom feeders typically have a longer upper jaw and undercut lower jaw, making it easier to eat food that is beneath them.

Specialized Jaws

• Although fish jaws follow a consistent pattern, there is much variation among them. Those that consume large prey generally have powerful cutting teeth, to bite their meal into pieces that can be swallowed. Others, which eat smaller prey, have much smaller teeth or even areas of fine, mesh-like teeth to hold their prey as it's swallowed. Those, like the parrotfish, which must chew through shell to reach their prey, often have a tough, bony beak. Many sharks have jaws that are held in place by connective tissue and can be projected forward for a better grip.

Pharyngeal Jaws

• In addition to their main jaws, most omnivorous or carnivorous fish have teeth in their throats to help hold and swallow their prey. These are called pharyngeal jaws, and they usually serve double duty as part of the gill mechanisms' structure. These are the bony rib-like sections often seen in fish as they're being removed from the hook. The throat muscles can move these and help pull food down the fish's gullet. An odd variation on this mechanism can be seen in moray eels, which have a full second pair of jaws in their throats, which strike forward to hold their prey.