Fish jaw

Most bony fishes have two sets of jaws made mainly of bone. The primary oral jaws open and close the mouth to catch and handle their food, while a second set of pharyngeal jaws at the back of the throat help process the food and move it toward the stomach. These jaws play an important role in how fish eat and survive.

Cartilaginous fishes, such as sharks and rays, have only one set of oral jaws made of cartilage instead of bone, and they do not have pharyngeal jaws. Their jaws are made of upper and lower parts and can have many teeth. These fishes can grow new teeth and replace old ones as they wear out, similar to a moving belt.

Jaws first appeared in ancient fish called placoderms and spiny sharks during the Silurian period, about 430 million years ago. Scientists believe that early jaws were not used for eating but helped these fish breathe better by moving water over their gills. Over time, jaws became important for feeding and are now a key feature in all vertebrates, including humans. The development of jaws was a major step in the history of animals because fish without jaws had a harder time surviving.

Skull

The skull of fishes is made from a series of loosely connected bones. Lampreys and sharks only have a cartilaginous structure, with their upper and lower jaws being separate. Bony fishes have extra dermal bone, forming a more complete skull roof in lungfish and holost fish.

Jawless fish have a simpler skull structure, made from cartilaginous elements that only partially enclose the brain. Cartilaginous fish, such as sharks, also have simple skulls. Their cranium forms a case around the brain, with an opening at the top called a fontanelle. The front part of the cranium includes a plate of cartilage called the rostrum and capsules for the olfactory organs. Behind these are the orbits, and then capsules for the inner ear. The skull tapers towards the rear, ending with a condyle that connects to the first vertebra. There are also smaller openings in the cranium for the cranial nerves. The jaws are made from separate hoops of cartilage, distinct from the main cranium.

In ray-finned fishes, the skull has undergone changes. The roof of the skull is usually well formed, though the exact relationship to the bones of tetrapods is unclear. The upper jaw is often formed from the premaxilla, with the maxilla located further back, and a bone called the symplectic linking the jaw to the rest of the cranium.

While the skulls of fossil lobe-finned fish resemble those of early tetrapods, living lungfishes have a different structure. Their skull roof is not fully formed and consists of irregularly shaped bones. The upper jaw is formed from the pterygoids and vomers, which all have teeth. Much of the skull is made from cartilage, and its overall structure is simpler.

Oral jaws

In fish, the lower jaw is made of bone and connects to the skull. It includes several bones that work together, such as the dentary bone where most teeth are attached. Over time, many of these bones have changed or disappeared in different fish species.

The upper jaw is formed from two bones that fuse together. In bony fish, it holds teeth and helps with biting. Some fish, like billfish and sawfish, have special extended upper jawbones called rostrums. These rostrums help them catch prey by stunning or slashing at it.

Many fish, especially teleosts, can push their upper jaws outward to catch food. This helps them grab prey and pull it into their mouths without losing it. Different fish use various methods to achieve this jaw movement, which helps them catch food more effectively. The slingjaw wrasse, for example, can extend its jaws quite far to catch smaller fish and crustaceans.

Pharyngeal jaws

Pharyngeal jaws are a second set of jaws found in the throat of most bony fish. They are different from the main jaws used to bite and are supported by muscles instead of having a joint.

One interesting example is the moray eel. Most fish cannot move their pharyngeal jaws, but moray eels can. This helps them because they live in tight spaces where they cannot easily swallow food the usual way. When a moray eel catches something, it first uses its main jaws to grab it. Then, its pharyngeal jaws move forward to hold the prey and pull it down the eel's throat so it can be swallowed.

Cartilaginous jaws

Cartilaginous fishes like sharks, rays, and skates have jaws made of cartilage instead of bone. These jaws need to be very strong because they face a lot of pressure when catching food. They are covered with tiny, hexagonal plates called "tesserae", made of calcium salts, which make them as strong as bone in other animals.

Sharks usually have one layer of these plates, but bigger sharks, such as the bull shark, tiger shark, and the great white shark, can have two to five layers in their jaws. This extra layering helps their jaws stay strong. The front part of a shark’s snout, called the rostrum, has flexible cartilage that can absorb impacts.

Teeth

See also: Shark tooth and Animal tooth development

Jaws in most bony fish hold simple pointed teeth, but there are many exceptions. Some fish, like carp and zebrafish, only have teeth at the back of their throats. Sea horses, pipefish, and adult sturgeon have no teeth at all. Special genes called Hox genes help control how teeth form in fish.

Both sharks and bony fish grow new teeth their whole lives, but they do it in different ways. In sharks, teeth grow inside the jaw and move forward in rows until they fall out, like a moving belt. Shark teeth are not attached directly to the jaw but sit in the gums. Some sharks may lose thousands of teeth over their lifetime. In most bony fish, teeth are replaced one at a time. However, in piranhas and pacus, all the teeth on one side of the jaw are replaced together.

The shape of a shark’s teeth depends on what it eats. Sharks that eat hard-shelled animals like mollusks and crustaceans have flat, strong teeth for crushing. Sharks that eat other fish have sharp teeth for holding onto their prey. Those that eat larger animals like mammals have pointed lower teeth and specially shaped upper teeth for cutting.

Examples

Salmon

Male salmon often change the shape of their jaws during their journey to lay eggs. These changed jaws, called kypes, curve strongly. Scientists aren't exactly sure why, but they may help the fish show they are stronger by grabbing another fish near its tail.

Cichlids

Most fish jaws are the same on both sides, but one special type of cichlid fish has jaws that twist to either the left or the right. This helps it bite off scales from other fish on the opposite side. Cichlids use their front jaws to catch food and their throat jaws to chew it. This lets them eat different kinds of food and live in many places.

Different cichlids have different teeth. Some have strong, flat teeth for cracking shells, while others have pointy teeth for grabbing slippery prey. Some even have small teeth for eating algae, showing how their jaws help them survive.

Other

Stoplight loosejaws are small deep-sea fish with very wide mouths. Their lower jaws are special, with large fang-like teeth in the front and many small barbed teeth behind. They also have throat teeth that help guide food down.

Another deep-sea fish, the pelican eel, has jaws bigger than its body. These jaws are loose and open wide enough to swallow a fish larger than itself.

Distichodontidae are freshwater fish families. Some have upper jaws that can stretch out to catch meat, while others have fixed jaws and eat plants or tiny creatures.

Evolution

The earliest fish with jaws were the now extinct placoderms and the spiny sharks. Having jaws helped fish survive better than those without jaws. Most jawless fish went extinct during the Triassic period, but some, like hagfishes and lampreys, survived.

Jaws are thought to have developed from structures that supported the gills in early fish. These structures changed over time to form the upper and lower parts of the jaw. This change allowed fish to use their jaws for feeding, which became very important for many vertebrates. Some fish, like modern teleosts, have very complex jaws that help them catch prey.