Fish jaw

Source: Wikipedia, the free encyclopedia.

Skull of a generalized cichlid, showing a lateral view of the oral jaws (purple) and the pharyngeal jaws (blue)[1]
Dorsal view of the lower pharyngeal and oral jaws of a juvenile Malawi eyebiter showing the branchial (pharyngeal) arches and ceratobrachial elements (arch bones). The white asterisk indicates the toothed pharyngeal jaw. Scale bar represents 500 μm.[1]

Most

bony fishes have two sets of jaws made mainly of bone. The primary oral jaws open and close the mouth, and a second set of pharyngeal jaws are positioned at the back of the throat. The oral jaws are used to capture and manipulate prey by biting and crushing. The pharyngeal jaws, so-called because they are positioned within the pharynx, are used to further process the food and move it from the mouth to the stomach.[2][3]

teeth
. Cartilaginous fishes grow multiple sets (polyphyodont) and replace teeth as they wear by moving new teeth laterally from the medial jaw surface in a conveyor-belt fashion. Teeth are replaced multiple times also in most bony fishes, but unlike cartilaginous fishes, the new tooth erupts only after the old one has fallen out.

Jaws probably originated in the

buccal pump to pump water across the gills. The familiar use of jaws for feeding would then have developed as a secondary function before becoming the primary function in many vertebrates. All vertebrate jaws, including the human jaw, evolved from early fish jaws. The appearance of the early vertebrate jaw has been described as "perhaps the most profound and radical evolutionary step in the vertebrate history".[4][5] Fish without jaws
had more difficulty surviving than fish with jaws, and most jawless fish became extinct.

Jaws use

protrusion of the premaxilla,[6] leading to three main four-bar linkage systems to generally describe the lateral and anterior expansion of the buccal cavity in fishes.[6][7] The most thorough overview of the different types of linkages in animals has been provided by M. Muller,[8]
who also designed a new classification system, which is especially well suited for biological systems.

Skull

The

holost
fish.

The simpler structure is found in

jawless fish, in which the cranium is represented by a trough-like basket of cartilaginous elements only partially enclosing the brain, and associated with the capsules for the inner ears and the single nostril.[9]

foramina for the cranial nerves. The jaws consist of separate hoops of cartilage, almost always distinct from the cranium proper.[9]

In

ray-finned fishes, there has also been considerable modification from the primitive pattern. The roof of the skull is generally well formed, and although the exact relationship of its bones to those of tetrapods is unclear, they are usually given similar names for convenience. Other elements of the skull, however, may be reduced; there is little cheek region behind the enlarged orbits, and little, if any bone in between them. The upper jaw is often formed largely from the premaxilla, with the maxilla itself located further back, and an additional bone, the symplectic, linking the jaw to the rest of the cranium.[9]

Although the skulls of fossil lobe-finned fish resemble those of the early tetrapods, the same cannot be said of those of the living lungfishes. The skull roof is not fully formed, and consists of multiple, somewhat irregularly shaped bones with no direct relationship to those of tetrapods. The upper jaw is formed from the pterygoids and vomers alone, all of which bear teeth. Much of the skull is formed from cartilage, and its overall structure is reduced.[9]

Oral jaws

Lower

Oral jaw from side and above of Piaractus brachypomus, a close relative of piranhas

In vertebrates, the lower jaw (

articular bone forms the articulation with the skull proper. Finally a set of three narrow coronoid bones lie above the prearticular bone. As the name implies, the majority of the teeth are attached to the dentary, but there are commonly also teeth on the coronoid bones, and sometimes on the prearticular as well.[11]

This complex primitive pattern has, however, been simplified to various degrees in the great majority of vertebrates, as bones have either fused or vanished entirely. In

Cartilaginous fish, such as sharks, do not have any of the bones found in the lower jaw of other vertebrates. Instead, their lower jaw is composed of a cartilaginous structure homologous with the Meckel's cartilage of other groups. This also remains a significant element of the jaw in some primitive bony fish, such as sturgeons.[11]

Upper

The upper jaw, or maxilla[12][13] is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible (lower jaw), which is also a fusion of two halves at the mandibular symphysis. In bony fish, the maxilla is called the "upper maxilla," with the mandible being the "lower maxilla". The alveolar process of the maxilla holds the upper teeth, and is referred to as the maxillary arch. In most vertebrates, the foremost part of the upper jaw, to which the incisors are attached in mammals consists of a separate pair of bones, the premaxillae. In bony fish, both maxilla and premaxilla are relatively plate-like bones, forming only the sides of the upper jaw, and part of the face, with the premaxilla also forming the lower boundary of the nostrils.[14] Cartilaginous fish, such as sharks and rays also lack a true maxilla. Their upper jaw is instead formed from a cartilagenous bar that is not homologous with the bone found in other vertebrates.[14]

Some fish have permanently protruding upper jawbones called

laterally
(sideways).

  • Fish with rostrums (extended upper jawbones)
  • Sailfish, like all billfish, have a rostrum (bill) which evolved from the upper jawbone
    Sailfish, like all billfish, have a rostrum (bill) which evolved from the upper jawbone
  • The paddlefish has a rostrum packed with electroreceptors
    The
    electroreceptors
  • Sawfish have an electro-sensitive rostrum (saw) which is also used to slash at prey
    Sawfish have an electro-sensitive rostrum (saw) which is also used to slash at prey

Jaw protrusion

derived teleosts, the enlarged premaxilla is the main tooth-bearing bone, and the maxilla, which is attached to the lower jaw, acts as a lever, pushing and pulling the premaxilla as the mouth is opened and closed. These protrusible jaws are evolutionary novelties in teleosts that evolved independently at least five times.[16]

The premaxilla is unattached to the neurocranium (braincase); it plays a role in protruding the mouth and creating a circular opening. This lowers the pressure inside the mouth, sucking the prey inside. The lower jaw and maxilla (main upper fixed bone of the jaw) are then pulled back to close the mouth, and the fish is able to grasp the prey. By contrast, mere closure of the jaws would risk pushing food out of the mouth. In more advanced teleosts, the premaxilla is enlarged and has teeth, while the maxilla is toothless. The maxilla functions to push both the premaxilla and the lower jaw forward. To open the mouth, an adductor muscle pulls back the top of the maxilla, pushing the lower jaw forward. In addition, the maxilla rotates slightly, which pushes forward a bony process that interlocks with the premaxilla.[17]

Teleosts achieve this jaw protrusion using one of four different mechanisms involving the ligamentous linkages within the skull.[18]

Lips of a humphead wrasse
The sling-jaw wrasse has the most extreme jaw protrusion of all fishes.
Slingjaw wrasse protruding its jaw – YouTube
  • Mandibular depression mechanism: The depression of the lower jaw (mandible) pulls or pushes the premaxilla into protrusion via force transmission through ligaments and tendons connected to the upper jaws (e.g. Cyprinus, Labrus).[18] This is the most commonly used mechanism.
  • Twisting maxilla mechanism: The depression of the mandible causes the maxilla to twist about the longitudinal axis resulting in the protrusion of the premaxilla (e.g. Mugil).[18]
  • Decoupled mechanism: Protrusion of the premaxilla is accomplished through elevation of the neurocranium causing the premaxilla to move anteriorly. Movements of the neurocranium are not coupled with the kinematics of the upper jaw (e.g. Spathodus erythrodon),[18][19] allowing for more versatility and modularity of the jaws during prey capture and manipulation.
  • Suspensorial abduction mechanism: The lateral expansion of the suspensorium (a combination of the palatine, pterygoid series, and quadrate bones) pulls on a ligament which causes the premaxilla to protrude anteriorly (e.g. Petrotilapia tridentiger).[18][19]

Some teleosts use more than one of these mechanisms (e.g. Petrotilapia).[18]

Wrasses have become a primary study species in fish-feeding

protractile mouths, usually with separate jaw teeth that jut outwards.[20] Many species can be readily recognized by their thick lips, the inside of which is sometimes curiously folded, a peculiarity which gave rise the German name of "lip-fishes" (Lippfische).[21]

The nasal and mandibular bones are connected at their posterior ends to the rigid neurocranium, and the superior and inferior articulations of the maxilla are joined to the anterior tips of these two bones, respectively, creating a loop of 4 rigid bones connected by moving joints. This "four-bar linkage" has the property of allowing numerous arrangements to achieve a given mechanical result (fast jaw protrusion or a forceful bite), thus decoupling morphology from function. The actual morphology of wrasses reflects this, with many lineages displaying different jaw morphology that results in the same functional output in a similar or identical ecological niche.[20]

The most extreme jaw protrusion found in fishes occurs in the

slingjaw wrasse, Epibulus insidiator . This fish can extend its jaws up to 65% the length of its head.[22]
This species utilizes its quick and extreme jaw protrusion to capture smaller fishes and crustaceans. The genus this species belongs to possess one unique ligament (vomero-interopercular) and two enlarged ligaments (interoperculo-mandibular and premaxilla-maxilla), which along with a few changes to the form of cranial bones, allow it to achieve extreme jaw protrusion.

Pharyngeal jaws

Moray eels
have two sets of jaws: the oral jaws that capture prey and the pharyngeal jaws that advance into the mouth and move prey from the oral jaws to the esophagus for swallowing

gill arch
which no longer has a respiratory function. The first four arches still function as gills. Unlike the oral jaw, the pharyngeal jaw has no jaw joint, but is supported instead by a sling of muscles.

Pharyngeal jaw of an asp carrying some pharyngeal teeth

A notable example occurs with the moray eel. The pharyngeal jaws of most fishes are not mobile. The pharyngeal jaws of the moray are highly mobile, perhaps as an adaptation to the constricted nature of the burrows they inhabit which inhibits their ability to swallow as other fishes do by creating a negative pressure in the mouth. Instead, when the moray bites prey, it first bites normally with its oral jaws, capturing the prey. Immediately thereafter, the pharyngeal jaws are brought forward and bite down on the prey to grip it; they then retract, pulling the prey down the moray eel's gullet, allowing it to be swallowed.[23]

All vertebrates have a pharynx, used in both feeding and respiration. The pharynx arises during development through a series of six or more outpocketings called

endoskeletal support through the contribution of neural crest cells.[24]

Cartilaginous jaws

tesserae", which are crystal blocks of calcium salts arranged as a mosaic.[25]
This gives these areas much of the same strength found in the bony tissue found in other animals.

Generally sharks have only one layer of tesserae, but the jaws of large specimens, such as the bull shark, tiger shark, and the great white shark, have two to three layers or more, depending on body size. The jaws of a large great white shark may have up to five layers.[26] In the rostrum (snout), the cartilage can be spongy and flexible to absorb the power of impacts.

In sharks and other extant

articulated with the upper. The arrangement of soft tissue and any additional articulations connecting these elements is collectively known as the jaw suspension. There are several archetypal jaw suspensions: amphistyly, orbitostyly, hyostyly, and euhyostyly. In amphistyly, the palatoquadrate has a postorbital articulation with the chondrocranium from which ligaments primarily suspend it anteriorly. The hyoid articulates with the mandibular arch posteriorly, but it appears to provide little support to the upper and lower jaws. In orbitostyly, the orbital process hinges with the orbital wall and the hyoid provides the majority of suspensory support. In contrast, hyostyly involves an ethmoid articulation between the upper jaw and the cranium, while the hyoid most likely provides vastly more jaw support compared to the anterior ligaments. Finally, in euhyostyly, also known as true hyostyly, the mandibular cartilages lack a ligamentous connection to the cranium. Instead, the hyomandibular cartilages provide the only means of jaw support, while the ceratohyal and basihyal elements articulate with the lower jaw, but are disconnected from the rest of the hyoid.[27][28][29]

Teeth

Inside of a shark jaw where new teeth move forward as though on a conveyor belt
See also: Shark tooth and Animal tooth development

Jaws provide a platform in most

tooth initiation.[32][33]

While both

homologous organs.[39] Some sharks lose 30,000 or more teeth in their lifetime. The rate of tooth replacement varies from once every 8 to 10 days to several months, although few studies have been able to quantify this. In most species of bony fish, teeth are replaced one at a time as opposed to the simultaneous replacement of an entire row. However, in piranhas and pacus, all the teeth on one side of the jaw are replaced at a time.[40]

Tooth shape depends on the shark's diet: those that feed on

mollusks and crustaceans have dense and flattened teeth used for crushing, those that feed on fish have needle-like teeth for gripping, and those that feed on larger prey such as mammals have pointed lower teeth for gripping and triangular upper teeth with serrated edges for cutting. The teeth of plankton-feeders such as the basking shark and whale sharks are very small.[41][42]

Examples

Salmon

Open mouth of a salmon showing the second set of pharyngeal jaws positioned at the back of the throat
Kype of a spawning male salmon

Male

caudal peduncle) of an opponent.[43][44]

Cichlids

Dorsal view of right-bending (left) and left-bending (right) jaw morphs[45]

Fish jaws, like vertebrates in general, normally show

parasitic scale-eating cichlid Perissodus microlepis. The jaws of this fish occur in two distinct morphological forms. One morph has its jaw twisted to the left, allowing it to eat scales more readily on its victim's right flank. The other morph has its jaw twisted to the right, which makes it easier to eat scales on its victim's left flank. The relative abundance of the two morphs in populations is regulated by frequency-dependent selection.[45][46][47]

In cichlids generally, the oral and pharyngeal teeth differ with different species in ways that allow them to process different kinds of prey. Primary oral jaws contain teeth which are used to capture and hold food, while pharyngeal jaws have pharyngeal teeth which function as a chewing tool.

Lower jawbone with molariform teeth (Ctenochromis horei)
Lower jawbone with conical teeth (giant cichlid)

This allows for different nutritional strategies, and because of this, cichlids are able to colonize different habitats. The structural diversity of the lower pharyngeal jaw could be one of the reasons for the occurrence of so many cichlid species. Convergent evolution took place over the course of the cichlid radiation, synchronous with different trophic niches.[48] The pharyngeal jaw apparatus consists of two upper and one single lower plate, all of which have dentations that differ in size and type.[49] The structure of the lower pharynx is often associated with the species of food of the species.[50]

In order to crack shellfish considerable force must be generated, which is why cichlids that feed on molluscs (e.g. the cichlid bass, Crenicichla minuano), have molariform teeth and a strengthened jawbone bone. To grab and bite prey not armoured with shells, predators need conical, bent back teeth.[51] Herbivorous cichlids also have structural differences in their teeth. Cichlids that specialise in algae (e.g. Pseudotropheus) tend to have small conical teeth. Species that feed on pods or seeds require large conical teeth for chewing their food.[52]

Other

Stoplight loosejaw
Relative to its size the stoplight loosejaw has one of the widest gapes of any fish
Closeup of jaw
The pelican eel jaws are larger than its body.

Stoplight loosejaws are small fish found worldwide in the deep sea. Relative to their size they have one of the widest gapes of any fish. The lower jaw has no ethmoid membrane (floor) and is attached only by the hinge and a modified tongue bone. There are several large, fang-like teeth in the front of the jaws, followed by many small barbed teeth. There are several groups of pharyngeal teeth that serve to direct food down the esophagus.[53][54]

Another deep sea fish, the pelican eel, has jaws larger than its body. The jaws are lined with small teeth and are loosely hinged. They open wide enough to swallow a fish larger than the eel itself.

Distichodontidae are a family of fresh water fishes which can be divided into genera with protractile upper jaws which are carnivores, and genera with nonprotractile upper jaws which are herbivores or predators of very small organisms.[55]

Evolution

Vertebrate classes
spiny sharks
.
See also: Evolution of fish

The appearance of the early vertebrate jaw has been described as "a crucial innovation"

cyclostomes, the jawless hagfishes and lampreys that did survive, have yielded little insight into the deep remodelling of the vertebrate skull that must have taken place as early jaws evolved.[58][59]

The customary view is that jaws are

teleosts.[62]

Dunkleosteus terrelli
↑ Spiny shark

It is now accepted that the precursors of the jawed vertebrates are the long extinct bony (armoured) jawless fish, the so-called

spiny sharks.[66]

Placoderms were a

dermal armour[71] suggesting that Dunkleosteus terrelli was perfectly adapted to prey on free-swimming, armoured prey like arthropods, ammonites, and other placoderms.[72]

Early Permian about 290 million years ago.[73]

The original selective advantage offered by the jaw may not be related to feeding, but rather to increased respiration efficiency.

suction feeding and jaw protrusion, resulting in highly complex jaws with dozens of bones involved.[75]

Jaws are thought to derive from the

hyoid arch, which braces the jaw against the braincase and increases mechanical efficiency. While there is no fossil evidence directly to support this theory, it makes sense in light of the numbers of pharyngeal arches that are visible in extant jawed (the Gnathostomes), which have seven arches, and primitive jawless vertebrates (the Agnatha
), which have nine.

articular bone, which forms part of the jaw joint in all tetrapods except mammals.[76]

See also

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Further reading

External links

External videos
video icon Video of a slingjaw wrasse catching prey by protruding its jaw
video icon Video of a red bay snook catching prey by suction feeding
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