page_title: Fish jaw
section_title: Teeth
hierarchical_section_title: Fish jaw / Teeth
caption_attribution_description: English: The internal side of a shark sp. mandible, showing sequential growth arrangement of teeth.
context_page_description: 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.
  Cartilaginous fishes, such as sharks and rays, have one set of oral jaws made mainly of cartilage. They do not have pharyngeal jaws. Generally jaws are articulated and oppose vertically, comprising an upper jaw and a lower jaw and can bear numerous ordered teeth. Cartilaginous fishes grow multiple sets 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 pharyngeal arches supporting the gills of jawless fish.
context_section_description: Jaws provide a platform in most fishes for simple pointed teeth. Lungfish and chimaera have teeth modified into broad enamel plates with jagged ridges for crushing or grinding. Carp and loach have pharyngeal teeth only. Sea horses, pipefish and adult sturgeon have no teeth of any type. 
  In fish, Hox gene expression regulates mechanisms for tooth initiation.
  However, sharks continuously produce new teeth throughout their lives via a drastically different mechanism. Shark teeth form from modified scales near the tongue and move outward on the jaw in rows until they are eventually dislodged. Their scales, called dermal denticles, and teeth are homologous organs.
  Shark teeth are embedded in the gums rather than directly affixed to the jaw, and are constantly replaced throughout life. Multiple rows of replacement teeth grow in a groove on the inside of the jaw and steadily moving forward as though on a conveyor belt. 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. In most species, teeth are replaced one at a time as opposed to the simultaneous replacement of an entire row, which is observed in the cookiecutter shark.
  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 are small and non-functional.
caption: Inside of a shark jaw where new teeth move forward as though on a conveyor belt
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