Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest know sharks date from more than 420 millions ago, before the time of the dinosaurs. Since that time, sharks have diversified in 440 species. They eat, plancton, squid, and small fish by filter feeding. They generally do not live in freshwater, with a few exceptions. such as the bull shark and the river shark. wich can both live both in seawater and freshwater. They breathe through five to seven gill slits. Sharks have a covering of dermal denticles that protect their skin from damge and also have parasites that improve their fluid dynamics so the shark can move faster. They have several sets of replaceable teeth.
Sharks teeth are embedded in the gums rather than directly affixed to the jaw, and are constantly replaced throghout life. Multiple rows of replacement teeth grow in a groove on the inside of thye jaw and steadily move forward as in a "conveyor belt"; some sharks lose 30,000 or more teethin their lifetime. The rate of tooth replacement varies from once every 8-10 to several months. In the most species teeth are replaced one at a time, except in cookiecutter sharks the entire row of teeth is replaced simultaneosly. Tooth shape depends on diet: sharks that feed mollusks and crustacean have dense flattened teeth for crushing, those that feed on fish have needle-like teeth for griping, and those that feed on larger prey such as mammals have pointed lower teeth for griping and triangular upper teeth of plankton feeders such as the basking shark are smaller and non-fuctional.
Varying tail shapes have envolved in sharks adapted for differents enviroments. Tail vary considerably between species. The tail provides thrust, making speed and aceleration dependent on tail shapes. Sharks possess a heteroceral caudal fin in which the dorsal portion. This is because the vertebral column extends into that dorsal potion, providing a greater surface area muscle attachment. This allow more efficient locomotion among these negatively buoyantcartilaginous fishes. By contrast, most bony fishes possess a homocercal caudal fin.
The tiger's shark tail has a large upper lobe which delivers maximum power for slow cruising or sudden burst of speed. The tiger shark must be able to twist and turn in the water easily when hunting to support its varied diet, whereas the porbeagle, which hunts schooling fish such as mackerel and herring has a large lower lobe to help it keep pace with its fast-swiming prey. Some tail adaptions have other purposes. The thresher feeds on fish and squid, which it herds and stunts with is powerful and elongated upper lobe.
Like other fish, shark extract oxygen from seawater as it passess over their grills. Unlike other fish, shark gill slits are not covered, but lie in a row behind the head. A modified slit called a spiracle lies just behind the eye; the spiracle assits water intake during respiration and plays as a major role in bottom-dwelling sharks. While at rest, most sharks pump water over the grills to ensure a constant supply of oxygenated water. A small number of species have lost the ability to pump water through their gills and must swim without rest. These species arf obligate ram ventilators and would presumably asphyxiate if unable to move. Obligate ram ventilation is also true of some pelagic bony fish species.
The respiration and circulation process begins when deoxygenated blood travels to the shark's two-chambered heart. Here the shark pumps blood to its gills via the ventral aorta artery where it branches into afferent brachial arteries.
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