Sharks (superorder Selachimorpha) are a type of fish with a full cartilaginous skeleton and a highly streamlined body. They respire with the use of five to seven gill slits. Sharks have a covering of dermal denticles that protect their skin from damage and parasites and improve fluid dynamics. They have several sets of replaceable teeth. Sharks range in size from the small dwarf lanternshark, Etmopterus perryi, a deep sea species of only in length, to the whale shark, Rhincodon typus, the largest fish, which grows to a length of approximately and which feeds only on plankton, squid, and small fish through filter feeding.

The bull shark, Carcharhinus leucas, is the best known of several species that swim in both seawater and freshwater, as well as in deltas.

Physical characteristics

main: Physical characteristics of sharks

Skeleton

The skeleton of a shark is very different from that of bony fish and terrestrial vertebrates. Sharks and other cartilaginous fish (skates and rays) have skeletons made from cartilage, which is a flexible and dense connective tissue, but they are still considered bones. They function in the same way as human bones do. Like its relatives, rays and skates, the shark's jaw is not attached to the cranium. The jaw's surface, like its vertebrae and gill arches, is a skeletal element that needs extra support due to its heavier exposure to physical stress and its need for extra strength. It has therefore a layer of unique and tiny hexagonal plates called "tesserae", crystal blocks of calcium salts arranged as a mosaic. This gives these areas much of the same strength found in real and much heavier bony tissue.

Generally there is only one layer of tesserae in sharks, but the jaws of large specimens, such as the bull shark, tiger shark, and the great white shark, have been found to be covered with two to three layers or more, depending on the body size. The jaws of a large white shark may even have up to five layers.

In the rostrum (snout), the cartilage can be spongy and flexible to absorb the power of impacts.

The fin skeletons are elongated and supported with soft and unsegmented rays named ceratotrichia, filaments of elastic protein resembling the horny keratin in hair and feathers.

Respiration

if unable to stay in motion. (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 blood is pumped to the shark's gills via the ventral aorta artery where it branches off into afferent brachial arteries. Reoxygenation takes place in the gills and the reoxygenated blood flows into the efferent brachial arteries, which come together to form the dorsal aorta. The blood flows from the dorsal aorta throughout the body. The deoxygenated blood from the body then flows through the posterior cardinal veins and enters the posterior cardinal sinuses. From there blood enters the ventricle of the heart and the cycle repeats.