Sharks (superorderSelachimorpha) are a type of fish with a full cartilaginousskeleton 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, Etmopterusperryi, a deep sea species of only 17 centimetres (7 in) in length, to the whale shark, Rhincodontypus, the largest fish, which grows to a length of approximately 12 metres (39 ft) and which feeds only on plankton, squid, and small fish through filter feeding.
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.