Sharks | Selachii
Sharks are one of the oldest animal species on earth having outlived the dinosaurs with fossil records dating them back 400 million years. Most sharks live in saltwater environments, although two species can survive in freshwater and saltwater. In their ecosystems, the carnivorous shark is usually the top of their food chain, but are being threatened by human activities and hunting. The largest fish in the world is the whale shark, capable of growing to 40 feet (12.2 m) in length. Sharks have multiple rows of teeth that are embedded in their gums instead of their jaws, and are constantly losing and replacing their teeth—sharks can lose and replace up to 30,000 teeth in their lifetime.
Sharks do not have any bones in their bodies, as their skeleton system is made up of cartilage and connective tissue. This classifies them as Chondrichthyes fish. Cartilage is flexible and has about half of the normal density found in bones.
The diet of a shark varies from shark to shark depending on the species, habitat, and available prey. There are over 400 species of sharks. Most sharks are carnivorous and predators, while some are planktivorous. Sharks aren’t picky and are able to adjust their diet to what is available in order to survive.
It is not known if sharks are able to sleep, but they do seem to have periods of rest. Sharks need to keep water moving over their gills to receive oxygen. Some sharks need to keep moving all the times to keep water over their gills, while others have spiracles, an opening behind each eye, that allows them to breath while they are still.