Sharks do not sleep in the way that land animals such as humans do, but sharks do experience quiet periods and undergo what is called sleep-swimming. It was once believed that all sharks had to stay awake constantly in order to move water over their gills to breathe.
Sharks rest in an approximation of what is termed sleep in land animals, though they do it with their eyes open. However, some sharks are nocturnal, which means they are active at night and rest during the day, and others rest when they are well fed and safe.
Sharks are found in every ocean in the world and are sometimes found out of their habitat in rivers and streams. There are around 368 species of sharks in the world.
Sharks appear to have evolved from fish between 400 and 450 million years ago, during the Early Devonian period. The earliest known sharks were fairly small and measured just 12 to 16 inches long. They appear to have lived near or on what is now Antarctica.
According to the Shark Foundation, sharks are found in most of the Earth’s oceans and seas. The highest shark biodiversity occurs in the warm waters of the tropics, but many species are found in Arctic and Antarctic waters as well. Some sharks, for example bull sharks, ...
Sharks live in every ocean of the world, with most species occupying waters no deeper than 7,000 feet. The Portuguese dogfish is the deepest-living shark and can be found at a depth of 12,000 feet. Some sharks, such as the bull shark, are capable of surviving in fresh w...
Tiger sharks live in both open and coastal ocean areas at depths as low as 460 feet. They are commonly located in tropical and sub-tropical waters such as the Gulf of Mexico, the Caribbean Sea, Australia and New Zealand.