Sharks have a highly streamlined, swimmable appearance; their entire body is covered with shield scales, which, in addition to protecting them from injury or parasites, enhance their hydrodynamics and allow them to swim faster. Sharks have 5-7 gill slits on the side of the body for breathing. They have several sets of replaceable teeth. Sharks are found in waters all over the world, even at depths of 1,000 meters (3,300 feet). A few sharks, such as the lowfin mako or the ragged-tooth shark, live in both freshwater and saltwater.
Sharks have quite sensitive sense organs and can even smell fish blood from thousands of meters away. They have the ability to sense electricity and can detect prey hidden under the sand. Its species include weasel sharks, big blue sharks, mako sharks, thresher sharks, hammerhead sharks, megalodon sharks, etc. Different species have different foods, some are carnivorous and can devour seals and sea turtles, while some only filter-feed plankton. Most sharks have been greatly reduced in number by human activities. Sometimes the constant rotor rudder on the stern of the boat will stir up the fish to produce a bloody odor, so there are many sharks following the tail to eat.
Modern sharks appeared in the Jurassic to Cretaceous periods, and teeth of the order Porbeagle can be found in the early Cretaceous. The latest shark to evolve was the hammerhead shark, which fossilized in the Oligocene. During the Miocene, sharks of the family Otodontidae evolved to very large sizes, including the famous megalodon. Megalodon sharks are estimated to have reached a maximum length of 23 meters (75 ft) and an average of 16-20 meters (52-66 ft), far exceeding the largest great white sharks at 8 meters (26 ft) and the whale sharks at 12.65 meters (42 ft). The internal parts of sharks consist of flexible cartilage rather than bone. Sharks have a cartilaginous skeleton with five to seven gill slits on either side of the head; these gills are used for breathing and extracting oxygen from the water.