They are characterized by being large predators. Sharks range from small species from the deep sea, to the whale shark, the largest of the fish, which is believed to measure a length of 18 m and feed only on plankton. The Sardinian shark can move to fresh water and some shark attacks have occurred in rivers. Some of the larger species, especially the mako shark and the white shark, are partial endotherms, able to partially maintain their body temperature above that of the aquatic environment in which they live. Of the more than 375 species of sharks found in the world’s oceans, the species responsible for most of the unprovoked attacks on humans are the white shark, the tiger shark, the Sardinian shark and the Carcharhinus leucas.