SAILORS used to call sharks "sea dogs" until the 16th century. In 1569 a shark was exhibited in London and after that the name shark was used more generally.
Until then shark referred to the large sharks of the Caribbean Sea.
There are more than 440 species of shark, some of which reach 11m though they feed only on plankton and small squid.
Sharks are not mammals. They are cold-blooded fish and have a cartilaginous skeleton that makes them unique among other fish, which have bony skeletons. - amazingfactsworld.com