They believe there are probably too many sharks to count, but estimate around 400 million species and over a billion sharks altogether.  There are lots of different kinds, all but two kinds, the River Shark and the Bull Shark, stay in the ocean.  These two species can also be found in fresh water.  Sharks have been around as long as dinosaurs, some say longer, but around 450 million years.  The largest kind of shark is a whale shark, about 60 feet long, and the smallest is a dwarf lantern shark, approximately 7 inches long.  Sharks don't have skeletons.  They have cartilage not bones, like people have in their ears and noses.  Sharks are my favorite ocean creature but I'd rather read about them than swim with them.  The new words I learned from reading this Wonderopolis report were Cartilage:  the connective tissue that sharks are made of instead of bones.  Mariners:  Old time sailors.   Recreational:  Do something for fun.