The dwarf lantern shark (Etmopterus perryi) is smaller than a human hand.

Unlike bony fish, sharks do not have gas-filled swim bladders for buoyancy. Instead, they rely on a large liver filled with oil.

Bull sharks can thrive in both salt and fresh water. They have been known to travel up the Mississippi River as far as Alton, Illinois, about 700 miles (1100 km) from the ocean.

Until the 16th century, sharks were known to mariners as "sea dogs". This is still evidential in several species termed "dogfish," as well as the porbeagle.

The U.S. records by far the most unprovoked shark attacks--an average of 45 per year over. However, only 1.3 percent of these incidents are fatal--0.6 deaths per year.

Shark litters vary in size, from a single pup to over a hundred.

The sharpnose sevengill shark has large, fluorescent green eyes.

The shortfin mako has an extremely streamlined body shaped like a torpedo that helps minimize drag as it cruises through the water. It can reach top speeds of 60 miles per hour.

Shark teeth are constantly replaced throughout life. Multiple rows of replacement teeth grow in a groove on the inside of the jaw and steadily move forward like a conveyor belt; some sharks lose 30,000 or more teeth in their lifetime.

Thresher sharks use their impressive tails to stun their prey with sharp overhead tail slaps. Their scythe-like tails make up half the length of their body.