It was after the 16th century that the 'sea dog' as it was known earlier, came to be named 'shark' after the Mayan word 'xook'.  Sharks have been around for aeons and the Shark Callers of Kontu in Papua New Guinea have made us aware that the man-shark relationship has a hoary past. Sharks do not normally attack people but are capable of being ferocious. 
     
With fully cartilaginous skeletons over streamlined bodies, sharks respire through  five or seven gill slits. Dermal denticles that cover them protect the skin and improve their fluid dynamics. From the small dwarf lanternshark to the great big white shark, they come in all sorts of shapes and sizes.

Feeding mostly on  plankton, squid and other fishes sharks are found in both salt and fresh water as also in deltas.  In the absence of swimming bladders sharks rely on their large livers filled with oil to remain buoyant. In addition they employ dynamic lift to maintain depth and sink when they stop swimming.

Sharks need to constantly swim in order to breathe and can't sleep very long, if at all, or they will sink. However certain shark species have spiracles that force water across their gills allowing them stationary rest on the ocean bottom. Some sharks, if inverted or stroked on the nose, enter a natural state of tonic immobilty.

Maximum shark ages vary by species. Most sharks live for 20 to 30 years, while the spiny dogfish species lives a record lifespan of more than 100 years. Sharks are nature's hardy predators and humans have been fascinated by them since times immemorial. Shark videos helps you learn more about these fantastic creatures of the waters.