Skin - A sharks skin is made out of placoid scales. They are tooth like structures which are abrasive if rubbed from back to front. There are different scales on all parts of the body, they include rounded , shield like, keeled diamond shaped, quadrangular shaped structures. Sharks have a variety of shapes and sizes of teeth, with each shark possessing a unique kind of tooth. Sharks teeth are finely serrated and razor sharp for efficiency.
Sharks also have a conveyor belt system of dental replacement.
When an old tooth breaks off or is worn down, about every eight days, a new tooth directly behind the old tooth is there to replace it. Buoyancy - Sharks are truly the fighter jets of the sea, in constant motion to avoid sinking to the bottom. Unlike bony fishes sharks do not have a swim bladder. Instead sharks have a large liver that has a specific gravity of 0.926, sea water has a specific gravity of 1.025, which gives them neutrally buoyancy. The shark also has another feature that gives it positive buoyancy, it's large pectoral fins.