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Lack of Swim Bladder
Bony fish contain a swim bladder, an organ that fills with gas and provides buoyancy. Bony fish, being denser than water, would sink without the swim bladder. It allows them to float in the water. One of the traits of cartilaginous fish is the lack of a swim bladder. An oil-filled liver provides them with an alternative to maintain buoyancy, but for the shark it is not enough. If a shark stops swimming it sinks to the bottom. The solution for the shark is to swim constantly.
Gills
All fish use gills to obtain oxygen from water. Cartilaginous fish possess five to seven gill slits along the side of their heads. Unlike bony fish with gill covers, cartilaginous fish slits remain open at all times. Water must continually be passing through the slits. The continual swimming of the shark allows this to occur. Blood vessels in the gills remove oxygen from the water, allowing the fish to breathe.
Scales
Sharks possess scales, called dermal denticles, which give the fish a smooth appearance. The scales produce a texture that feels like sandpaper. Also known as placoid scales, these scales resemble teeth when examined closely. Their structure is the same as teeth; an outer layer of enamel, middle of dentine and pulp cavity. The scales are fixed in size and do not grow with the fish. As the fish grows more scales are added.
Cartilage
The defining trait of cartilaginous fish is the lack of bones. The bones are replaced by cartilage. Due to this lack of bone the muscles connect directly to the tough skin. This connection, when combined with cartilage that is lighter and less dense than bone, gives sharks and other cartilaginous fish a streamlined and sturdy body that moves effortlessly through water. Less energy is expended in swimming, which is important for a species that swims all the time.