Sharks are members of the Class Elasmobranchiomorphi. They are related to the rays and skates. Like all other members of this class they have a cartilage skeleton. The Shark shown here is a White shark, one of the most effective predators on earth.
This pelagic predator is adapted for constant movement. It is fusiform shaped to reduce drag and very streamlined. Sharks depend on ram ventilation to breath. Unlike most boneyfish which can pump their opercular (gill) flaps to move water over their gills, sharks depend on foward motion to provide a flow of water running over their gills. If they stop moving they suffocate.
Sharks also differ from most boney fish in that they do not possess a swim bladder. They are slightly heavier than salt water and have a tendency to sink. They counteract this by the shape of their bodies and the shape of their fins. The tail of a shark is heterocercal. The top half of their tail is longer than the bottom half. As a result the top of their tail provides more thrust than the bottom forcing their tail up. The front half of their body is wedge shaped. The angle of the slope from nose to jaw forced water down pushing the head up. The forces working on the head and tail counteract the sharks natural tendency to sink. The pectoral fins of a share are fixed ridgidly to the sides fo the body. All though they cannot move foward and backwards they can twist up and down like the flaps on plane. This allows them to steer.
Click next to see one swimming