Fins & Swimming
Most sharks are designed for efficient motion through the water. They have three types of median fins (dorsal, anal, and caudal) and two sets of paired fins (pelvic and pectoral). Swimming is achieved by side-to-side undulations of the caudal, or tail fin, and often part of the trunk; these motions propel the shark forward.
Unlike those of bony fish, shark fins generally have broad bases, and are fleshy and relatively inflexible. The tail fin is the driving force, dorsal and anal fins provide stability, and pectoral fins (along with the broad snout) provide lift and diving control, somewhat like airplane wings or the planes of a submarine.
As with any other aspect of shark biology, there are differences in fins among different species. For example, the hammerhead shark’s expanded head provides some lift, so they have smaller pectoral fins. Blue sharks and oceanic whitetips have long pectoral fins for increased lift in the pelagic environment.
Pectoral fins have other uses as well. They can be angled downward to turn or descend quickly. They can be used for display of social signals. But they cannot be used to make a shark swim backwards, like they can in many bony fishes. The small pelvic fins provide some lift, too.
Dorsal fins stabilize the shark, keeping it from rolling to the side and helping it swim in a straight line. The trailing edge of the first dorsal fin may create a low pressure area extending to the tail, increasing the efficiency of the tail’s forward thrust and helping to conserve energy.
Skin covering also plays a role in swimming. Whereas bony fish usually have flat, round, overlapping scales, sharks’ scales (denticles) have a structure similar to teeth. On fast swimming sharks, denticles have sharp peaks and small grooves running from front to back, helping water flow over the body more efficiently. The denticles of some sharks even have cross-ridges and pits (like a golf ball) to further increase swimming efficiency. Like teeth, denticles are continuously shed and replaced throughout a shark’s life.