Sharks are ocean fishes with skeletons made of cartilage. They are found worldwide. There are 440 species of shark, varying drastically in size and shape. Sharks have evolved to possess anatomical features that suit their hunting lifestyle.
Fins
Fins provide stability, steering, lift, and propulsion. Different types of fins serve different purposes. Dorsal fins are found on the sides of the shark. They provide stability, keeping the animal from rolling from side to side. Pectoral fins project upward behind the head, aiding in steering and lift. The caudal fin is composed of an upper and lower lobe. The caudal fin makes up what would commonly be considered the tail region. It propels the shark through the water.
Skeleton
Shark skeletons are composed entirely of cartilage. Cartilage is lighter and more buoyant than bone, yet retains the strength a shark requires. It allows the shark to move with great speed and flexibility in its underwater environment. Shark backbones and jaws contain calcified cartilage, which is bone-like, and provides additional strength in these important areas.
Eyes
The eyes of a shark contain a special layer called the tapetum lucidum. This layer allows sharks to see well in low-light situations. Sharks also possess a nictitating membrane. The nictitating membrane covers the shark’s eye when it passes closely to another object. It also engages when a shark is attacking prey.
Gills
Sharks posses five to seven sets of gills on the sides of their heads. Oxygenated water passes through the shark’s mouth, into the pharynx and then into the gills. Gas exchange occurs in the gills, providing the shark with oxygen. Carbon dioxide is expelled into water, which passes out through the gill slits.
Teeth
The appearance, shape and quantity of shark teeth vary widely by species. They are not anchored in the jawbone, but attached to the tooth bed, a structure that continually grows outward. Shark teeth grow in rows. As the tooth bed moves outward, newer, sharper teeth replace older teeth.
Apullae of Lorenzini
The appulae of Lorenzini are small pores found on the shark’s head. They are visible to the naked eye, and part of a large sensory network that resides beneath the shark’s skin. Apullae of Lorenzini allow sharks to detect magnetic fields of other fish, helping them detect well-hidden prey. Research appears to indicate that they also improve detection of slight shifts in water temperature.