Sharks have certain anatomical features in common. Sharks belong to the group Chondrichthyes, which are fish with the following characteristics: jaws, paired fins, paired nostrils and a skeleton made of cartilage. Sharks along with most other chondrichthyan fishes, such as stingrays, electric rays, skates, sawfish and guitarfish, are collectively known as elasmobranchs. Elasmobranch means "strap gills," which refers to the five to seven gill slits an elasmobranch has on each side of its head.

CLASSIFICATION

Chondrichthyes are simply fish, except that they have the following characteristics : a cartilage skeleton, jaws, paired fins and nostrils. Chondrichthyes can also be divided into two subclasses, which are namely Holocephaiis and Elasmobranchiis.

SKELETON

Chondrichthyans differ from osteichthyans. Osteichthyans are bony fishes, whose skeletons are heavily calcified. In other words, they contain a lot of calcium. A shark's skeleton is made mostly of cartilage than can be strengthened by deposits of minerals in areas subjected to more stress such as the jaws and vertebrae. Cartilage is an ideal tissue for sharks for several reasons. First, cartilage is lighter than bone, which is important for sharks since they have no swim bladder. Most bony fish utilize a gas-filled swim bladder for buoyancy, so if the sharks have bones instead of cartilage, they will sink easily. Second, cartilage is a relatively flexible material, giving tensile force to swimming and turning movements. Many sharks can virtually turn on a dime, but their skeletal structure and fins are much less elaborately articulated than in bony fish. Bony fish have a more complex bone infrasructure, so naturally, they are more flexible. This combined with the lack of a swim bladder means that the bodies of sharks are on the whole less maneuverable than those of many bony fish. Lastly, cartilage can grow throughout the life of a shark, resulting in their large number of teeth and their humongous size.

SHAPE

Most sharks are characterized by a fusiform (rounded and tapering at both ends) body. Their streamlined, cylinder-shaped body shape reduces drag and requires a minimum energy to swim.

FINS

Sharks are good swimmers; the wide spread of the pectoral fins and the upward curve of the tail fin provide lift, and the sweeping movements of the tail provide drive.

EYES

Although sharks cannot make out details very clearly, their light-sensitive eyes can see well in dim light. The nictitating membrane can be raised to control the amount of light entering the eyes. They may have colour vision.

EARS

Sharks are extremely sensitive to sound. Hair cells in the inner ear convert sound waves to nerve impulses and send them to the brain.

SKIN

The shark's lateral-line system allows it to pick up disturbances in the water, such as the low-frequency vibrations that are given off by a thrashing wounded fish. The system is made up of fluid-filled canals just below the skin. Sensory cells in the canals respond to changes in water currents, creating nerve signals that are sent to the shark's brain.

TEETH

Sharks teeth are arranged in parallel rows. They are constantly replaced throughout their lives. Thin, pointed teeth, found in the mako, are for grabbing and holding prey; serrated, wedge-shaped teeth for cutting are found in the great white shark; and small conical teeth found in the nurse shark are for crushing the shells of crabs and mollusks on which these bottom dwellers feed. A rough formula for calculating the size of a shark, using its teeth, is to measure the length of one side of the tooth in inches, then multiply by ten to calculate the total length of the shark in feet. However, this estimate only applies to large triangular-shaped teeth.

"TONGUE"

All sharks (and "bony" fish) have "tongues", though they aren't much like ours, or any other tetrapod (the "four-limbed" non-fish vertebrates) for that matter. Therefore, shark and fish "tongues" aren't called tongues at all--they are called basihyals. The basihyal is found on the floor of the mouth of sharks and fishes, just as our tongue is placed on the floor of our mouths. Whereas tongues in tetrapods are generally large (or long), flexible and extremely useful, the basihyal is basically just a small, stout, and only vaguely tongue-like piece of cartilage. It is the forward-most part of the basibranchial, a cartilaginous bar-like structure running down the midline of the shark or fish's chest that supports the lower gill-related bones.

REPRODUCTION

For the smaller and more flexible types of fish, the males often coil themselves around the females.Whereas in larger and more rigid species, the male would orient himself parallel and head-to-head with the female. During mating,the males often bite females on the pectoral fins or the middle of the back to hold onto them.This happens in many different species. Because of this, females often bear scars or marks on their fins. Upon examination, these marks show they have been made by the upper jaw teeth of the males. Some sharks lay eggs from which young sharks, called pups, hatch. Other species give birth to live pups. Pups fend for themselves from the first day, growing slowly to adult size over as long as 15 years. In certain species of the elasmobranchs, males have longer, narrower teeth than females. In some female sharks, such as the blue shark ( Scientific name:Prionace glauca), the skin on the back and flanks are more than twice as thick as the skin on the males.

FINDING PREY

Sharks use three primary methods to find their prey. First, they can pick up very slight sounds through vibrations in the water when an object moves. Second, they have sensitive spots on their snouts that look like skin pores, but are really special glands that detect small amounts of electricity given off by animals. Third, they have a powerful sense of smell, so strong that it can pick up one molecule of blood in a million molecules of water. A keen sense of smell helps sharks find food. Through touch, hearing, and sight, they detect movement in the water. A special organ on the snouts of sharks, called the ampullae of Lorenzini, allows them to detect weak electrical voltages from living creatures or ocean currents.