Today there are at least 370 species of sharks worldwide. Like all fish, sharks are vertebrates, but ichthyologists place them in a separate class from most bony fish because the shark’s skeleton is made of cartilage instead of bone. Sharks also have five to seven gill slits on each side of the head, allowing each gill to vent separately into the surrounding water. Bony fish, in contrast, have on each side of their bodies one gill opening that is covered by a bony plate called the operculum.
Sharks also lack the gas-filled swim bladders of most bony fish. Instead, sharks have evolved a different means of maintaining buoyancy: They have extremely large livers that contain oils that are lighter than water. These oils, coupled with the cartilaginous skeleton, make sharks almost neutrally buoyant.
Swimming abilityNot all sharks must swim constantly to force water over their gills for respiration. Some can actively pump water over their gills and occasionally rest motionless on the bottom. Many bottom-dwelling sharks pump water over their gills most of the time.
Sharks must literally swim or sink, however, because their bodies are slightly denser than water, and they require forward motion to stay afloat. Sharks have a number of physical adaptations that make them exceptionally efficient swimmers.
Sensory abilityAs sharks swim, they constantly sample the water for odors and sounds. They can detect odors at a few parts per million and are attracted by low-frequency vibrations. Feeding is dependent on vision and the detection of electrical fields.
Their well-developed visual system functions well in high and low light; a special structure in the eye called the tapetum lucidum increases their sensitivity in low light. At close range, the shark’s electroreception system comes into play. Receptors located in pores on the shark’s snout and lower jaw can detect tiny electrical fields created by the prey’s muscular movement.
Feeding and digestionA shark swallows its food whole or in chunks. Once the shark is satiated, it may not eat again for several weeks. As the food is digested, it passes through the intestine, which has a spiral valve structure unique to sharks. This valve increases the interior surface area of the intestine for more efficient absorption of nutrients. Sharks are opportunistic feeders and will often eat whatever is available.
TeethShark teeth come in as many shapes and sizes as sharks do. They also say something about the shark’s diet. Some sharks are specialized predators; their teeth are adapted for efficient capture of their preferred prey. Others eat whatever is available, and their teeth are amply suited for many types of food.
The great white uses its triangular, serrated, bladelike teeth for grabbing and biting off chunks of large fish and marine mammals. At the other end of the spectrum, the smooth dogfish uses its flat teeth for crushing the shells of mollusks and crustaceans. Others, like the mako or the sand tiger, have narrow, pointed teeth for impaling and holding onto prey small enough to swallow whole.
SkinSharks have placoid scales, or denticles, which are tiny, bony projections implanted in the shark’s skin. They come in many shapes and sizes but usually completely cover the shark’s skin like a coat of armor. Their main functions are protection and reducing drag as the shark’s body slices through the water. Denticles give the shark’s hide a rough texture like sandpaper.
ReproductionSharks have a number of reproductive strategies. Some enclose fertilized eggs in tough, leathery egg cases that are released into the water for subsequent development and birth. Some females retain eggs within their bodies and hatch the young internally, so they are born alive and fully formed.
Others have a sophisticated placental arrangement similar to that of mammals. Many sharks take 10 to 20 years to mature sexually, and they produce as few as one pup at a time. A number of species are estimated to live for 40 or 50 years.