Although there are many kinds of sharks, they all are similar anatomically. A shark’s digestive system begins at the mouth, which is filled with teeth. Shark teeth are continuously produced, and at any time a shark may have 3,000 teeth arranged in six to 20 rows. As older teeth are lost from the front rows, younger ones move forward and replace them. Teeth are adapted to specific kinds of food. Depending on their species, sharks may have thin, daggerlike teeth for holding prey; serrated, wedge-shaped teeth for cutting and tearing; or small, conical teeth that can crush animals in shells.
The internal skeletons of sharks are made of cartilage, a lightweight and flexible bonelike material. Their external surfaces are very tough and rugged. Sharks have extremely flexible skin that is covered with placoid scales, each of which is pointed and has a rough edge on it. Shark fins are rigid and cannot be folded down like the fins of bony fish.
Like other aquatic organisms, sharks get the oxygen they need to live from the water. Compared to air, water contains a small percentage of dissolved oxygen. Surface waters may contain five milliliters of oxygen per liter of water, dramatically less than the 210 ml of oxygen per liter of air that is available to land animals. To survive, fish must be very efficient at removing and concentrating the oxygen in water.
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| Shark Anatomy (image source: seaworld.org) |
In aquatic organisms, gills carry out the function of lungs in terrestrial animals. To respire, sharks pull water in through their mouths and spiracles, holes on top of their heads. The water passes over their gills and exits through the gill slits on the sides of the head. Most species of sharks can pump water over their gills by opening and closing their mouths. Some sharks, the “ram ventilators,” must swim continuously to move water over their gills. Oxygen in water is picked up by tiny blood vessels in the gills, then carried to the heart, a small two-chambered, S-shaped tube. From there, oxygenated blood is pumped to the rest of the body.
Sharks fertilize their eggs internally. Males transfer sperm to females using modified pelvic fins. Some species are oviparous, which means the female lays fertilized eggs. Shark eggs may be deposited in lagoons or shallow reef water, where they incubate for six to 15 months. Many of the eggs’ cases are equipped with hairy or leathery tendrils that help hold them to rocks or plants. Other species are viviparous, so the embryos develop inside the mother and are born alive. Several species are ovoviviparous, which means that the embryo develops inside an egg within the female’s body. The egg hatches inside the mother, the hatchling eats the yolk and any unfertilized eggs, then is born alive.
Shark populations are relatively small compared to other kinds of fish. One reason is because shark reproduction rates are low. Unlike fish and many of the invertebrates, a female shark produces only a few offspring each year. In addition, the gestation period, time when the embryo develops inside the mother, of viviparous species is long.