introduction THINK OF A SHARK When you think of a shark what do you picture Perhaps you see a sleek speedy predator or an insatiable hunter with a frightening mouthful of vicious jagged teeth This book may change those images Animal species evolve to find and catch food to avoid being eaten by predators and to reproduce with their own kind Amazingly these three basic needs have fostered physical and behavioral adaptations that are as varied as the pouches of modern day marsupials the shells of snails and the venom glands of pit vipers Among different shark species we find an amazing array of variations sharks that live in fresh water sharks that emit their own light sharks with heads shaped like hammers or saws or tails shaped like scythes filter feeding sharks that eat tiny plankton sharks that can walk flat sharks that are nearly invisible against the ocean bottom and even sharks that are parasites on the most enormous ocean animals Sharks live all over the world in oceans and even in some bodies of fresh water Some species live in all oceans while others live only in certain locations Tropical species inhabit warm waters and temperate species live in the part of the ocean that’s between the tropics and the cold polar seas There are species that live near the ocean’s surface others live lower in deeper water Some live close to shore and some live far from land WHAT IS A SHARK So what is a shark really First sharks are fish cold blooded vertebrates animals with spinal columns that live in water breathe with gills and swim with fins Sharks are different from the fish we call the bony fishes though Where bony fish as well as mammals birds reptiles and amphibians have skeletons made of bone a shark’s skeleton is made of a tough connective tissue called cartilage Your skeleton is made of bone but you have cartilage in other parts of your body in your joints at the tip of your nose and in your ears for instance A shark’s scales are different than the roundish overlapping scales of a bony fish too The scales of a bony fish grow as the fish grows In fact if you looked closely at a bony fish scale you’d see circular growth marks like the 4
rings on a tree stump On the other hand a shark’s scales are more like tiny teeth In fact they are called dermal denticles Dermal comes from the Greek term for skin and denticle comes from Latin word for tooth Dermal denticles give a rough like sandpapery texture to a shark’s skin As a shark grows it develops more scales Teeth are the tools of the trade for these predators so they have to be sharp and strong Sharks are constantly growing new teeth rows and rows of them form just behind the first row of functional teeth When a shark’s tooth is worn and loose it falls out and a newer sharper tooth replaces it If you could do that you wouldn’t have to worry about cavities or dentist appointments you’d just grow a new tooth PARTS OF A SHARK To identify a shark species you need to know the names of a shark’s body parts Sharks have dorsal fins on top and a tail fin called a caudal fin The large paired fins closest to the shark’s head are its pectoral fins and a second smaller set of paired fins farther back on its body are called pelvic fins Some sharks also have a single fin far back on the underside called an anal fin Humans take in oxygen from the air Sharks take in oxygen from the water Unlike the in and out flow of air through our lungs the flow of water over a shark’s gills is one way only For most sharks water goes in through the mouth for all sharks it goes out through the gill slits just behind the head Most sharks have five pairs of gill slits although some species have six or seven pairs SO MANY SHARKS One thing that’s true of all sharks living and extinct is that we have a lot to learn about them Whether it’s through the discovery of new shark fossils observations of shark behavior or scientific experiments researchers continue to study sharks and their place in the world 5
