单选题 Shark Facts
There are almost 400 different kinds of sharks. Each kind of shark looks different, has a unique diet, and behaves differently. There are sharks in all four oceans of the world. Some sharks are longer than a school bus, while others are so small they can live in fish tanks. Sharks come in all kinds of colors. Most of the time, their skin color helps them blend in with their surroundings. But, some'sharks that live in the deepest part of the ocean actually have parts that glow in the dark. Most sharks live in salt water, but some can live in fresh water. All sharks are unique, or have different qualities that make them so special.
Sharks are actually a type of fish. There are some similarities as well as differences between sharks and typical fish. Shark skeletons are made of cartilage. Fish skeletons are made of bones. Cartilage is the bendy, tough substance in people's ears and noses. Like other fish, sharks have gills. Gills are how fish breathe. Unlike fish, people use lungs to get oxygen from the air. Fish get oxygen from the water using gills. Water needs to move over the gills so that sharks and fish get enough oxygen. To keep the water moving, most sharks need to swim in water that has a very strong current.
Sharks have lots of teeth that are arranged in many rows, rather than in just one row like people. The teeth from the outside row gradually fall out, and teeth from the next row take their place. Some sharks will lose 30,000 teeth in a lifetime! Each species of shark has different kinds of teeth. This is because different sharks eat different kinds of food. Some sharks eat food as small as plankton(浮游生物) while other sharks eat animals as big as sea turtles. Most sharks do not eat very often. Some sharks will go weeks between meals. Sharks are at the top of the ocean food chain, which means that no animal is hunting them for food. This position is very important because sharks keep the other animal populations from growing out of control. This process helps ensure there is diversity, or many types, of life in the oceans.