Sharks Perform. a Service for Earth's Waters It is hard to get people to think of sharks as anything but a deadly enemy. They are thought to 【51】______ people frequently. Although these fish perform. a valuable 【52】______ for earth's waters and for human beings, business and sport fishing are threatening their 【53】______. As a result, some sharks are at risk of disappearing from Earth. Warm weather may influence both fish and shark 【54】______. Many fish swim near coastal areas because of their 【55】______ waters. Experts say sharks may follow the fish into the same areas, 【56】______ people also swim. In fact, most sharks do not purposely charge at or bite humans. They are thought to mistake a person 【57】______ a sea animal, such as a seal(海豹) or sea lion. That is why people should not swim in the ocean when the sun goes down or comes up, because those are the times when sharks are looking for 【58】______. Experts also say that bright colors and shiny jewelry(珠宝) may cause sharks to attack. A shark has an extremely good sense of 【59】______, with which it can find small amounts of substances in water, such as blood, body liquids and chemicals 【60】______ by animals. These powerful senses help sharks find their food. Sharks eat fish, any 【61】______ sharks, and plants that live in the ocean. Medical researchers want to learn more about the shark's body defense and immune(免疫的) 【62】______ against disease. Researchers know that sharks 【63】______ quickly from injuries. They study the shark in hopes of finding a way to fight human disease. Sharks are important for the world's oceans, as they eat injured and diseased fish. Their 【64】______ activities mean that the numbers of other fish in ocean waters do not become too 【65】______. This protects the plants and other forms of life that exist in the oceans.