| {{B}}Sharks Perform a Service for
Earth's Waters{{/B}} It is hard to get people to think of sharks as anything but a deadly enemy. They are thought to {{U}} (51) {{/U}} people frequently. Although these fish perform a valuable {{U}}(52) {{/U}} for earth's waters and for human beings. Yet business and sport fishing are threatening their {{U}}(53) {{/U}}. As a result, some sharks are at risk of disappearing from Earth. Warm weather may influence both fish and shark {{U}}(54) {{/U}}. Many fish swim near coastal areas because of their {{U}}(55) {{/U}} waters. Experts say sharks may follow the fish into the same areas, {{U}} (56) {{/U}} people also swim. In fact,most sharks do not purposely charge at or bite humans. They are thought to mistake a person {{U}}(57) {{/U}} 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. Those are the times when sharks are looking for {{U}}(58) {{/U}} Experts also say that bright colors and shiny jewelry(珠宝)may cause sharks to attack. A shark has an extremely good sense of {{U}}(59) {{/U}} ,with which it can find small amounts of substances in water, such as blood, body liquids and chemicals {{U}} (60) {{/U}} by animals. These powerful senses help sharks find their food. Sharks eat fish,any {{U}}(61) {{/U}} 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) {{U}} (62) {{/U}} against disease. Researchers know that sharks {{U}} (63) {{/U}} 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. They eat injured and diseased fish. Their {{U}} (64) {{/U}} activities mean that the numbers of other fish in ocean waters do not become too {{U}} (65) {{/U}}. This protects the plants and other forms of life that exist in the oceans. |