Choose and cross (X) the correct answer.

The following text is for questions 1 to 7.

    The shark, who plays an important role in maintaining ecosystems, is a fish that lives in the sea, particularly in warm waters. Like all fish, sharks breathe through their gills.

    There are about 375 species of sharks. The smallest shark is called the dwarf dogfish, less thari 20 centimetres long. The huge whale shark can be more than 15 metres long. Sharks usually eat fish and shellfish, but great white sharks sometimes eat seals, dolphins, whales, and other marine mammals.

    Sharks are vertebrates, animals with a backbone. However, a shark's skeleton is not made of bone. It is made of a bendable material called carti lage. A shark's teeth are set into its gums. While eating, a shark often loses teeth. But, there are always rows of new teeth growing behind the first set. As a shark loses teeth, new ones move forward to replace them.

7. " .... an important role in maintaining ecosystems ...." (Paragraph 1)

The underlined word means ....