Read the passage: A coral reef is a large underwater structure made of living and dead corals. These reefs form in tropical seas between 30° north and south of the equator, where they shelter many fish and other animals. Most present-day reefs grew after the last ice age, when rising seas flooded the continental shelves and coral communities built reefs upward as the water rose. Because corals need warm, sunlit water, they grow near the surface, which makes them vulnerable to human activity. Pollution from boats and coastal building clouds the water and cuts sunlight; toxic substances can bleach and kill corals. Many countries now restrict construction near reefs and regulate shipping to protect them. In the passage, the word 'vulnerable' most nearly means