Passage: "Coral reefs cover a tiny fraction of the ocean floor, yet they support about a quarter of all marine species. In the Philippines, reefs provide food and income to millions of people in coastal towns. Rising sea temperatures cause corals to lose the algae that give them colour and food, a process called bleaching. Corals can recover if temperatures return to normal quickly, but repeated bleaching can kill entire reefs." According to the passage, about what share of marine species do coral reefs support?