There’s an old paradox about coral reefs: They occupy tiny parts of the ocean, and nutrient-poor parts of it at that, and yet they’re home to a third of the world’s fish species. Millions of people depend on them for food. What accounts for their dazzling productivity?

Ever since Charles Darwin first recognized the paradox, scientists have struggled to resolve it. One theory suggests that the sloping topography of reefs captures and concentrates nutrients and microscopic plankton from the surrounding waters. Another focuses on the role of sponges and other invertebrates in recycling dead organic matter into something that can be eaten.

Now, marine scientists from Canada, France, Australia, and the U.S. believe they have found another part of