Why Coral Reefs Are Cities Built by Animals the Size of Pinheads

The tropical ocean, in most places, is close to empty. The clear blue water that makes a tropical postcard is clear because it holds so little: few nutrients, little plankton, not much to eat. Marine scientists call these waters oligotrophic, which is a polite way of saying underfed. Yet in the middle of that blue emptiness there are places where life is stacked in layers so thick that a single diver could never count it all. Fish by the thousand, crabs in every crevice, eels, sponges, sea slugs in colors that look invented. In 1971 the ecologist Eugene Odum described this puzzle as "an oasis in a desert ocean." Scientists have been working on it ever since.

The numbers are startling. Coral reefs cover less than one percent of the ocean floor. Yet they are home to roughly a quarter of all marine species. Imagine a town-sized patch of land that somehow housed a quarter of the world's population, and you begin to see why researchers talk about reefs as underwater cities.

Like any city, a reef has architecture, residents, a supply system, and a fragile economy. Also like any city, it was built by workers, and the workers here are animals, many of them no larger than a pinhead. This episode is about how those workers build, who they depend on, and what happens when the arrangement fails. It is a story about one of the most successful partnerships in the living world, and about how quickly a partnership that old can come apart.