How can a reef teem with life in water that is almost empty?

A reef teems with life because it does not waste anything. Tropical surface water is very poor in nutrients, so the reef survives by recycling them, passing nitrogen and other building blocks around its community many times instead of letting them drift away. Corals, algae, sponges and animals all catch, trade and reuse what little arrives.

The puzzle is real. Coral reefs cover less than 0.1 percent of the world's ocean, about half the area of France, yet they are home to at least a quarter of all marine species. Charles Darwin described reefs in a book published in 1842, noting that they were found in some tropical areas but not others. The puzzle that is sometimes named after him asks how such a rich community can live in water so low in nutrients. It is called Darwin's paradox, and a reef has been compared to an oasis in the desert.

Why is the water so poor? The sun heats the tropical surface all year, and a stable layer of warm water floats on cooler water below. When living things die they sink and rot in the deep, releasing nitrogen and phosphorus, but in the tropics those nutrients do not return directly to the surface. Microscopic plants at the surface use up what is there, and little is replaced.

The reef answers with teamwork. Corals catch plankton at night and share nitrogen with their algae. Sponges filter drifting plankton and excrete nutrients that corals can use. Algae and seaweed on the reef pass nutrients to the corals. Rough coral surfaces help too, because waves break against them and let corals reach passing nutrients. And the structure itself, full of cracks and crevices, gives many species a home.