Many coral reefs are dying. This one is exploding with life.

Once a year, after dark, a bit of magic happens in the ocean.

In tropical waters worldwide, large chunks of coral — those colorful rocklike structures in shallow, coastal waters, each a colony of living animals — start puffing out hundreds of little pearl-sized balls. Some are pink. Others are red, orange, or yellow. For a few minutes, the ocean is a snow globe, and then the balls float away.

This phenomenon, known as spawning, is how many corals reproduce. Each ball is a bundle of eggs and sperm from an...