CANBERRA, Australia—Australia’s Great Barrier Reef is spawning in an explosion of color as the World Heritage-listed natural wonder recovers from life-threatening coral bleaching episodes. Scientists on Tuesday night recorded the corals fertilizing billions of offspring by casting sperm and eggs into the Pacific Ocean off the Queensland state coastal city of Cairns. The spawning event lasts for two or three days. The network of 2,500 reefs covering 134,000 square miles suffered significantly from coral bleaching in 2016, 2017, and last year. The bleaching damaged two-thirds of the coral. Gareth Phillips, a marine scientist with Reef Teach, a tourism and educational business, is studying the spawning as part of a project to monitor the reef’s health. “It is gratifying to see the reef give birth,” Phillips said in a statement on Wednesday. “It’s a strong demonstration that its ecological functions are intact and working after being in a recovery phase for …