What gets coral in the mood?
By Amy Leinbach Marquis
You’ve never seen the ocean like this before. In one of its showiest annual events, dozens of coral species erupt eggs and sperm in a massive, synchronized spawning. “It’s a wild time under water,” says Matt Patterson, a Park Service ecologist who witnessed coral spawning in 1996 while diving in Dry Tortugas National Park in Florida. “The small fish have a feeding frenzy, and the big fish come out to eat the small fish. Suddenly you’re part of the food chain and hoping there isn’t a bigger fish behind you.”
Scientists are still trying to figure out what signals the event—theories point to calm winds, monsoon cycles, spring and fall equinoxes, and a precise dose of sunlight. New evidence shows that Acropora milepora, a reef-building coral found in the Indian and Pacific Oceans, actually has a primitive form of eyesight, thanks to photoreceptors that detect moonlight—the amount of which is another possible trigger.
The discovery could provide clues to coral spawning that occurs oceans away at Biscayne National Park in Florida, every August, several days after a full moon.
The morning after a release, thousands of eggs—some fertilized, some not—sit like Pepto Bismol on the water’s surface. Within a few days, embryos develop into swimming coral larvae in search of the right reef to call home. Patterson likens it to the colorful sheets in Christo’s modern art displays.
But Patterson—who coordinates natural resource monitoring in seven national park units in Florida and the Caribbean—isn’t just there to admire art. There are real applications to studying coral reproduction—most notably, coral rehabilitation via captiverearing programs that help rebuild damaged reefs.
In some cases, scientists capture larvae, nurture them for a few weeks, and then release them back into the wild. Or they might capture the eggs and sperm, rocking them gently in giant coolers until they form larvae that can build colonies in aquariums. But getting the conditions right is no small task. Take Michael Henley, an invertebrate aquarist at the National Zoo in Washington, D.C., who attempted to rear elkhorn coral, a threatened species that ranges from north of Biscayne National Park to South America. Henley placed 12,000 larvae in a 350-litre aquarium with underwater jets, 400-wattt lamps, an advanced filtering system, and plenty of food and algae. A couple months later, however, there were just two surviving larvae, each a millimeter long.
Scientists continue to look to the oceans for clues, where synchronized reproduction is commonplace. But corals are among the most cryptic of marine life. The larvae are microscopic—about 2,000 can fit into a Coke bottle filled with ocean water—and as they disperse in the ocean, it’s nearly impossible to track them. When they settle on a reef, they often nestle into crevices, out of sight. And just 5 to 10 percent survive long enough to grow an exoskeleton.
So when a reproduction event fails to create much new reef growth, it’s hard to know why. Could it be that corals aren’t producing eggs due to environmental stress? Do larvae no longer have a place to settle because of seaweed overgrowth? Are toxins in the water halting larval development?
The search for answers is ongoing at Hawaii’s Kalaupapa National Historical Park, where reef building from reproductive events appears low compared to marine systems nearby. But Kalaupapa’s monitoring program is in its infancy, and trends may not emerge for at least a decade.
Biscayne National Park faces a different set of challenges, where storms and wayward boats can shatter reefs. Normally, broken corals would amount to little more than dead, drifting tumbleweeds on the ocean floor—but by nursing these sprigs back to health in captivity, scientists can grow large enough pieces to restore wild reefs. “The nice thing is, you’re not pulling pieces off a healthy reef to repopulate another,” Patterson says.
But even the best methods can’t replace every reef. “It’s not a solution for mass coral bleaching and large-scale disease outbreaks,” says Wade Cooper, a doctoral student at Florida’s University of Miami. “If the environment is going downhill you can try to replant corals, but they’ll die just like the wild ones did. First you have to get the environment right.”
Environmental changes like global warming and ocean acidification are causing the world’s corals to perish at an alarming rate—a scary trend when you consider that reefs protect shorelines from strong storms, deter beach erosion, and offer countless resources to fishing communities. But research also shows that by easing environmental stressors and boosting marine protections, reefs rebound. And national parks are the perfect place to start.
“Parks are our laboratories, and some of our greatest areas of exploration,” says Jason Bennis, NPCA’s marine policy manager. “They provide a great opportunity for scientists to unravel mysteries that are widespread in the marine world.”