Another Blow for the Future of Corals

Certain species of coral release their eggs and sperm with perfect synchronization. But a new study suggests their incredible timing is starting to slip.

Coral release their eggs and sperm, in a wonder of synchrony.
In a wonder of synchrony, certain coral species release their eggs and sperm all at once, and likely have for eons. (Tom Shlesinger)

Tom Shlesinger knew what happens when corals have sex, but neither the books he had read nor the videos he had seen had prepared him for actually witnessing the event.

It happened on a summer night in 2015. Shlesinger had been snorkeling in the Gulf of Eilat for a few frustratingly uneventful hours when he saw a coral release small pink globules—packed with eggs and sperm. As these drifted upward, more globules emerged nearby. Within minutes, as if someone had flicked an unseen switch, thousands of corals had released their reproductive cells, which rose through the water in a kaleidoscopic blizzard. Small fish darted in to feast. Bigger fish arrived to eat the smaller fish. “It was a complete celebration of life,” says Shlesinger, who works at Tel Aviv University. “If I wasn’t floating in the sea, I would have fallen to my knees.”

Corals are tiny animals living within rocky fortresses of their own creation. Being immobile, most reproduce by simultaneously releasing millions of eggs and sperm, in the hope that a few will find one another amid the vast, diluting ocean. Those liberated cells are only good for a few hours, so timing is everything. If the corals are to make a new generation, they must spawn at the same time. They discern the right month by the temperature of the sea and the strength of the sun. They know the right night from the phase of the moon. The setting sun cues the right hour. Pheromones help them coordinate to the minute. The synchrony of this mass orgy is so precise that certain species spawn at the same predictable times every year, and likely have for eons.

Or, at least, they used to. “It’s all regulated by the environment,” Shlesinger says, “and when the environment is going through so much change, you’d expect that these processes will also change in response.”

Sure enough, Shlesinger and his colleague Yossi Loya have found that three common coral species in the Red Sea have lost their rhythm. Their timing is off; their unison is breaking. Rather than releasing a majestic unified blizzard of eggs and sperm at precise moments, they now spawn in pathetic, erratic drizzles across weeks and months. “It doesn’t look promising for those species,” Shlesinger says.

“This study is heartbreaking,” says Shayle Matsuda of the Hawaii Institute of Marine Biology. “This is something we’ve all worried might be true.”

“It’s definitely the case that the probability of fertilization goes way down if they don’t spawn at the same time, and really at the same time,” says Nancy Knowlton, a coral researcher at the Smithsonian National Museum of Natural History. There’s a huge evolutionary pressure for them to be synchronized, “so to not be synchronized is a really big thing.”

Shlesinger first realized something was wrong when he and Loya tried to rear some well-known coral species in large outdoor aquariums. To their surprise, some didn’t spawn at the expected times—an abnormality that Shlesinger initially blamed on his setup. “It took me two or three years to realize that things are completely off,” he says.

Across 225 nights spread over four years, Shlesinger snorkeled through the Gulf of Eilat for hours, recording the number of spawning corals. Several species, he realized, had abandoned their regular year-on-year patterns. It’s not that they had shifted to a different time window. One of them spawned at a full moon in 2015, but at a new moon in 2016. Two others spawned in fits, on almost every night over several months. “They were completely unsynchronized,” he says. “Every night, I just saw a few individuals spawn, and they released just a few drops of material. It’s meant to blast into the ocean, but I just saw dribbles.”

On land, climate change has repeatedly screwed with nature’s schedules, making once-synchronized species out of step with each other. In parts of Alaska, brown bears are abandoning their usual feasts of salmon. Songbirds are struggling to feed their chicks as hatching periods become uncoupled from gluts of insects. Earlier snowmelts mean that snowshoe hares are becoming dangerously conspicuous as their still-white fur contrasts against exposed earth. Shlesinger’s work “supports the idea that natural patterns in the ocean are changing in similar ways to what we’ve observed on land,” says Erika Woolsey of the Stanford Center for Ocean Solutions.

The worst bit about this is that no one really knows why it’s happening. Light pollution from nearby cities could throw the corals off, as could plastics, pesticides, or human-made pollutants that resemble hormones. Then again, Shlesinger found that corals in isolated nature reserves had become just as desynchronized as those growing near urban areas. Whatever’s making them misfire is widespread in its reach, and rising temperatures seem like a likely culprit. “I wouldn’t be surprised if we hear about this phenomenon elsewhere in the next few years,” Shlesinger says. (Woolsey adds that scientists might be able to use crowdsourced data from dive-tour operators, Facebook groups, and apps like iNaturalist to check on coral-spawning patterns around the world.)

But even in the Red Sea, not all corals are affected. Some corals fertilize their eggs internally and release fully formed larvae, instead of spawning en masse. And among the mass-spawners, some are still keeping to their usual schedules. “It’s not that everything is going to hell,” Shlesinger says. But the species that have gone out of sync are already paying the price: For two species, Shlesinger couldn’t find a single juvenile. If that trend continues, those populations seem destined to age, and eventually disappear.

To make matters worse, corals are already beset by a multitude of other threats. Storms pound them, diseases afflict them, and pollution weakens them. Acidifying water makes it harder for them to build their reefs. Warming waters and relentless heat waves force them to expel the algae that provide them with nutrients and color, bleaching and eventually killing them. In some places, these mass-bleaching events are happening too often for the reefs to recover. And if, on top of that, corals can’t produce new generations to replace those that are lost, their already perilous future looks a little bleaker. “The situation with corals is so dire that every little thing makes it harder for them to bounce back between big mortality events,” Knowlton says.

Parts of the Red Sea, however, have been largely unaffected by bleaching events. The corals there are “super corals” that have long since adapted to the high temperatures that reefs elsewhere in the world are now dealing with. That’s a good thing, but Shlesinger’s study shows that apparently thriving reefs could be silently compromised.

“Usually, when we survey a coral reef, we measured things like the percentage of live coral cover, or the diversity of species,” he says. “If I used this approach on the same reef I was working with, I would have said that it’s perfect. They look good, and the species with the breakdown in synchrony are some of the most abundant. But in a decade or two, maybe I won’t be able to tell you the same. And in a few more decades, I might have to tell you that they’re not there anymore.”