Coral eggs are rich in waxy fat, which provides energy during development, and buoyancy, helping them float to the ocean surface during spawning. Photo by Heyward & Negri, AIMS
Once a year, shortly after a full moon, many corals undergo a wild and colorful sex spectacle known as broadcast spawning. During this time, coral colonies spawn like a snowstorm, releasing a blizzard of brightly colored bundles containing eggs and sperm into the open ocean.
Just before that spawning occurs, the coral polyps -- individual organisms that make up coral -- turn pink. Soon after, eggs that range in color from light pink to red are ejected from the polyps, and float upward, like tiny helium balloons, to the water surface, where they bob buoyantly, waiting to get fertilized. But unlike nearly every other animal, these eggs have no protective membrane surrounding them, which means the fertilized embryos are extremely fragile, especially during the first 12 hours of development.
In a study published online Thursday in the journal Science, two Australian scientists have found that when exposed to even small waves, many of these embryos will break into genetically identical pieces, each with the ability to develop into its own larvae. The eggs can clone much like human identical twins.
"We found that when the embryos broke apart, the stem cells were able to reassemble or continue to develop individually into very small swimming larvae, and these larvae were able to attach to the ocean surfaces and develop into coral polyps," said Andrew Negri of the Australian Institute of Marine Science, a study co-author. "In humans, the chance of having identical twins might be 1 in 250, but on a windy night, during spawning on a reef, the chance of finding identical twins is much higher."
But let's back up. Every coral colony is made up of these tiny organisms called polyps, which bind together to build the skeletal structure we recognize as coral. Each polyp has a gut and a mouth surrounded by tentacles.
"These polyps do nearly everything you and I do," said Daphne Fautin, professor of biology at the University of Kansas. "They eat, they defecate, they respire, they reproduce sexually. And they do all these things without a heart, without a head, without lungs, without gonads. They're really remarkable animals."
But some three-quarters of the world's coral reefs are at risk, due to overfishing, acidification, ocean pollution and warming, among other factors.
Sea stars can also create larval-like clones, but the scale of the cloning and the link to weather events observed among the corals shows a different process, Negri said. "There's literally billions of these naked embryos at the surface of the ocean, and the interaction with the weather can potentially produce billions of clones."
Negri and co-author Andrew Heyard documented such cloning events among four different families of Pacific corals, all of which produce extremely fragile embryos. After spawning, they collected embryos and poured them down a 30-centimeter water drop, in an attempt to simulate small ocean white caps, or "the most gentle turbulence of the smallest breaking wave," Negri said. Waves of the size simulated, he said, occur on roughly half of spawning nights.
Nearly 50 percent of the embryos they exposed to water waves fragmented, and many went on to reorganize and develop into larvae and eventually juvenile corals, the study showed.
While most corals are "broadcast spawners," they posses the ability to reproduce in a staggering number of ways. Many corals are hermaphrodites, but some are males, releasing only sperm into the water, and some are females, releasing only eggs. Still other coral snatch up sperm from the ocean and fertilize their eggs internally, releasing a developing larvae rather than an unfertilized egg.
Coral can also reproduce asexually by making fragments of themselves -- simply, a piece breaks off of an adult colony, reattaches to a different part of the reef and continues to grow.
"They have a lot of reproductive plasticity," said Abby Renegar, a researcher with Nova Southeastern University's National Coral Reef Institute. "I think this paper is further evidence of that characteristic of corals, and that reproductive plasticity."
Renagar added that she's probably seen the cloning occur under the microscope in her lab, but didn't realize what she was looking at. "This is really, really cool," she said of the study. "It's something that's never been seen before in any animal kingdom."
But still unknown is whether this ability to fragment and clone will benefit the many coral at risk worldwide.
"We think fragmentation must have advantages for corals, but we have no idea whether this susceptibility to fragment is an advantage or a disadvantage in a rapidly changing climate," Negri said.
Fautin said she suspects it will do little to help coral already suffering from bleaching and die-offs. "If the situation that has resulted in the deaths of corals doesn't change, then putting more out there isn't going to change things either, because they're going to suffer the same fate," she said.
But it could add important new information to the research, Negri said. "The more we know about the modes of reproduction and the capabilities and ways [coral] has adapted to survive over hundreds of millions of years, the better chance we have to protect them in the future."
