March 30, 2009 -- To switch from male to female and back again without the help of surgery is a feat that only a handful of organisms can accomplish, including some types of fish, shrimp, snails and worms. Now, a new study adds mushroom corals to the list.
It is the first study to show that any coral can change sex in either direction, let alone both.
Understanding why and when some corals make the switch may eventually help scientists protect them from the stresses of a changing environment. For now, the study remains a fascinating window into the biology and evolution of these corals.
"We know in detail the reproductive patterns of more than 500 coral species, but no one reported before on the fact that some coral species may change sex," said lead author Yossi Loya, a zoologist at Tel Aviv University. "I believe this was quite a big surprise to all coral reef scientists."
Mushroom corals belong to a family called Fungiidae. They are solitary, mobile species that live throughout the Pacific and Indian Oceans. Mushroom corals are abundant and diverse, but how they reproduce is something scientists haven't known much about.
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To learn more, Loya and a colleague traveled to a patch reef near Okinawa, Japan. The reef is home to tens of thousands of mushroom corals, representing a dozen species.
In 2004, the researchers collected, weighed, measured, and tagged about 15 individuals from two species. Each coral then got its own aquarium in the lab.
That July, about five days after the full moon, the mushroom corals did what many corals do -- simultaneously release sperm and eggs. In the ocean, these gamete explosions produce larvae that drift off to become new corals. In the lab, the scientists collected the gametes and looked at them under a microscope. Then, they returned the corals to the sea.
Initial analyses showed that each coral produced either sperm or eggs. Some types of corals are hermaphroditic, with both male and female parts. But mushroom corals appeared to be just one or the other.
The researchers repeated the same experiment in 2006 and 2007 -- with both the same individuals and new ones. The results grew increasingly surprising.
In 2006, about 25 percent of one species and 50 percent of the other had changed sex since they'd been tagged two years earlier. In 2007, 80 percent of the corals had changed sex from the year before. A quarter of those had reverted back to the sex they had originally been in 2004. Results appeared in the journal Proceedings of the Royal Society B.
"They went back to their notebooks because they thought they had made a mistake," said Robert van Woesik, a marine biologist at the Florida Institute of Technology in Melbourne. "We never realized in our wildest dreams that these corals can undergo sex changes. This is really exciting."
When mushroom corals are small, it makes more sense to be male, Loya said, because it takes less energy to produce sperm than to produce eggs. When the corals reach some critical size, however, it's better to be female.
Some plants do the same thing, Loya said, making his study interesting form an evolutionary perspective. Corals may look plant-like, but they belong to the Animal Kingdom.
The transition from male to female seems to be a natural progression with growth, van Woesik added. But the fact that the corals sometimes switch back from female to male, might be a sign that they are in distress and need to conserve resources.
The oceans face a lot of stressors these days, from pollution to climate change. If environmental pressures push too many mushroom corals towards maleness, a skewed sex ratio could threaten their future.
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TreeHugger: New Research Indicates Some Corals Are Taking the Heat and Surviving
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