| Stressed female corals become male|
|A new study has found that mushroom corals can switch from male to female and back again, the first to show that coral can change sex in either direction.|
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," says lead author professor 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.
To learn more, Loya and a colleague travelled 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% of one species and 50% of the other had changed sex since they'd been tagged two years earlier.
In 2007, 80% 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.
The results appear in the journal Proceedings of the Royal Society B .
"They went back to their notebooks because they thought they had made a mistake," says Professor Robert van Woesik, a marine biologist at the Florida Institute of Technology.
"We never realised 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, says Loya, 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.
Loya says some plants do the same thing, making this study interesting from 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 adds.
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.
Ocean organisms face stress from a number of source, including pollution and climate change. If environmental pressures push too many mushroom corals towards maleness, a skewed sex ratio could threaten their future.