Coral Tumbleweeds

Corals by nature are stoic creatures, huddling together in their stony reefs to resist the ocean's currents and turbulence. But scientists now have found that when the sea gets too rough or becomes choked with sediment, some corals don't just sit around--they break away from the mother reef in a kind of escape pod to start a new reef. This newly discovered form of colonization is reported in today's issue of Nature.

Most corals are made up of small, soft-bodied polyps encased in a calcium carbonate shell, or calyx. Each armored polyp fuses with adjacent ones, forming a continuously growing network, the structure of which varies by species. Once a year, the polyps reproduce sexually, producing larvae that drift and swim until they settle somewhere advantageous for a new colony. But in turbulent waters, fragile coral larvae can get buried in muck and perish. "If you have a lot of physical disturbances and sedimentation, [the larvae] have a low chance of surviving," says Yossi Loya of Tel Aviv University.

Now Loya and his colleagues have discovered that at least two corals--Favia favus in the Red Sea and Oculina patagonica in the Mediterranean Sea--have developed a unique way to cope in rough seas. Instead of developing gonads and reproducing, a few polyps, calyx and all, disengage from surrounding ones and raise themselves above the coral surface on a mineral stalk. When a strong current breaks them off, these armored exploring parties roll, like a tumbleweed, to a new home. Their large size and calyx, Loya says, helps the polyps survive a thrashing and form a new colony.

Other corals are known to jettison their fleshy polyps, particularly when under stress, but these are the first species known to "take their skeletons with them," says Paul Sammarco, a coral-reef ecologist at the Louisiana Universities Marine Science Consortium in Chauvin. "It's a good strategy," he adds. Loya says he's confident that this new mode of propagation "will be found anywhere there's disturbance" and in more than these two coral species.