The way pulsating corals open and close their soft, feathery tentacles in a rhythmic grabbing motion is what earned them their name. First described nearly 200 years ago by Jean-Baptiste Lamarck—the naturalist most noted today for his ideas on evolution—these soft corals are very common in the coral reefs of the Red Sea. However, it was long uncertain why they spent energy vigorously pulsating. Now researchers report in the Proceedings of the National Academy of Sciences that these motions could help greatly help their symbionts with photosynthesis.
All these corals, known as the Xeniidae, are hosts to photosynthetic algae that provide essential nutrients and live off the waste of the corals. Marine ecologist Maya Kremien at the Hebrew University of Jerusalem and her colleagues reasoned these pulsations would mix up the water around the corals in ways that might help their symbionts. The algae generate oxygen that, in large enough concentrations, might stymie the chemical reactions of photosynthesis, and pulsating tentacles might act like a fan to clear away oxygen-rich water.
The scientists analyzed the coral Heteroxenia fuscescens from the coral reef of Eilat in the Red Sea. “In some places these corals form extensive pulsating carpets on the bottom of the reef,” Kremien said.
After watching colonies of the corals with an underwater infrared-sensitive camera night and day for about two weeks, the researchers found these corals pulsated more than 95 percent of the time, resting only for short intervals, usually in the late afternoon, when the sunlight was less than half as intense as it was at its strongest. In lab experiments, “I was really astonished by the fact that photosynthesis by the coral’s symbiotic algae was 10 times higher during pulsation compared with rest,” Kremien said. “Pulsation modulates the flow and creates perfect conditions for high photosynthesis rates.
Kremien and her colleagues also shone lasers at the reef as part of a system that continuously scanned how water flowed around pulsating corals. The lasers illuminated natural particles surrounding the corals, revealing how they whirled to and fro as the corals pulsated. Water flowed away from the corals about twice as fast when they pulsated than elsewhere in the reef, significantly reducing the chance each coral polyp would suck in water it or another polyp spewed out.
All in all, the ratio of oxygen-generating photosynthesis compared to oxygen-guzzling respiration in the pulsating coral greatly surpassed those of nonpulsating soft and stony corals. In other words, pulsation apparently yields great energy benefits for these corals, and could help explain the enigmatic scarcity of food seen in their digestive cavities.
“It was very exciting to connect the pieces of the puzzle and realize that we understand what’s going on here,” Kremien said.
As to why the corals also pulsate at night when the algae do not photosynthesize, the researchers speculate the pulsations might help keep each polyp from taking in waste-laden water from its neighbors as well as improve their chances of snagging dissolved nutrients.