Although largely sedentary, corals are extravagant builders. Over millions of years, the shells of these tiny animals can accumulate into sprawling structures like Australia’s 80,000-square-mile Great Barrier Reef. Corals grow and conquer new territory in two ways. Individual polyps- -mature coral animals--divide asexually, producing clones; coral polyps also reproduce sexually, releasing their eggs and sperm into the water. The resulting generation of free-swimming larvae eventually anchor themselves to the seafloor, mature into polyps, and begin a new reef.
Biologists have believed that all corals reproduced in these ways. But marine biologist Esti Kramarsky-Winter at Tel Aviv University recently found corals that use a very different propagation strategy. Shallow water corals in the Red Sea and on the Mediterranean coast of Israel eject fully formed polyps from their midst to pioneer colonies.
While diving in the Gulf of ‘Aqaba in Israel, Kramarsky-Winter came upon some vigorous colonies of the coral Favia favus. She soon discovered that these colonies, in addition to spawning sexually like most corals, grew crops of fully formed polyps on short stems made of calcium carbonate. Somehow the polyps pop off their stems, and the ejected animals, carried by currents, start new colonies as far as ten feet from the parent reef. When Kramarsky-Winter harvested a few polyps and brought them back to the lab, they cloned new colonies within two months. She has since found another coral, Oculina patagonica, that has popping polyps.
Kramarsky-Winter doesn’t yet know how the popping mechanism works, but its advantages are clear. The release of mature polyps, she says, helps coral compensate for the hazards of sexual reproduction. Heavy waves can foil a coral colony’s attempts to spawn, sweeping away immature larvae before they have a chance to settle down and begin cloning a new colony. The polyp emerges fully formed, so it has many fewer stages to go through than a larva, says Kramarsky-Winter.