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Sex in the reef: How to tell coral species apart


For a few nights each summer—when the water temperature, tidal and lunar cycles are just right—hundreds of coral species in the Caribbean spawn at the same time. This maritime orgy holds the secret to understanding how coral species continue to evolve and maintain their genetic diversity.

Caribbean Acropora species A. cervicornis (left) and A. palmata (right).

Corals reproduce both sexually and asexually, but they also interbreed to some extent and hybrids result. In a new study, scientists at Harvard University in Cambridge, Massachusetts, used novel molecular techniques to determine how genetically distinct three Acropora species really are. The method could help conservationists identify and protect different species from extinction.

Marine biologists Steven V. Vollmer and Stephen R. Palumbi analyzed DNA from the staghorn coral Acropora cervicornis, the elkhorn coral Acropora palmata, and Acropora prolifera, sometimes called fused staghorn. By comparing DNA sequences from two genes and mitochondrial DNA, they determined that one of the three corals—A. prolifera—is a hybrid.

With one copy of each gene from each of the other two species, A. prolifera is a first-generation hybrid and does not represent a distinct species. These hybrids are what the Harvard team calls sterile mules. "They have little genetic impact on either parent species," the researchers write in Science.

Scientists have different views about what makes a coral a distinct species, and whether hybrids are also able to produce sexually and eventually evolve into a distinct species. Vollmer and Palumbi observe that the potential for hybrid A. prolifera to reproduce sexually is very low and that "hybrid breakdown occurs in later generations."

Acropora prolifera from Puerto Rico are first-generation descendents of A. palmata and A. cervicornis. The hybrids can be bushy or palmate depending on which species provided the egg and mitochondrion.

Reef-building corals may live for several decades or centuries, so these "immortal" hybrid lines accumulate over time. They vary in appearance, depending on which species provided its egg and mitochondrion to the hybridization event. This is why corals seem to diversify without actually increasing the number of species on the reef.

Coral spawning—a natural phenomenon when multiple coral species release their gametes at the same moment—is triggered by specific environmental conditions including high water temperatures, the tidal and the lunar phase. Often, other critters join the action and spawn as well. Branching corals like elkhorn and staghorn tend to spawn for about one hour between the fourth and sixth night after the full moon.

Little pink colored balls consisting of both sperm and eggs—held together by mucus—detach from the corals and float towards the surface where they fall apart because of the water movement. When the sperm and eggs are released they meet with either sperm or eggs from other colonies and fertilization takes place. The resulting larvae drift in the ocean currents generally for three to four weeks before settling on an open area of hard bottom or bare substrate. They will never move again.

Coral spawning has become a tourist attraction for divers in reefs all over the Caribbean.

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Vollmer, S.V. & Palumbi, S.R. Hybridization and the evolution of reef coral diversity. Science 296, 2023-2025 (June 14, 2002).

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