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The hidden genetic diversity of corals corresponds to different lifestyles

The stony corals that build reefs hid their diversity from view. A genetic analysis of the most widespread reef coral in the Indo-Pacific revealed that instead of being a single species (Pachyseris speciosa), they were actually four distinct species of coral, according to researchers on April 2 in Current Biology.

Coral reefs are the condominiums of ocean biodiversity, supporting more species per square meter than any other marine habitat. Understanding which coral species foster that biodiversity and how those corals behave is vital to caring for them, especially because a warming climate threatens global ocean biodiversity (SN: 5/6/20). “Just knowing what’s there is key to tracking what we’re missing out on,” says Rebecca Vega-Thurber, a marine microbiologist at Oregon State University in Corvallis, who did not participate in the new study. The results suggest that other corals that are thought to be a single species may be much more diverse than the researchers realized.

Using a combination of diving equipment and remotely operated vehicles, marine biologist Pim Bongaerts of the California Academy of Sciences in San Francisco and colleagues tested more than 1,400 P. speciosa corals from the ocean surface to 80 meters. In the laboratory, the samples looked identical and their internal structures were indistinguishable in scanning electron microscope images. However, their genomes, their complete books of genetic instructions, revealed that corals had diverged millions of years ago. That made sense for one of the species in the Aqaba Gulf of the Red Sea, which was geographically separated from the others. But the other three newly identified species lived together on the same reefs in the waters of South Asia. If the corals lived together, why one didn’t outperform the other two, the team wondered.

Examining habitat data from their dives, the researchers found that the three different coral species favored different depths of water, with one abundant in the upper 10 meters and the other two blooming further down. The three coral species also had different concentrations of algae and photosynthetic pigments, suggesting that they had different strategies for housing their food-providing algae mates. And spawning times for these three species have also been slightly extended. One released most of its gametes five days after the full moon, another seven days later, and the third at nine days and counting. Spawning separation could help each species ’eggs and sperm connect with their correct species.

Marine biologists Pim Bongaerts and Norbert Englebert collect coral samples during a dive on Holmes Reef in the coral sea in northern Australia.David Whillas

This study is the first to show how a set of cryptic reef corals divide their shared ecological space – by depth, physiology, and spawning time, says Bongaerts. "All of these cryptic lineages are occurring, but they have been largely ignored from an ecological point of view."

The results open the door to the possibility that many other doppelgänger corals are multiple species that coexist thanks to ecological differences, says Christian Voolstra, genome of reefs at the University of Konstanz in Germany. "There is a minimal chance that they will choose the unicorn, but I doubt it very much. This article shows that in all probability there is a huge diversity of reef corals with different ecologies."

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