Similarities in the appearance of coral reefs does not mean the reefs will respond to climate change in similar ways, according to a team of researchers that assessed the reproductive state of coral reefs amid rising ocean temperatures.
While examining the lobe coral species Porites lobata in the Eastern Pacific Ocean off the West Coast of Central America, Iliana Baums, an associate professor of biology at Penn State University, realized that a separate, genetically distinct coral known as Porites evermanni was living amid the P. lobata.
"That surprised us," Baums said. "These two lineages look identical and we thought they were all the same coral species, but evermannihas a very different genetic makeup. We knew about P. evermanni -- it's not a new species -- but we didn't expect to find it in the Eastern Pacific, which is a suboptimal environment for coral. Typically you find P. evermanni in the waters of the Hawaiian Islands."
Upon further investigation of the two coral types, Baums and her colleagues found that the coral species respond to their environment in different ways.
A process known as bleaching occurs when the single-celled algae living on coral breaks down as a result of increasing water temperature. The researchers observed that P. evermanni was less susceptible to bleaching than P. lobata in the Eastern Pacific waters.
"If water temperatures continue to rise, and they surely will, coral species that succumb to bleaching more easily will die," Baums said. "So we're going to see a shift in the relative abundance of these two species."
The team went on to find that P. evermanni was capable of cloning itself through asexual reproduction, while P. lobata did not. Furthermore, the cloned P. evermanni, on average, had more tiny mussels living within the coral colony's skeletons. Their presence is marked by the key-shaped holes they leave in the coral.
The researchers then wanted to determine the connection between the P. evermanni's ability to clone itself and the higher numbers of mussels living inside the coral. It turned out that biting triggerfish helped the team reach their conclusions.
"That was the missing piece," Baums said. "We realized that triggerfish were eating mussels inside the coral skeletons, and to get at the mussels the fish have to bite the coral. Then they spit the fragments out, and those fragments land on the ocean floor and grow into new colonies.
"This is what's fascinating," Baums continued. "No one has ever realized how important fish might be in helping corals reproduce, and here we have evidence that triggerfish attacks on P. evermanni result in asexual reproduction -- the coral fragments cloning themselves. Conversely, the other coral lineage, Porites lobata, has fewer mussels and reproduces sexually through its larvae."
Regarding the bleaching effect, the researchers postulated that either different types of algae are growing on the two species, or that one species is simply more tolerant to warmer water.
"The good news in all of this is that some of these corals are true survivors, especially in the Eastern Pacific," Baums said. "It's a rough place for coral to live but they are still hanging around. So if we can figure out how to slow down climate change and keep identifying some hardy corals, we can do something about preserving coral reefs."
Baums and her colleagues' research is scheduled to appear in the Proceedings of the Royal Society B.