December 14, 2013
Species Diversity In Coral Reefs Hints At How Coral Responds To Climate Change
redOrbit Staff & Wire Reports - Your Universe Online
It is widely known that coral reefs are being put in jeopardy by rising water temperatures caused by climate change. A team of marine biologists, led by Penn State University, has made a surprising discovery that suggests that very similar looking coral species actually differ in how they survive in harsh environments.
Coral reefs are a vital part of coastal ecologies. They protect shorelines from battering hurricanes and generate millions of dollars in recreational revenue each year. Reefs also provide habitat for a wide variety of seafood, while serving as a discovery ground for new drugs and medicines.
The international team included Jennifer Boulay, a Penn State graduate student; Jorge Cortes, professor at the University of Costa Rica; and Michael Hellberg, associate professor of biological sciences at Louisiana State University. The scientists sampled the lobe coral Porites lobata in the Eastern Pacific Ocean off the West Coast of Central America. They genetically analyzed the samples, revealing differences among various sample locations. The analysis revealed an unexpected pattern suggesting two separate lineages of coral that look deceivingly similar and sometimes live together in the same location.
The genetic data confirmed that not all the samples were Porites lobata. The team found that some of the samples belonged to the species P. evermanni, instead. "That surprised us," Baums said. "These two lineages look identical and we thought they were all the same coral species, but evermanni has 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."
Investigating the two species to see if they differed in the way they live, Boulay found that P. evermanni was less susceptible to bleaching — which occurs when the symbiotic relationship that corals share with single-celled algae breaks down as a result of an increase in water temperature — than P. lobata. "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."
Other important differences were found: P. evermanni had many genetically identical clones, which means that this species is reproducing asexually by breaking apart, although P. lobata did not. Additionally, P. evermanni, on average, plays host to many more tiny mussels that live within the coral colonies' skeletons. The mussels form keyhole shaped holes by poking through the surface of the colonies.
The team investigated P. evermanni's ability to clonally reproduce and its interactions with the mussels and other members of the reef community in the Eastern Pacific. This line of inquiry was based on Cortes' memory of a colleague reporting that some corals are a target of biting triggerfish. "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 continues. "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, has fewer mussels and reproduces sexually through its larvae."
Baums explains that the benefit of asexual reproduction is that corals living in a harsh environment such as the Eastern Pacific might have a hard time finding partners for sexual reproduction. "It takes two to tango so you need a partner," she said. "In areas of the Eastern Pacific that are so harsh that only a few individuals can survive, it might be easier for the coral to clone itself, ensuring that the offspring can survive as well."
Two possible explanations exist for the differences in bleaching. First, the types of algae living in the coral species is different, and one of them can withstand a hotter temperature. "Just like in your garden -- the tomatoes like the heat more than the cauliflower does," said Baums.
Second, the difference is not in the algae but in the corals themselves. "In the literature there's been a lot of attention paid to how different algal species react to increases in temperature and whether, if a coral species could switch to a hardier alga, it could survive hotter temperatures," Baums said.
The researchers found evidence, however, that suggests a different scenario. Bleaching still differs even though the two coral species have the same algal species, suggesting it's the coral host that contributes to bleaching.
"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."
Their findings have been published in Proceedings of the Royal Society B.