A tale of two corals: genetic relationships of coral reefs in Micronesia

By Medhavi Ambardar, Department of Zoology, Oklahoma State University

When we think of corals, many of us picture the giant colonies that make up reefs. But the giant reefs have small beginnings, all starting with tiny larvae that go on great journeys, swept up in ocean currents, but also swimming a bit. When the larvae travel, they sometimes end up very far from where they started. So what determines where the corals end up? Sarah Davies, a PhD student at the University of Texas at Austin set to find out.  Amazingly, she found that a 50-year-old theory of island biogeography explained why corals from islands near one another were genetically more similar than corals from distant islands. With the rapid decline of coral reefs around the world, it is becoming increasingly necessary to understand the genetic relationships between different coral populations. The reefs are especially important to the marine fishes that use them for habitat and for the humans who rely on the fish for food.

An Acropora coral next to a size and color standard. Photo by Sara Davies. 

The larvae of many coral species are produced through fertilization of sperm and eggs released into the water by mature corals. At that point, the larvae start to travel. Although they can move a little bit, they are primarily transported along ocean currents. The larvae eventually receive an environmental cue that tells them that they are at a reef, and it’s time to settle. “That’s the biggest ‘decision’ of a coral’s life,” Davies said. “It’s like deciding to rent a house for your whole life. That’s it. You’re there forever. You can’t move.” Once the larvae settle, they change into a polyp and start remodeling their houses by secreting a hard calcium carbonate skeleton. They add on more and more rooms and eventually become the giant colonies that we know as coral reefs.

Davies studies the genetic connectivity and population structure of two coral species: Acropora hyacinthus and Acropora digitifera. She does this work in Micronesia and several other islands in the western Pacific Ocean.

Davies works on nine different islands so that she can test island biogeography theory, a fundamental theory in ecology developed in the late 1960’s by Robert MacArthur and E.O. Wilson. The islands she works at—from west to east—are Palau, Ngulu, Yap, Guam, Chuuk, Pohnpei, Kosrae, Kwajalein, and Majuro. The theory predicts that an island that is far from the mainland will receive fewer immigrants than an island that is near the mainland. In the instance of the reefs, the “mainland” would be the Coral Triangle, a large area of corals in western Pacific. This includes Palau, but not the other islands that Davies works on. The immigrants here are the little larvae that move on the currents and eventually make up mature coral colonies. Based on the theory, Davies expected that corals from two distant islands would be genetically more different than corals from neighboring islands because fewer larvae would make it to distant islands.

Davies and a field assistant collected several samples of A. hyacinthus and A. digitifera colonies at each island either by snorkeling or SCUBA diving to the reefs and breaking off small pieces of the corals. They collected 50 samples at each of the 22 reef sites. They then completed the tedious but important task of ensuring that there was no cross-contamination of samples. Because Davies was looking at genetic relationships, a misplaced sample could destroy her results. To avoid cross-contamination, Davies and her field assistant spent countless hours, painstakingly placing each and every coral sample into its own tube. Back in the lab, Davies and a large troupe of undergraduate students under her supervision amplified 12 microsatellite loci, which are short segments of DNA that can show how closely related different populations are from one another.

For the first species, A. hyacinthus, corals from Palau were genetically similar to corals from the neighboring islands of Ngulu and Yap. Likewise, this same species from Chuuk and its neighbor Pohnpei had few genetic differences. These results fall in line with island biogeography; corals from islands that are physically close are genetically close.

Surprisingly, the corals of A. hyacinthus from the island of Kosrae were more similar to those from distant Palau and Yap than they were to corals from the much closer island of Chuuk. These results differed from the other results, and were unexpected. If these corals followed the predictions of island biogeography theory, corals from Kosrae would be more closely related with corals from Chuuk than with corals from Palau and Yap. Davies suspects that some factor other than distance, such as differing ocean currents, might account for this pattern.

For the second species, A. digitifera, Davies found that with increasing distance between islands, genetic differences between the corals increased as well. For example, the corals from Palau and neighboring Ngulu were quite close in terms of genetic distance, while corals from Palau had a much larger genetic distance from those at faraway Chuuk. Again, these results follow island biogeography theory.

An outer reef on Chuuk (and Herbert, the traveling gnome). Photo by Sara Davies. 

Davies was so surprised by the way the genetic data were perfectly mapped based on island distances that she analyzed the data again. “I re-did it, maybe ten times, just to make sure!” she laughed. That’s when it sunk in that, yes, these corals were following the pattern predicted by MacArthur and Wilson nearly 50 years ago. In scientific research, the findings are not always this nice, neat, and linear; sometimes the findings are the exact opposite of what is expected. But not in this case.

Overall, the genetic relationships of the corals followed the predicted pattern: the genetic relationships between coral populations at the different islands were predicted by island distance. But Davies was curious about the case that didn’t quite follow the pattern. It seems that, even though they are very similar ecologically, A. hyacinthus and A. digitifera have different abilities in how they move to different locations. Now the question becomes what causes these different abilities? Davies’ next endeavor is to answer that very question. She will do this by using a computer program to try to predict dispersal patterns. The program will incorporate ocean current patterns and display on the computer screen dispersal connections from island to island. These results will help to paint a more robust picture of how the corals move through the ocean to get to where they settle.

The coral reefs in Micronesia are highly valuable to marine life as well as humans. Many fishes and marine invertebrates make their homes among the reefs. The people on the islands intimately rely on the corals, as much of their diet consists of fish, and they are very mindful of how human activity can impact the reefs. But coral reefs are declining worldwide at an alarming rate and need to be managed. Davies stresses that management of the corals needs to be done at the appropriate scale. Coral larvae can settle on reefs far from where they originated. This means that one reef could be a source of larvae for another reef. If one reef is well-managed, but receives many larvae from a nearby reef that isn’t, the first reef might suffer. But if scientists can detect which reefs are genetically related by examining where the larvae travel on their long trips, they can propose effective management plans for related reefs.

The work that Davies does has an expansive reach. On one hand, it brings us a step closer to utilizing coral reef population genetics for conservation purposes. On the other, it tests the fundamental predictions of island biogeography. In the end, the implications of Davies’ study are almost as expansive as the journeys of the coral larvae themselves.

Medhavi Ambardar completed her MS at the University of Wisconsin-Milwaukee, and is currently a PhD student at Oklahoma State University. She spends much of her time studying (and communing with) eastern bluebirds in order to tease apart the interactions between the testosterone, nest defense, and reproductive success. Apart from research, she enjoys writing, baking, running, rock climbing, and spending time with her dog.