Coral migration, like that of other species, is halted by the Eastern Pacific Barrier.

A coral species found in abundance from Indonesia eastward to Fiji, Samoa, and the Line Islands rarely crosses the Eastern Pacific Barrier toward the coast of the Americas, according to a team of researchers led by Iliana Baums, assistant professor of biology at Penn State. Charles Darwin hypothesized in 1880 that most species could not disperse across the marine barrier, and Baums’ study is the first comprehensive test of that hypothesis using coral. Her findings have important environmental and economic implications.

The Eastern Pacific Barrier (EPB)—an uninterrupted 4,000-mile stretch of water with depths of up to 7 miles—separates the central from the eastern Pacific Ocean. In his writings, Darwin termed the barrier “impassable,” and later scientists have confirmed that many species of marine animals cannot cross this oceanic divide.

As Baums explains coral dispersion, “The adult colonies reproduce by making small coral larvae that stay in the water column for some time, where currents can take them to far-away places. But the EPB is a formidable barrier because the time it would take to cross it probably exceeds the life span of a larva.”

To test coral’s ability, Baums and her team chose a particularly hearty species called Porites lobata. “Compared with other species, Porites lobata larvae seem able to survive for longer periods of time,” she explains. “This species also harbors symbionts in its larvae that can provide food during the long journey. In addition, the adults seem able to brave more extreme temperatures, as well as more acidic conditions. So, if any coral species is going to make it across, it is this one.”

groupings of genetically diverse coral around the Eastern Pacific barrier
Baums Lab, Penn State University

A map of the Pacific Ocean showing where coral samples were collected. Circles of the same color indicate corals that are genetically similar. Clipperton Island has corals that are genetically similar to those found throughout the central Pacific, suggesting that larvae from the central Pacific traveled eastward to populate Clipperton. Blue arrows indicate cool ocean currents; red arrows indicate warm ocean currents. The inset image shows the study species, Porites lobata.

A map of the Pacific Ocean showing where coral samples were collected. Circles of the same color indicate corals that are genetically similar. Clipperton Island has corals that are genetically similar to those found throughout the central Pacific, suggesting that larvae from the central Pacific traveled eastward to populate Clipperton. Blue arrows indicate cool ocean currents; red arrows indicate warm ocean currents. The inset image shows the study species, Porites lobata.Baums Lab, Penn State University

She and her team hypothesized that coral larvae originating in the central Pacific might be pushed along the North Equatorial Counter Current, which flows from west to east and becomes stronger and warmer in years with an El Niño Southern Oscillation event—a climate pattern that occurs about every five years.

“Coral larvae are not very mobile,” Baums notes. “So the only way coral larvae originating to the west of the barrier could travel to the east is along an ocean current, and warming of a current like the North Equatorial Counter Current would help larvae survive. If coral have traveled along this current in the past, we should find populations that are genetically similar living from the Galapagos to Costa Rica, Panama, and Ecuador.”

The team members collected samples from both sides of the Eastern Pacific Barrier and performed genetic tests using special markers called microsatellites—repeating sequences of DNA that are informative for the purpose of distinguishing among individuals.

“We found that Darwin was right: the EPB is a very effective barrier,” Baums says. “For the most part, samples we found to the east are genetically dissimilar to those we found to the west. This means that coral larvae originating in the central Pacific simply are not making it across the ocean to the Americas.”

The only exception, the team found, was a relatively small population of Porites lobata living near Clipperton Island, which is located just north and west of the Galapagos. The samples collected there were genetically similar to samples found throughout the central Pacific, indicating that the species had migrated there from the west relatively recently.

“Interestingly, the coral that are lucky enough to cross the EPB to Clipperton Island stay there and don’t go any farther,“ Baums notes. “In other words, we find that Porites lobata are not migrating south and east to the Galapagos after making it to Clipperton. We believe this is because these coral are adapted to the warmer conditions that their parents enjoyed to the west of the EPB.”

The team’s findings bear significantly on species-conservation efforts, and on the economic stability of the eastern Pacific. The Galapagos, Costa Rica, Panama and Ecuador all rely heavily on tourism. Tourism, in turn, relies on healthy reefs that divers can visit and the sale of shellfish and lobster—species that are maintained, in large part, by the presence of coral communities.

”The take-home message is that coral populations in the eastern Pacific need to be protected,” Baums says. “That is, in the event of any large-scale coral crisis, we cannot count on coral populations in the eastern Pacific being replenished by larvae from the west.”

Iliana Baums, Ph.D., is assistant professor of biology, baums@psu.edu. Other researchers who contributed to the study include Jennifer Boulay and Nicholas R. Polato at Penn State and Michael E. Hellberg at the Louisiana State University. This research was funded by the National Science Foundation and published in the journal Molecular Ecology.

Last Updated September 19, 2012