Charles Darwin Was Right - Some Species Can't Make The Trip
August 28, 2012

Eastern Pacific Barrier Is A Difficult Route For Coral Species To Travel

April Flowers for - Your Universe Online

In 1880 Charles Darwin hypothesized that most species could not disperse across the Eastern Pacific Barrier (EPB). A new study, led by Iliana Baums, assistant professor of biology at Penn State University, is the first comprehensive test of that hypothesis using coral.

The study, to be published in Molecular Ecology, found that a coral species in abundance from Indonesia eastward to Fiji, Samoa and the Line Islands rarely crosses the EPB towards the Americas. This has important implications for climate-change research, species preservation efforts and the economic stability of the eastern Pacific region, including the Galapagos, Costa Rica, Panama and Ecuador.

The EPB is a 4,000 mile stretch of uninterrupted water with depths up to 7 miles. It separates the central from the eastern Pacific Ocean.  Darwin considered this barrier "impassible," and since his time scientists have confirmed that many marine species cannot cross this divide.

Until now, however, researchers had not performed a comprehensive analysis of the impact of the barrier on coral species.

"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," Baums said. "But the EPB is a formidable barrier because the time it would take to cross it probably exceeds the life span of a larva."

The team chose a hearty coral species, Porites lobata, to test whether coral larvae are able to travel across the barrier.

"Compared with other coral species, Porites lobata larvae seem able to survive for longer periods of time; for example, the weeks that are required to travel across the marine barrier," Baums said. "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."

The hypothesis is 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 Nino Southern Oscillation Event, which occurs about every five years.

El Nino / La Nina are climate patterns of warming and cooling that occur across the tropical eastern Pacific Ocean. The warming, El Nino, is accompanied by high air surface pressure in the western Pacific while the cooling, La Nina, is coupled with low air surface pressure in the western Pacific. Extremes of this pattern's oscillations can cause extreme weather such as floods and droughts in many regions of the world.

Baums explains that coral larvae are not very mobile, so the only way that larvae originating to the west of the barrier could travel to the east is along an ocean current. The team began looking for coral populations on both sides of the barrier with genetic similarities.

They collected coral samples of the Porites lobata species from both sides of the EPB 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 said. "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."

There was one outlier result. The team found one small population of Porites lobata living near Clipperton Island, which is located just north and west of the Galapagos. The genetic markers of this population were very similar to those found throughout the central Pacific, indicating that the species had migrated there from the west rather recently.

"Interestingly, the coral that are lucky enough to cross the EPB to Clipperton Island stay there and don't go any farther," Baums explained. "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; for example, near the Line Islands, Fiji, and Samoa."

"Coral reefs thrive in shallow water in areas where the annual mean temperature is about 64 degrees Fahrenheit," Baums said. "The eastern Pacific tends to be much cooler; in part, because of a process called upwelling -- a phenomenon that occurs when winds stir up cold, deep ocean water, pulling it to the surface. Clipperton Island may provide a similar-enough environments to the Central Pacific, but the Galapagos area simply may be too cool."

The findings of this study could have important implications for the economic stability of the eastern Pacific, the conservation efforts in the region, and perhaps for the impact of climate change on tropical ecosystems.

The Galapagos, Costa Rica, Panama and Ecuador all rely heavily on tourism, which 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 said. "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."

Such a coral crisis is not that unlikely. During the El Nino events that occurred from 1982 — 83 and 1997 — 98, some reefs experienced a 90 % loss. They were eventually able to bounce back, but a stronger El Nino event, caused by global warming, could spell extinction for some species of coral.

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.

The research was funded by the National Science Foundation.