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Coral rarely crosses Eastern Pacific Barrier, as Darwin predicted, says study

by Adrian Bishop 28 Aug 2012
Coral rarely crosses Eastern Pacific Barrier, as Darwin predicted, says study

Baums collects samples of Porites corals in the Pacific Ocean. Credit: Joshua Feingold, Nova Southwestern University

Coral rarely crosses the Eastern Pacific Barrier, just as Charles Darwin predicted 130 years ago, say researchers.

The study led by Iliana Baums, of Penn State University, America, is the first to test Darwin's 1880 assertion that that most coral species could not cross the barrier near the US coastline.

The results of the paper being published in the journal Molecular Ecology, has vital implications for climate-change research, conservation and the economy of eastern Pacific, including the Costa Rica, Panama, Ecuador and the Galapagos.

The Eastern Pacific Barrier (EPB) is a 4,000-mile stretch of water up to seven miles deep that separates the central and eastern Pacific.

Darwin claimed the barrier was "impassable" and although scientists have since confirmed many marine species cannot cross the divide, a full analysis has not been carried out on coral.

Iliana Baums says, "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 if coral larvae could cross the barrier, Iliana Baums and colleagues chose the reliable Porites lobata species. "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," she says.

"This species also harbours 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 researchers suggested that coral larvae from the central Pacific might be pushed along in the North Equatorial Counter Current that flows west to east and is made stronger and warmer in years with an El Niño Southern Oscillation event every five years or so.

"Coral larvae are not very mobile, 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 travelled along this current in the past, we should find populations that are genetically similar living from the Galapagos to Costa Rica, Panama, and Ecuador," says Iliana Baums.

The team collected samples of the Porites lobata coral from both sides of the Eastern Pacific Barrier and carried out genetic testing with special microsatellite markers that repeat DNA sequencing and help distinguish individuals.

"We found that Darwin was right: the EPB is a very effective barrier," says Iliana Baums. "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 one exception was a small population of Porites lobata near Clipperton Island, north and west of the Galapagos that were genetically similar to samples in the central Pacific, suggesting it had recently migrated there from the west.

"Interestingly, the coral that are lucky enough to cross the EPB to Clipperton Island stay there and don't go any farther," Iliana Baums explains. "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.

"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 environment to the Central Pacific, but the Galapagos area simply may be too cool."

The results of the study have important implications for the economic stability of the eastern Pacific, conservation and climate change on tropical ecosystems.

The Galapagos, Costa Rica, Panama, and Ecuador all rely heavily on tourism, part of which requires healthy reefs for diving and the sale of shellfish and lobster that are aided by the coral.

"The take-home message is that coral populations in the eastern Pacific need to be protected.

"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 says as the Earth's surface warms, such a crisis in coral reefs is possible.

During the El Niño Southern Oscillation events from 1982-1983 and 1997-1998, some reefs suffered a 90-percent loss. Although they eventually recovered, a stronger El Niño event might cause some coral species to become extinct.

Other researchers who took part in the study included Jennifer Boulay and Nicholas R. Polato from Penn State University and Michael E. Hellberg at the Louisiana State University.

The study was financed by the National Science Foundation.

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