28 August 2012, 09:39 BST
Charles Darwin was right, again, but this time his predictions focused on a somewhat different genre than we normally associate with the man; this time, it's the ocean.
Researchers have found that Darwin may in fact have been right on the money when he labelled the Eastern Pacific Barrier (EPB) "impassable" in the 1880s.
The researchers tested one of the hardiest species of coral found in abundance around Indonesia, eastward to Fiji, Samoa, and the Line Islands, to see whether it could make the jump across the EPB. The results of the study has important implications for species-preservation efforts, the economic stability of the eastern Pacific region, and an impact on climate-change research.
Lead author Iliana Baums collects samples of Porites corals in the Pacific Ocean.
The EPB is an uninterrupted 4,000 mile stretch of water that separates the central from the eastern Pacific Ocean and can dive down to depths of up to 7 miles. Darwin wrote in 1880 that this barrier was "impassable", but this is the first time that researchers have comprehensively tested this theory by using coral.
"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," said Iliana Baums, an assistant professor of biology at Penn State University. "But the EPB is a formidable barrier because the time it would take to cross it probably exceeds the life span of a larva."
An adult lobe coral (Porites lobata) colony provides shelter for a myriad of fishes and invertebrates.
The researchers chose a particularly hearty species of coral called Porites lobata to test this theory.
"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."
Baums and her team made the assumption that coral larvae originating in the central Pacific could feasibly be pushed along the North Equatorial Counter Current, which flows from west to east and that becomes warmer and stronger in years with an El Niño Southern Oscillation event.
"Coral larvae are not very mobile," Baums said. "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."
Adult lobe coral (Porites lobata) colonies can grow to be several hundred years old, providing habitat to small reef dwellers.
The team collected samples of the coral Porites lobata from both sides of the Eastern Pacific Barrier and performed genetic tests.
"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 exception, a relatively small population of Porites lobata that were living near Clipperton Island, located just north-west of the Galapagos Islands. The samples collected at Clipperton were genetically similar to samples found throughout the central Pacific Ocean, indicating that the species had migrated to Clipperton 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."
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.
"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 environment to the Central Pacific, but the Galapagos area simply may be too cool."
As mentioned, these findings have wide ranging implications, including the economic stability of the region. The Galapagos Islands, Costa Rica, Panama, and Ecuador, are all heavy tourist destinations with a thriving coral reef diving business, and the sale of shellfish and lobster, species that acquire a lot of help from healthy 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."
Baums explained that, especially as the Earth's surface continues to warm, such a crisis to coral reefs is not unlikely. During the El Niño Southern Oscillation events that occurred from 1982 to 1983 and from 1997 to 1998, some of the reefs experienced a 90-percent loss. Although they ultimately were able to bounce back, a stronger El Niño event might spell extinction for some coral species.
Source: Penn State University
Image Source: Baums lab, Penn State University & Joshua Feingold, Nova Southwestern University
Source: Planet Save