Baums collects samples of Porites corals in the Pacific Ocean. Credit: Joshua Feingold, Nova Southwestern University |
A
coral species that is 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,
an assistant professor of biology at Penn State University. Darwin
hypothesized in 1880 that most species could not disperse across the
marine barrier, and Baums’s study is the first comprehensive test of
that hypothesis using coral. The results of the scientific paper, which
will be published in the journal Molecular Ecology,
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
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 had termed this barrier
“impassable” and, since Darwin’s time, scientists have confirmed that
many species of marine animals cannot cross this oceanic divide.
However, until now, 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.”
To
test whether or not coral larvae are able to travel across the barrier,
Baums and her team chose a particularly hearty species called Porites lobata. “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 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 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.”
The team members collected coral samples of the Porites lobata
species 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 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.”
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 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
F,” 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.”
The
team’s findings about the ability of coral to travel across the marine
barrier have important implications for the economic stability of the
eastern Pacific, the region’s species-conservation efforts and, more
broadly, for the impact of climate change on tropical ecosystems. 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 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% loss. Although they ultimately were able to
bounce back, a stronger El Niño event might spell extinction for some
coral species.
In addition to Baums, 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.
Source: Penn State University