Coral reefs discovered off the coast of Japan's Tsushima Island are the northernmost coral reef ever found on Earth. The discovery was announced in July 2012.
Credit: Kaoru Sugihara
When you think of coral reefs, you probably picture scuba divers gliding through warm, crystal-clear waters. And for the most part, you'd be right: more than 90 percent of the world's coral reefs are located in the tropics.
Now researchers in Japan have found what is — so far — the world's northernmost coral reef. Located off the coast of Tsushima Island at 34 degrees north latitude, the newly discovered reef is far different from other coral reefs and is 217 miles (350 kilometers) north of most others in the region.
"Coral reefs have been believed to develop under warm-water settings — at least 18 degrees Celsius [64 degrees Fahrenheit] in winter. This setting is 13 degrees Celsius in winter [55 degrees Fahrenheit], which is unbelievably low," Hiroya Yamano, a researcher at Japan's National Institute for Environmental Studies, told OurAmazingPlanet. "The species, and thus seascape, is completely different from normal reefs."
A team led by Yamano found a similar reef off the coast of Japan's Iki Island in 2001, and until now, that reef was the planet's northernmost. The newfound Tsushima Island reef is 43 miles (70 km) north of the Iki Island reef. [Images: Dazzling World of Coral, Unveiled]
Yamano's team tracked down this reef after poring over old manuscripts and interviewing local residents. Their sleuthing paid off, and they eventually found the 4,300-year old reef in one of Tsushima's murky inner bays.
"The water in this bay is basically turbid" — cloudy with lots of suspended particles — "and water temperatures are low, especially in winter," Yamano said.
Both of those qualities make life difficult for most corals. But this reef is composed of a genus of corals called Favia, a massive brown coral type. Most reefs are made up of corals from the genus Acropora, which can be a variety of colors and grow in branching or platelike polyps. Favia species tend to tolerate cold, turbid waters better than Acropora do, Yamano said.
So why did this reef start building itself in such an unfriendly environment? The team isn't sure, but Yamano thinks the Tsushima Warm Current, a stream of warm water flowing along the northwestern coast of Japan, probably helped transport coral larvae northward into the turbid inner bays of Iki and Tsushima islands. Yamano thinks there may be many more undiscovered reefs in similar settings throughout the region.
Changing species, changing climate?
Reefs like this one might help researchers measure ecosystem changes in warming oceans.
Although the Tsushima and Iki reefs both formed in very cold waters and predominantly house Favia coral species, Acropora corals have begun settling near the reefs over the last 20 years. Comparing coral species in older parts of the reef to newly arrived corals might help scientists determine how climate change and warming waters are affecting these reef ecosystems, Yamano said.
His team's findings were published online July 12 in the journal Geology.