Soft Corals "Melting" Due to Warming Seas, Expert Says

Mati Milstein
for National Geographic News
July 13, 2007
Soft coral communities in tropical waters may literally be melting away because of bleaching events, which have been dramatically accelerated by global warming, a leading expert says.

Unlike their hard coral cousins, soft corals have no stony outer skeletons to leave behind when they die. Instead their fleshy branches extend uncovered from reefs, where they grow alongside hard corals.

This means that soft corals simply vanish in response to environmental stresses.

"I have observed sites before and after bleaching in Okinawa, Japan, and it was remarkable to see a massive disappearance of soft corals," said marine biologist Hudi Benayahu, head of the Porter School of Environmental Studies at Tel Aviv University.

"You can't imagine this was the same site. Just two years passed and the entire area was deserted, lifeless."

Soft corals are common throughout the Indo-Pacific Ocean, where Benayahu focuses his research. But efforts to cataloge the world's soft corals are still underway, and many species remain unknown to scientists.

Benayahu and other experts say that if bleaching continues to intensify, entire soft coral species may go extinct before they are even discovered.

Extreme Bleaching

The causes of coral bleaching are roughly the same for hard and soft corals.

Slight changes in water temperature or salinity can damage the mutually beneficial relationship between coral and the algae—called zooxanthellae—that live in their tissues.

Such stresses cause the algae to "jump ship," and without them the corals eventually die. In hard corals, the loss of living animals inside the exoskeletons causes once colorful reefs to appear stark white.

(See related photos of healthy coral reefs.)

But because the exoskeletons remain, hard corals can sometimes regenerate after a bleaching event.

For soft corals, which are much more sensitive, bleaching usually wipes them out for good.

"Once soft corals disappear, the entire ecosystem is threatened," Benayahu said.

"Many organisms [such as reef fish] are associated with the corals, and once the host disappears, all the associated organisms will disappear as well."

In addition, harmful macro-algae—which compete with corals—can settle on the barren reefs, preventing coral larvae from regrowing the colonies.

Soft corals once covered 50 to 60 percent of some of the sites Benayahu studies. This figure has now dropped to an average of five percent.

"I'm afraid we've already lost knowledge of the real diversity of some sites," he said.

"There is such a huge gap in our knowledge of soft corals …. But it's too late, we have now actually missed the boat."

Ounce of Prevention

Benayahu and other experts agree that bleaching events, such as the one that caused the worst coral die-off on record in 1998, have been intensifying as the world's climate changes.

(Related news: "Global Warming Has Devastating Effect on Coral Reefs, Study Shows" [May 16, 2006].)

Nick Polunin, a marine scientist at the U.K.'s Newcastle University, said continuing damage to coral reefs due to bleaching could affect food webs and cause the localized extinction of fish species.

He also raised the possibility that the loss of just one pivotal species in a given reef environment—a so-called keystone species—could dramatically affect the entire ecosystem.

"When you lose one of those, then you could be on much more difficult ground" in terms of recovery, Polunin said.

Maoz Fine, of the Interuniversity Institute for Marine Sciences at Eilat, Israel, said global warming will subject most reefs to deadly temperature increases by the year 2030.

Preparing reefs now for dramatic climate change should be the most important task, he said.

"If we want to see reefs in the near future, we must remove all other disturbances such as overfishing, increased pressure from tourists, sewage, and so on," he said.

"Many acute disturbances can be prevented, and this will definitely increase the resilience of reefs."

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