Digestive Acts of Sea Cucumbers May Be Dissolving Coral Reefs

Sea cucumbers secrete acidic compounds that may be weakening the structure of the coral communities in which they live.

The main component of a coral reef is calcium carbonate (CaCO3), a mineral that makes up the corals and much of the sand and rubble around them. Many of the organisms living on the reef either add to or absorb CaCO3 from the surrounding environment. In a healthy reef this would be in balance, but when the calcium carbonate is out of whack, the reef may be unhealthy and could cease to grow.

The researchers studied a part of Australia's Great Barrier Reef known as One Tree Reef. In one specific area, called DK13, they found lots of sea cucumbers. They collected these squishy animals and studied them in the lab.

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Life of a reef

Researchers measure coral reef growth by measuring how much calcium carbonate is in the surrounding waters. If CaCO3 levels drop, it means the reef is growing. The researchers found that the digestive action of the sea cucumbers increases the CaCO3 levels in the waters, and may account for about half of the total nighttime increase in calcium carbonate.

The sea cucumbers survive by scouring the seafloor for food. They suck in sand and rubble and send it through their digestive system, filtering out the edible particles and sending the rest of the rocks and sand on its way. During this process, the sea cucumbers produce acids to dissolve the carbonate-based sand, which produces soluble calcium carbonate minerals that are released into the surrounding waters.

"This CaCO3: You can argue that it is not 'the reef,' as it is not coral and seems not to be reef-building," study researcher Kenneth Schneider, of the Carnegie Institution for Science at Stanford University, told LiveScience. "You can also claim that the sea cucumber recycles these materials to make them available for other organisms ... and I will fully agree with you that sea cucumbers have an ecological role in recycling."

But in times of stress or changing ocean acidification, this process might get sticky.

Times of trouble

"In a case where the ability of reef organisms to precipitate CaCO3 will decrease — say, due to ocean acidification or thermal stress — ... that sand and rubble that the sea cucumbers dissolve may hurt the reef to a point where a reef is eroding and its stability weakens," Schneider said in an email.

"The reef is built mainly by corals and calcifying algae, but between the structure sand and rubble accumulate," he wrote. "This material fills up gaps in the reef structure and creates the sand and rubble on the reef floor at the same time it adds stability to the reef structure."

The calcium carbonate may not be completely a bad thing: It could help buffer this secluded reef from the increasingly acidic ocean — "helping to maintain the overall health of the coral reef," Schneider said. "Although sea cucumbers may play a part in reef dissolution, they are also an important part of an incredible marine environment."

The study was published Dec. 23 in the Journal of Geophysical Research.

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