Sea Cucumber Poop Could Revitalize Coral Reefs

In one reef, three million sea cucumbers released 64,000 metric tons of nutrient-packed poo back into the ecosystem

The black sea cucumber Holothuria atra is found in shallow waters along reefs and uses sand to coat itself for camouflage and protection from the sun. (Philippe Bourjon, CC BY-SA 3.0, via Wikimedia Commons)
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Sea cucumbers (Holothuroidea) are strange, cylindrical marine invertebrates closely related to sea urchins and starfish. They have no eyes, resemble a colossal chubby worm, and use their anus for both breathing and defecating. They spend their lives scooting around the seafloor, scouring sediment for food, and excreting it back into the ecosystem. With 1,250 unique species of sea cucumbers in the ocean, all of that excess excrement adds up, and seems to play a significant role in coral reef ecosystems' health, according to a new study published last week in Coral Reefs.

Using drones and satellite images, researchers at Macquarie University, the University of Newcastle, and James Cook University found that sea cucumbers can amass over 64,000 metric tons of poop in a single year across one coral reef, according to a University of Newcastle Australia press release.

Like earthworms, sea cucumbers aerate ocean sediments by sucking up sediment like a vacuum, consuming some micro-organisms and then spitting the dirt back out, according to the University of Newcastle Australia. During this process, sea cucumbers also release other beneficial derivatives like nitrogen, ammonia, and calcium carbonate. Calcium carbonate is an essential component for coral formation, as it helps coral skeletons grow, reports Live Science.

Researchers analyzed drone footage to count sea cucumbers located at the southern end of the Great Barrier Reef on the 7.3 square-mile Heron Island reef in Australia. In total, three million sea cucumbers live within the shallow areas of the reef reports, Nicoletta Lanese for Live Science. Before scientists used drones to tally the number of sea cucumbers residing in a specific area, researchers would manually count them from a boat or by snorkeling, reports Rafqa Touma for the Guardian.

Then, study co-author Vincent Raoult, a reef ecologist at the University of Newcastle in New South Wales, and his team ran feeding experiments on the most abundantly found cucumber on the reef, the black sea cucumber (Holothuria atra) to see how much poop a sea cucumber produces in a day, reports Live Science. Each sea cucumber defecated 1.3 ounces of poop daily, or 30.8 pounds of poop in a year, reports Live Science.

"Every hour we would sit there, and take a spoon, and collect the little [fecal] pellets these sea cucumbers would produce," Raoult tells the Guardian.

In total, the amount dung excreted by the three million sea cucumbers found in the Heron Island Reef is approximately equal to the mass of five Eiffel Towers, reports the Guardian. While this amount of poop seems enormous, sea cucumbers and their excrement are vital for healthy coral reefs.

Despite their critical roles in reef ecosystems, the warty worms face threats of extinction from overfishing. The sea cucumber is considered a delicacy in some cultures and is valued at $80 per kilo, reports the Guardian. Researchers hope that their study can be expanded to count and manage global sea cucumber populations.

"It's very hard though for scientists to have a sense of what the loss of a species might be if we don't know the scale of their role in the ecosystem," says study co-auther and Macquarie University holothurian expert Jane Williamson in a statement. "We're advocating that the functional role of sea cucumbers on coral reefs is likely to be much more substantial than previously thought, and therefore greater attention needs to be directed to their management and ecology, particular when it comes to overharvesting of reefs that are already compromised."

About Elizabeth Gamillo
Elizabeth Gamillo

Elizabeth Gamillo is a science journalist based in Milwaukee, Wisconsin. She has written for Science magazine as their 2018 AAAS Diverse Voices in Science Journalism Intern.

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