Sea Cucumbers: The Excremental Heroes of Coral Reef Ecosystems

Historically, one of the major problems scientists have faced when trying to assess the importance of sea cucumbers (and their excrement) in the reef ecosystem is the difficulty in assessing just how many sea cucumbers there are in a given area, said Jane Williamson, the study's lead author and head of the Marine Ecology Group at Macquarie University.

Previous research used footage from boats or information collected by divers to estimate sea cucumber numbers, said Williamson. But boats stir up the water, making it difficult to see the animals, and divers can collect information over only relatively small areas, resulting in a high degree of uncertainty when their observations were used to extrapolate the population of the entire reef.

So Williamson and her team, which included coral reef geomorphologist Stephanie Duce, remote sensing expert Karen Joyce, and marine ecologist Vincent Raoult, wanted to try a different method. Using images captured by drones, the team surveyed sea cucumbers over tens of thousands of square meters in two different geomorphic zones (the inner and the outer reef flats). Researchers then used satellite imagery to determine the area of each of these geomorphic zones to extrapolate the number of sea cucumbers present on the entire reef. These methods indicated that there were more than 3 million sea cucumbers on the reef flats surrounding Heron Island Reef.

The team also collected dozens of individual sea cucumbers to observe their bioturbation rates—that is, how much each sea cucumber pooped in a given day. On average, each sea cucumber produced about 38 grams of poop in 24 hours. Using this information, along with their estimates of the reef's sea cucumber population, researchers determined that on a single reef, sea cucumbers produced more than 64,000 metric tons of poop per year—more than the weight of five Eiffel Towers.

By measuring how much individual sea cucumbers pooped per day and estimating the number of sea cucumbers on the reef using drones and satellite images, researchers determined how much poop sea cucumbers contributed to the Heron Island Reef. Credit: Associate Professor Jane Williamson et al., 2021, Macquarie University; Dr. Stephanie Duce, James Cook University; Dr. Karen Joyce, James Cook University; and Dr. Vincent Raoult, University of Newcastle. https://doi.org/10.1007/s00338-021-02057-2

The Importance of Excrement

Scientists think that all of that poop plays an important role in ecosystem health as well as in the biogeochemical cycles of the reef.

"Sea cucumbers can be considered like a long sausage, almost," said Williamson. "Sediment goes in and sediment comes out.… By eating the sediment and then pooping it out again, they're actually aerating the sediment, which makes the sediment a healthier place for other animals to live, like small crabs or polychaetes, which are worms, or small mollusks that live inside the sediment in the surface layer."

Sea cucumbers are also involved in the nitrogen cycles of the reef ecosystem. As sea cucumbers eat and excrete sediment, "they're releasing nitrogen that's trapped in between the sediments," said Williamson. "So this is really important because nitrogen in particular is a limiting nutrient on coral reefs.… The corals need nitrogen, and the algae need nitrogen, everything sort of locks it up really quickly when it's available, so the sea cucumbers are doing them a big favor in terms of the growth rate of these organisms."

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Sea cucumbers could even help protect coral reefs against one of the harmful side effects of climate change: ocean acidification. "The oceans are becoming more acidic, which means that the calcium carbonate which makes the skeletons of the corals and things is less available and in some cases is actually dissolving off the corals." In addition to releasing nitrogen, sea cucumbers also increase the availability of calcium carbonate as they eat their way through the sediment, said Williamson. "So for the sea cucumbers to release more calcium carbonate that's been trapped in the sediments into the environment that the corals and other animals can use is super important."

"These little sausages are playing a really key role that people just don't think about," said Williamson.

Steven Purcell, a marine scientist at Southern Cross University in Lismore, Australia, who was not associated with the study, said that more than 70 countries harvest sea cucumbers. Because these animals are of great ecological value, it's important to keep tabs on their numbers to make sure they're not being overharvested. He noted that drone surveillance techniques like the one used in this paper could also be used to assess populations of other exploited shallow-water reef species, like giant clams.

This story originally appeared in Eos and is republished here as part of Covering Climate Now, a global journalism collaboration strengthening coverage of the climate story.