Sea cucumber poo could save a Queensland reef

According to an article in The Guardian, sea cucumbers could play a vital - and greatly underappreciated - role in the maintenance of healthy reefs.

Interested in the role sea cucumbers play as bioturbators - species that make physical changes to the soils and sediments within their environments - a team of researchers from Australia’s James Cook, Macquarie and Newcastle universities co-authored a study into the subject.

“Sea cucumbers are not a charismatic animal,” reef ecologist at Newcastle University, and co-author of the study, Dr Vincent Raoult said. “Initially, people wouldn’t think they do much at all, and there has not been much research on the importance of these animals and their faeces in our reef systems.” In fact, in some other parts of the world sea cucumbers are actually being fished towards extinction, with seven species endangered and nine in the vulnerable category.

It turns out, much like common earthworms, sea cucumbers suck up sediment, ingest the nutritious micro-organisms living in the sand, and then expel the rest back out. This faeces then creates a safe habitat for organisms like crustaceans. It also releases nutrients, which fertilise the water and promote the growth of organisms like algae, which are eaten by other animals. And, critically, sea cucumber also releases calcium carbonate, a key component in the growth of coral skeletons.

Over 24 hours, the team - consisting of Raoult, holothurian expert Jane Williamson and remote sensing experts Karen Joyce and Stephanie Duce - collected the small faecal pellets of individual sea cucumbers in a tank. Once they knew how much poo was expelled by a single sea cucumber in one day, they could estimate the total bioturbation across any given reef. They just had to count the cucumbers. 

Using high resolution drone footage, the team mapped some 30,000 sq metres of Heron Island,  in the southern Great Barrier Reef, so that the sea cucumbers could be counted. From the images, the team estimated around three million sea cucumbers would be living on the 20 sq kilometre Heron Island reef - excreting some 64,000 metric tonnes per year!

And, in areas where sea cucumbers are fished for the luxury food trade, such as Asia and East Africa,  some are now covered in blue-green algae and black sediment.

Raoult added, “If we want to have healthier reefs, we can’t just ignore the fact that sea cucumbers are disappearing and focus solely on climate change, even though that is the major threat to coral reefs. We have to make sure that we address other issues such as overfishing as well.”