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How sea cucumbers act as little allies for disappearing coral reefs

MARY LOUISE KELLY, HOST:

The words coral reef evoke a riot of color and life, but the ecosystems are disappearing due to a barrage of threats. Now new evidence points to an ally in the quest to slow down coral die-off, a little creature called the sea cucumber. Science reporter Ari Daniel has more.

ARI DANIEL, BYLINE: Cody Clements is something of a coral gardener.

CODY CLEMENTS: You can just break off a branch from a coral, plant it into the sandy bottom, and it will grow into a whole new coral.

DANIEL: Over the years, Clements, who's a marine ecologist at Georgia Tech, has planted over 10,000 coral fragments across the South Pacific. But as he was gearing up for an experiment in 2018 in French Polynesia, something caught his attention. And it had to do with sea cucumbers, a marine invertebrate that's distantly related to starfish but looks like a pickle.

CLEMENTS: They come in various shapes, colors, and sizes. Some of them are very large.

DANIEL: They're slow-moving scavengers, and collectively they hoover up truckloads of sand to feed on algae, microbes and organic matter. And in this particular spot, there were quite a few of them.

CLEMENTS: I cleared the sea cucumbers out of the area so that I could have a uniform study site. And then I started seeing that the corals were starting to die from the base up. And I was just like, OK, this is pretty abnormal.

DANIEL: Clements wondered whether removing the sea cucumbers had had something to do with it. So he rang up his supervisor at the time, Georgia Tech marine ecologist Mark Hay. And Hay recalled an etching he'd seen years earlier in the Fiji Museum of an old sailing vessel.

MARK HAY: Exporting so many - I think it was hundreds of tons - of dried sea cucumber.

DANIEL: Nowhere near what Hay's seen in the modern ocean, where these squishy animals have been harvested as a delicacy to near-oblivion.

HAY: They must have been super-abundant at one time. And so we had wanted to say, OK, if there were that many of them, what were they doing? And what's their real role in the world?

DANIEL: With so few sea cucumbers in most places, though, there had been no way to answer these questions. But in French Polynesia, where Clements was working, there were a few bays with plenty to run a simple experiment.

CLEMENTS: We go around to natural sand patches that have the cucumbers and either remove them from the area or leave them in place and plant corals in that area and see what happens.

DANIEL: But Clements had a major setback within a few hours of entering the water at the start of the field season. A searing pain tore through his right hand. He looked down and saw a giant moray eel.

CLEMENTS: The eel had bitten me, pulled my hand down and was rag-dolling (ph) my hand, essentially.

DANIEL: Clements thought he might bleed out, but he made it back to his boat and then to shore. A top-notch hand surgeon in Tahiti reattached the thumb.

CLEMENTS: He did a really great job, I mean, compared to what it looked like the day of.

DANIEL: After that, the rest of the experiment went smoothly, and the results left no doubt. When sea cucumbers were removed, tissue death of the corals more than tripled, and whole colony mortality surged 15 times. The question was why. Hay thinks it's related to the vast volume of sand that sea cucumbers process.

HAY: We think of these sea cucumbers as little Roombas that run around and take sand in. They digest microbes out of it, and so the waste that would otherwise accumulate on the bottom is not being left there to heat up and grow microbes.

DANIEL: Less microbes mean less disease, the thinking goes, and healthier coral. But ocean warming and pollution encourage more microbes and more disease at a time when sea cucumbers have been overexploited. The research is published in the journal Nature Communications.

KAILEY PASCOE: Putting the two together, looking at coral disease and sea cucumber abundance, I thought was really unique.

DANIEL: Kailey Pascoe is a coral reef biologist currently in a Ph.D. program at the University of Arizona. She wasn't involved in the study but says it brings to mind a possible solution - the Hawaiian practice of kapu, basically a no-harvest time.

PASCOE: So no taking sea cucumbers to allow them to do their job filtering sand and microbes and bacteria for the corals' health.

DANIEL: Given all the threats corals face today, boosting sea cucumber numbers could provide a certain level of protection, perhaps giving the remaining reefs a fighting chance to survive. For NPR News, I'm Ari Daniel.

(SOUNDBITE OF MYA SONG, "ANYTIME") Transcript provided by NPR, Copyright NPR.

NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.

Ari Daniel is a reporter for NPR's Science desk where he covers global health and development.