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This often-overlooked sea creature may be quietly protecting the planet's coral reefs

This type of staghorn coral (<em data-stringify-type="italic">Acropora pulchra</em>)<em data-stringify-type="italic"> </em>appeared to benefit from the presence of sea cucumbers (<em data-stringify-type="italic">Holothuria</em> <em data-stringify-type="italic">atra</em>), a new study finds.
Terry Moore
Stocktrek Images / Science Source
This type of staghorn coral (Acropora pulchra) appeared to benefit from the presence of sea cucumbers (Holothuria atra), a new study finds.

Over the years, Cody Clements, a marine ecologist at Georgia Tech, has planted over 10,000 coral fragments across the South Pacific.

"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," explains Clements. "I have corals out there that I've planted and they were the size of my pinky — and now they're the size of a basketball."

As he was gearing up for an experiment in 2018 in French Polynesia off the island of Mo'orea, something caught his attention. It had to do with sea cucumbers — marine invertebrates that are distantly related to starfish but resemble soft pickles.

"They come in various shapes, colors and sizes," says Clements. "Some of them are very large." They're slow-moving scavengers, and collectively they hoover up truckloads of sand to feed on algae, microbes, and organic matter. (On a single seven-square-mile reef in Queensland, Australia, researchers previously found that the sea cucumbers there are processing the equivalent of more than five Eiffel Towers of sand each year.)

At the reef in Mo'orea that Clements was studying, there were quite a few sea cucumbers. Clements decided to clear them all from his study site to make things uniform for the experiment.

But a few days later, "I started seeing that the corals were starting to die from the base up," Clements says. "And I was just like, 'OK, this is pretty abnormal.'"

Clements wondered whether relocating the sea cucumbers had had something to do with it. And in new research published in Nature Communications, he and his colleagues demonstrate that when they removed sea cucumbers from a study patch, tissue death of Acropora pulchra, a species of staghorn coral, more than tripled. And mortality of the whole colony surged 15 times.

The reasoning, says Clements, is that sea cucumbers are like "these little vacuum cleaners on the reef that are cleaning things up," digesting and eliminating microbes that can lead to coral disease and demise — threats that are exacerbated by a warming and increasingly polluted ocean.

A hunch and a museum etching

Back in 2018, when Clements first suspected that clearing out the sea cucumbers may have triggered the mortality of the corals, he rang up his supervisor at the time — Georgia Tech marine ecologist Mark Hay.

Hay recalled an etching he'd seen years earlier in the Fiji Museum depicting an old sailing vessel transporting perhaps hundreds of tons of dried sea cucumbers. It's an amount nowhere near what he's observed in the modern ocean where these squishy animals have been harvested as a delicacy to near oblivion.

"And so they must have been super abundant at one time," says Hay. "And so we had wanted to [ask], 'OK, if there were that many of them, what were they doing? And what's their real role in the world?'"

With so few sea cucumbers in most places, however, there'd been no way to answer these questions. But off the island of Mo'orea where Clements was working, there were a few bays with enough of the critters to run a simple experiment.

This was the approach: The team would identify natural patches where sea cucumbers hung out. In some, they'd remove the resident sea cukes. In others, they'd leave them alone. In both, they'd plant corals and then watch what happened.

Sea cucumbers like the ones seen here on the sandy bottom behave like "little Roombas that run around," says marine ecologist Mark Hay. He thinks they consume the microbes that would otherwise make the corals pictured here sick.
/ Cody Clements
Cody Clements
Sea cucumbers like the ones seen here on the sandy bottom behave like "little Roombas that run around," says marine ecologist Mark Hay. He thinks they consume the microbes that would otherwise make the corals pictured here sick.

The question they were hoping to answer about coral health was this, says Clements: "Are they more likely to get sick when we've taken the cucumbers out?"

And so the study plan was hatched.

A rough and bloody start

In early 2022, Cody Clements was three hours into his field season. He was examining a large coral colony when a searing pain tore through his right hand. Clements looked down to find a giant moray eel had bit down and wasn't letting go. Instead, it was thrashing.

It all happened so fast, Clements recalls. And before he knew it, the eel — uncharacteristically — let him go. Clements thought he might bleed out, but fortunately he made it back to his boat and then to shore. The local hospital patched him up the best they could, but ultimately he went to Tahiti where a top-notch hand surgeon managed to reattach his thumb.

"He did a really great job! I mean, compared to what it looked like the day of," says Clements.

Ten weeks later, he was back in the water to begin his experiments anew. The rest of the field work went off without a hitch, and the results left no doubt. Wherever sea cucumbers were removed, there was "15-fold more death" of the whole corals, says Hay.

They ran a similar experiment in Palmyra Atoll — a small island about a thousand miles south of Hawaii with what Hay calls "no permanent human population."

"You have to get special permission to go within 50 miles of it," he said.

The reefs of Palmyra differed from Mo'orea in important ways — they're more isolated and therefore more intact.

"I remember getting in the water at this site that's called Crazy Corals," says Clements. "I literally gasped. I was like, 'Oh my god. I've never seen corals like this.'" Some were as large as buildings. Clements was immersed in a pageant of underwater life. "There aren't many places like that left in the world."

Clements and Hay found a similar result, though less pronounced, among the corals in Palmyra Atoll. "Part of the difference is that our time on Palmyra was limited," says Hay, "and the experiment did not run for as long." And of course, the Palmyra corals as a whole seemed to be in much better health.

Still, tissue mortality more than doubled without the sea cucumbers.

A slow-burning fuse

Hay thinks the reason for the increased mortality at both the Mo'orea and Palmyra reefs may be related to the vast volume of sand that sea cucumbers process.

"We think of these sea cucumbers as little Roombas that run around and take sand in," says Hay. "They digest microbes out of it. And so the waste that would otherwise accumulate on the bottom — it's not being left there to heat up and grow microbes, many of which could be pathogenic."

The idea is that fewer microbes mean less disease, which translates into healthier coral. But ocean warming and pollution encourage more microbes and more disease, especially as sea cucumbers have been overexploited.

Hay likens it to a slow-burning fuse that we lit 100 to 200 years ago when the massive harvests of sea cucumbers were well underway. What we may have been doing for decades, he argues, is removing these pudgy custodians of the reef at the same time we've introduced a barrage of other threats. "And all of a sudden," he says, "it's blowing up on us. We've lost huge amounts of corals in the last several decades."

"I have children that are in their mid to late 30's," Hay says. "I can't show them [a reef] anywhere in the Caribbean like when they were born."

Hay says a considerable amount of the global loss of corals is due to disease. The role that sea cucumbers play in suppressing coral mortality has been "just a missed part of the ecosystem that we didn't understand was important," he argues.

Kaylie Pascoe, a coral reef biologist currently in a PhD program at the University of Arizona, said the design of the study was elegant.

"We know sea cucumbers are the filters of the sea," says Pascoe, who wasn't involved in the study. "But putting the two together — looking at coal disease and sea cucumber abundance — I thought was really unique."

Pascoe appreciated that corals were planted for these experiments, but she'd like to know the impact that sea cucumbers have on corals growing naturally. Still, she says the research brings to mind a possible solution — the Hawaiian custom of "Kapu," which basically means a no-harvest time.

"Maybe that sort of practice could be applied to sea cucumbers," she says, to allow them to grow back and do their job — "filtering sand and microbes and bacteria for the coral's health, creating the habitat for all the marine organisms."

It's an idea that resonates for Cody Clements, especially for Holothuria atra, the particular species of sea cucumber that he examined in this study, which he and his colleagues argue has little economic value on its own.

"Sea cucumbers provide an extra level of insurance against the things that are causing coral decline," says Clements. "Doesn't mean it's gonna fix everything, but we want to give them as much of a fighting chance as we can."

Copyright 2024 NPR. To see more, visit https://www.npr.org.

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