Sea Cucumbers Save Corals

The “Roombas of the Sea”
help create a clean habitat for
coral reefs and their customers

All creatures of our God and King have their role to play—even the humble sea cucumber.

Drab, unappealing and slow, sea cucumbers have been disdained by divers in favor of brilliant tropical fish and coral reefs. Once sea cucumbers were recognized as edible, humans have pickled and fried them or eaten them raw. This has led to a thriving industry harvesting sea cucumbers for food. Why not? They’re drab, they’re ugly, and just sit there, easy to grab. What are they good for? Better off being eaten.

About a fourth of sea creatures live in and around coral reefs, even though they cover 1% of the ocean floor. For decades now, scientists and environmentalists have been sounding alarms about the death of coral reefs worldwide. They blame it on climate and human activities. Well, it’s partly true: humans have been stealing the reef’s janitors, and corals are getting sick.

‘Janitors’ of the Sea: Overharvested Sea Cucumbers Play Crucial Role in Protecting Coral (Georgia Tech, 26 Feb 2024). A serendipitous observation led to experiments that proved something interesting about sea cucumbers. They help corals.

The small, unassuming, sediment-eating organisms function like autonomous vacuum cleaners of the ocean floor. But, because they have been overharvested for decades for food and cannot reproduce effectively when in low densities, they are now rare and slow to recover following harvests. They have been gone so long that it wasn’t known exactly how important they are — until now. …

In first-of-its-kind research, [Mark] Hay [environmental biologist at Georgia Tech], along with research scientist Cody Clements, discovered a crucial missing element that plays a profound role in keeping coral healthy — an animal of overlooked importance known as a sea cucumber.

Members of the Echinoderm family, which includes sea stars, brittle stars and sand dollars, sea cucumbers are elongated creatures with leathery skin. Found worldwide, with about 1,700 species, they belong to a class called Holothurians. They spend their days sucking in sediment for organics and pooping out sand. But that’s the key: they are vacuum cleaners for coral reefs, removing disease germs and giving corals a clean environment. Listen to Professor Mark Hay talk about his findings in the embedded video clip in the press release.

“We knew that removing big predators has cascading effects that commonly change how ecosystems are organized and how they function,” said Hay. “What we didn’t know is what would happen following removal of detritivores — or as we like to call them, the janitors of the system.”

Sea cucumbers and corals enjoy a mutualistic partnership (symbiosis), where both members benefit from one another. Sea cucumbers don’t recover well when their density drops, Hay says, so overharvesting has depleted their numbers in many places.

Island of Mo’orea, French Polynesia, where the research was conducted. Island of Tahiti in distance. (DFC)

Dr Hay had overlooked their low population density as he tried to restore coral reefs by transplanting tens of thousands of corals. At one site in Tahiti, he made a crucial observation. He found that corals in the Tahiti islands looked healthy when large numbers of sea cucumbers were around. Controlled experiments established that corals suffered or thrived depending on the density of their Holothurian neighbors.

The presence of sea cucumbers seemed to suppress coral disease. They observed that corals without sea cucumbers present were 15 times more likely to die….

The experiment painted an alarming portrait. Sea cucumbers seemed to be a missing component of what had been, at one point, an intact ecological system. Before humans started harvesting these sediment-cleaning organisms, they had helped protect corals from disease.

Could the loss of coral reefs be due to a century of overharvesting sea cucumbers? If so, a solution to the coral crisis presents itself: create a sustainable abundance of sea cucumbers. With appropriate planning, they could help corals and satisfy human consumers, too.

Hay and Clements hope their findings will encourage communities to limit harvesting and begin to repopulate sea cucumber species. The species from their study are of little commercial or food value and could be cultured and released into the ocean. This would help to mitigate coral disease and help reefs worldwide return to health.

“Bringing these little guys back from the brink and drawing awareness to their value for ecosystems might improve the situation overall,” Clements said.

Dr Hay comments about needs to limit pollution and global warming, too, but to quantify those factors might require additional experiments. His own experiments improved the lot of corals 15-fold at his station in French Polynesia without solving climate or pollution concerns. Their work has been published open access in Nature Communications 26 Feb 2024, where the authors say,

Here we show, via field manipulations in both French Polynesia and Palmyra Atoll, that historically overharvested sea cucumbers strongly suppress disease among corals in contact with benthic sediments. Sea cucumber removal increased tissue mortality of Acropora pulchra by ~370% and colony mortality by ~1500%. … Historic overharvesting of sea cucumbers increases coral disease and threatens the persistence of tropical reefs. Enhancing sea cucumbers may enhance reef resilience by suppressing disease.

Sea cucumbers can be found in tidepools. (DFC)

As stewards of the biosphere, evolutionists and creationists should agree that our use of resources should be commensurate with the health of the ecosystem. It also makes good economic sense. Greediness leads to the Tragedy of the Commons, but wise utilization creates opportunities for sustainable use.

Incidentally, scientists have found an alternative habitat for many sea creatures that have produced thriving ecosystems: sunken ships (The Conversation, 18 Jan 2024). But whether created by accident or purposefully, those “artificial reefs” must be planned carefully to avoid unintended consequences.

Good for Hay, Clements and Georgia Tech for a Darwin-free research project that may lead to recovery of the world’s beloved coral reefs. Some follow-up questions arise from this story worth researching:

• How quickly can threatened reefs recover if sea cucumbers are supplied?
• Do their numbers automatically adjust to optimum levels when left alone?
• Aside from human consumption, what other factors threaten sea cucumber populations?
• Are there other threatened ecosystems where mutualistic partners are unrecognized?
• Why did it take so long for this discovery?
• The earliest coral is dated 500 million Darwin Years old, but the earliest sea cucumber is dated at 430 million years old. Could corals have survived for 70 million Darwin Years before sea cucumbers supposedly evolved?

Think of your own questions and discuss them in class or at home.