Underwater janitors prevent coral disease

Sea cucumbers benefit coral reef health

Coral reefs worldwide have lost approximately half their living cover since the 1950s, with scientists documenting a devastating 14 percent decline in just the past decade. However, researchers at the Georgia Institute of Technology have discovered an unlikely guardian that could help save these underwater ecosystems: the sea cucumber.

These cylindrical marine creatures, which belong to the same group as starfish and sea urchins, might hold the key to coral reef survival. In research conducted across French Polynesia and Palmyra Atoll, scientists Mark Hay and Cody Clements have demonstrated that sea cucumbers play a crucial role in protecting coral health.

The coral reef crisis deepens

Coral reefs support 25 percent of all marine species despite covering less than one percent of the ocean floor. These underwater rainforests provide critical habitats, coastal protection, and economic benefits worth billions of dollars annually.

Climate change has accelerated coral decline dramatically, with projections showing 70-90 percent of reefs could disappear if global temperatures rise 1.5°C above pre-industrial levels. The fourth global bleaching event, which began in 2023, has already impacted 84 percent of the world’s reefs.

Ocean warming, pollution, and disease have created a deadly combination for coral ecosystems. Many reef diseases are associated with sediment accumulation on the seafloor, where harmful bacteria can flourish and infect nearby corals.

Sea cucumbers function as “ocean janitors”

Sea cucumbers are soft-bodied marine animals that spend their lives moving across the seafloor. They continuously consume sand, sediment, and organic matter, processing this material through their digestive systems before excreting clean sediment.

“If you remove all the scum suckers in the great fish tank of Earth, you’re going to get a dirty tank eventually,” explained Clements in describing their ecological role.

The Georgia Tech team conducted controlled experiments by removing sea cucumbers from specific reef areas while leaving control areas untouched. Their results showed that sea cucumber removal increased coral tissue mortality by 370 percent and colony mortality by 1,500 percent.

The “mini-brain” of marine ecosystems

Beyond their cleaning duties, sea cucumbers serve as essential recyclers in marine ecosystems. Inside their bodies, organic matter breaks down and transforms into vital nutrients that are released back into the water column.

This nutrient cycling acts like a natural fertilizer for the diverse life forms inhabiting coral reefs. Without these creatures, reefs face a double threat: harmful bacteria accumulate without their cleaning action, and the nutrient cycle breaks down.

The inspiration for this research came when Hay saw a 19th-century ship etching in a Fiji museum, showing vessels carrying tons of dried sea cucumbers. This made him realize these now-rare creatures once covered shallow tropical ocean floors.

Experimental proof in tropical waters

The research team established monitoring patches in Mo’orea, French Polynesia, and Palmyra Atoll, planting corals in areas with and without sea cucumbers present. They observed the patches daily using GPS markers.

In areas without sea cucumbers, researchers frequently observed white band disease developing at coral bases, eventually killing entire colonies. This sediment-associated disease is common worldwide and represents a major threat to coral survival.

The experiments showed corals were “15 times more likely to die” in areas where sea cucumbers had been removed. The consistency of results across different locations and coral species strengthened the study’s conclusions.

Conservation implications and solutions

Sea cucumber populations have declined rapidly due to overharvesting for food and traditional medicine. High demand has created a significant threat to their survival, with ripple effects throughout marine food chains.

The research emphasizes urgent need for conservation measures. Protecting these “ecosystem engineers” requires regulating harvest quotas, establishing marine protected areas, and adopting sustainable trade practices.

The particular species studied, Holothuria atra, has little commercial value and could be cultured and released into ocean environments to help restore reef health.

Hope for reef restoration

Despite decades of coral decline, the discovery of sea cucumbers’ protective role offers new hope for conservation efforts. “Organisms like sea cucumbers give us insurance for another few decades, and focusing on their importance is something to try,” said Hay.

Clements, who has planted over 10,000 coral fragments throughout his career, sees sea cucumbers as providing “an extra level of insurance against the things that are causing coral decline.”

Successful reef restoration will require combined approaches: reducing global carbon emissions, controlling local pollution, protecting sea cucumber populations, and actively restoring damaged reef areas.

The study was published in the journal Nature Communications.

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