Coral reefs:- Growing sea cucumbers in underwater nurseries could be a way of restoring their services as “vacuum cleaners” of the ocean to protect the Asia-Pacific’s declining coral reefs, marine ecologists suggest.

Coral reefs play a crucial role in the region’s biodiversity, food security, employment, tourism, and medical research, but many reefs are suffering degradation due to pollution, ocean warming and overfishing.

Now, a team of researchers from the US has discovered that the overharvesting of sea cucumbers may also be contributing to the reefs’ demise.

“There is no way for managers and policymakers to protect coral reefs from bleaching because of acute warming, so this research suggests a new way to protect coral reefs by protecting sea cucumbers associated with coral reefs.”

Robert Steneck, professor emeritus of Oceanography, Marine Biology and Marine Policy at the School of Marine Sciences, University of Maine, US.

Sea cucumbers are marine invertebrates which have for centuries been found on the ocean floor and feed on microalgae, bacteria and detritus or bits of dead organisms.

In some Asian countries, they are considered a delicacy that can be pickled, fried or eaten raw. They also are used in traditional Chinese medicine, and medical researchers now are looking at their use for diabetes drugs and anti-cancer properties.

Sea cucumbers feeding next to corals. Photo courtesy of Dr. Cody Clements, School of Biological Sciences, Georgia Institute of Technology.

Co-author Mark Hay, a marine ecologist at the Georgia Institute of Technology, Atlanta, told SciDev.Net: “Humans have overharvested these detritivores, resulting in a removal of the janitors of the reef and this is causing increases in coral diseases.

“Coral disease is one of the major drivers of coral loss in some areas and it interacts with warming seas often stimulating microbial growth. The heat, plus the microbial growth buildup may increase coral losses to disease.”

Hay says this removal of “vacuum cleaners” from the reef has now been coupled with pollution and rising water temperatures from global warming.

Co-investigator Cody Clements, a fellow researcher at the university, explained: “That ecological fuse we lit long ago has finally reached the explosives of a dirtier and warmer fish tank earth because the sea is not there to help us clean up.”

The researchers believe it would be useful to start sea cucumber nurseries in affected areas.

“We are hopeful of stimulating such actions by conservation groups, fisheries managers in countries where reefs are declining or at risk,” Hay added, pointing out that this applies to “almost all countries with coral reefs.”

The pair’s study, published in Nature Communications, was replicated in two locations: Moorea, French Polynesia, a small island within sight of Tahiti, and the American protected Palmyra Atoll, also in the Pacific Ocean.

In each location, using different sea cucumber and different coral species, they excluded or included sea cucumbers and monitored the frequency and severity of coral diseases in these plots.

The researchers noted that the corals they planted got infected and died in areas where there were few or no sea cucumbers. However, this did not occur in areas with more sea cucumbers.

“This provided us an opportunity to test their functions in a natural setting,” Hay explained.

“We then repeated parts of the same experiment in a different reef system Palmyra Atoll using different sea cucumbers and coral species and found similar results.”

Hay says the findings demonstrate the importance of a missing trophic link on modern reefs.

Deron Burkepile, professor of Ecology, Evolution, and Marine Biology at the University of California, Santa Barbara, tells SciDev.Net: “When we think about how humans impact the oceans via overharvesting we typically think about the big charismatic organisms like sharks, grouper, and parrotfishes … Most of us overlook sea cucumbers, even though they act as the reefs’ vacuum cleaners.”

He notes that the corals used in the study create lots of homes for other organisms, “so, the sea cucumbers could dramatically influence the presence of a host of coral reef biodiversity”.

Robert Steneck, professor emeritus of Oceanography, Marine Biology and Marine Policy at the School of Marine Sciences, University of Maine, US, tells SciDev.Net says the study showed with “rigorous experiments” that sea cucumbers may play a critical role by controlling coral pathogens as they feed on coral reef-associated sediments.

“This is especially critical because coral disease is one of the greatest threats to reef corals,” he said.

Researcher diving in the Palmyra Atoll. Photo courtesy of Dr. Cody Clements, Research Scientist, School of Biological Sciences Georgia Institute of Technology, Atlanta

“There is no way for managers and policymakers to protect coral reefs from bleaching because of acute warming, so this research suggests a new way to protect coral reefs by protecting sea cucumbers associated with coral reefs.”

According to Steneck, it is “too soon to say how widely applicable the findings of this study are to coral reef ecosystems throughout the tropics, but it is certainly a new and exciting discovery.”

Margaret Miller, research director of the NGO SECORE International, said: “We struggle with the presence of sand sediments on reef surfaces that impair coral settlement and recruitment.

“This study suggests that sea cucumbers, which may have been functionally removed from our reefs via overharvest, could be an ally in removing and disinfecting these reef sediments to aid coral recruitment. Culturing them might be a next step.” AlphaGalileo/SP