Article image

Some fish have feces that is deadly to coral reefs

In a surprising turn of events, marine biologists at Rice University have discovered that feces from fish, which were previously believed to promote healthy coral reefs, may actually harm and even kill corals. 

The research, published in the open-access journal Frontiers in Marine Science, reveals that the feces of algae and detritus-consuming fish, known as grazers, contain high levels of coral pathogens. 

Conversely, the feces of coral-eating fish, or corallivores, have been found to harbor high levels of beneficial bacteria that could function as a “coral probiotic.”

“Corallivorous fish are generally regarded as harmful because they bite the corals. But it turns out that this doesn’t tell the whole story,” said study lead author Carsten Grupstra, who is now a postdoctoral researcher at Boston University and Woods Hole Oceanographic Institution.

In 2021, Grupstra, his doctoral advisor Adrienne Correa, and other members of her research group at Rice University made a groundbreaking discovery: feces from coral predators contain living symbiotic algae that corals rely on for survival. 

This new study builds upon that finding by examining data and evidence collected over two years of field research and laboratory experiments at Rice and the Moorea Coral Reef Long-term Ecological Research station in French Polynesia.

The research demonstrates that corallivore feces contain a significant amount of bacteria typically found in healthy corals under normal conditions. Meanwhile, grazer feces not only contain pathogenic bacteria but also have been proven to damage or kill living coral fragments in controlled laboratory experiments.

To comprehend why corals might benefit from the feces of their predators, Grupstra explained that it is crucial to consider that coral-eating fish do not completely consume their prey. These fish are constantly on the lookout for predators and follow a simple two-step process: they grab a mouthful of coral and then swim to a new location. As they move, they naturally disperse their feces, along with any beneficial organisms it contains, over a wide area.

According to Grupstra, the new study suggests that corallivore feces could serve as an essential source of beneficial microbes for corals. Drawing a comparison to human medicine, he said: “It’s analogous to fecal microbiota transplantation therapy in humans.”

How the study was done

The researchers collected fresh feces from corallivores and grazers during research dives, assaying the bacteria in the samples, and conducting controlled experiments to determine the impact of each type of feces on coral health.

The experiments involved placing pieces of coral in jars with microbe-free seawater and adding fresh feces from either corallivores or grazers to some jars. To explore whether the feces’ physical characteristics alone might damage the coral, sterilized fecal samples were added to other jars. 

The final group of jars, an experimental control, had nothing added. At the conclusion of the experiment, coral fragments from all jars were examined and categorized as apparently healthy, containing lesions, or dead.

What the researchers learned

The results showed that some feces could kill or smother corals. In most instances, the effect was localized, resulting in lesions on the coral fragment. In other cases, the entire fragment died. 

Grazers’ feces caused lesions or death in all jars, whereas corallivore feces produced fewer and smaller lesions and rarely led to fragment death. Sterilized feces produced comparable damage.

“The bacterial assays from our field samples helped explain the results from the laboratory experiments,” said Professor Correa. “We found coral pathogens were more abundant in grazer feces, and beneficial microbes were more abundant in corallivore feces.”

Grupstra emphasized the need for further research on how fish feces affect corals in the ocean. Damaging or beneficial effects could be limited if fecal pellets disintegrate or are consumed or removed by other organisms. 

A better understanding of the factors driving fecal effects could enable reef managers to implement treatments promoting beneficial effects or minimizing negative impacts.

“Together, these findings result in a more nuanced understanding of the roles of fish on coral reefs and may help us better understand the interactions that are happening on reefs around the world,” explained Grupstra. “Both corallivores and grazers have important ecological roles, and understanding those roles can help us better manage and conserve these important ecosystems.”

The research received support from the National Science Foundation (2145472 and 1635798), Rice University startup funds, and fellowships awarded by the Wagoner Foreign Study Scholarship Program and the Kirk W. Dotson Endowed Graduate Fellowship in Ecology and Evolutionary Biology.

The findings challenge the long-held belief that corallivorous fish weaken reef structures, revealing that their feces may actually have a beneficial effect on coral health. In contrast, grazer feces, previously believed to promote healthy reefs, have now been found to harbor harmful pathogens that can damage or kill corals. 

This discovery is an important step in understanding the complex relationships between various organisms within coral reef ecosystems and has the potential to significantly impact coral conservation efforts in the future.

More about impact of climate change on coral reefs

Coral reefs are among the most diverse and productive ecosystems on Earth, providing essential services like supporting fisheries, coastal protection, and fostering tourism. However, coral reefs are increasingly threatened by climate change, which impacts them in several ways:

Coral bleaching

One of the most visible and well-known impacts of climate change on coral reefs is coral bleaching. This occurs when corals experience stress from elevated sea temperatures, causing them to expel the symbiotic algae (zooxanthellae) living in their tissues. These algae provide corals with nutrients and are responsible for their vibrant colors. 

When corals lose these algae, they turn white or transparent, hence the term “bleaching.” Bleached corals are more susceptible to diseases and have reduced growth rates, reproductive capacity, and survival rates. If the stress persists for an extended period, bleached corals can die.

Ocean acidification

The increasing levels of atmospheric carbon dioxide (CO2) are also causing ocean acidification. As the ocean absorbs CO2, it leads to a decrease in pH levels, making the water more acidic. This acidification negatively impacts the ability of corals and other marine organisms to build and maintain their calcium carbonate skeletons. Reduced calcification rates can weaken coral structures, making them more susceptible to erosion and damage from storms.

Sea level rise

Climate change is causing global sea levels to rise due to the melting of polar ice caps and the expansion of seawater as it warms. Sea level rise can lead to increased sedimentation and reduced light penetration in coastal coral reefs, affecting their growth and overall health.

Changes in weather patterns

Climate change can alter weather patterns, leading to more intense and frequent storms and hurricanes. These events can cause physical damage to coral reefs, breaking apart coral colonies and removing large sections of the reef structure.

Changes in ocean currents

Climate change can also affect ocean currents, which play a vital role in the distribution of nutrients, heat, and larvae. Changes in currents can disrupt the supply of nutrients to coral reefs, impacting their growth and health.

To protect and conserve coral reefs, it is crucial to mitigate climate change by reducing greenhouse gas emissions and adopting sustainable practices. In addition, local measures such as improving water quality, protecting coral reefs from physical damage, and managing fisheries can help enhance the resilience of these vital ecosystems to climate change impacts.


Check us out on EarthSnap, a free app brought to you by Eric Ralls and

News coming your way
The biggest news about our planet delivered to you each day