Fish thought to help reefs have feces that's deadly to corals

Until recently, fish that consume algae and detritus -- grazers -- were thought to keep reefs healthy, and fish that eat coral -- corallivores -- were thought to weaken reef structures. The researchers found high levels of coral pathogens in grazer feces and high levels of beneficial bacteria in corallivore feces, which they say could act like a "coral probiotic."

"Corallivorous fish are generally regarded as harmful because they bite the corals," said Carsten Grupstra, a former graduate student at Rice and lead author of the open-access study in Frontiers in Marine Science. "But it turns out that this doesn't tell the whole story."

Grupstra received his doctorate in 2022, and is now a postdoctoral researcher at Boston University and Woods Hole Oceanographic Institution. In 2021, he, his doctoral adviser, Adrienne Correa, and others from her Rice research group discovered that feces of coral predators are packed with living symbiotic algae that corals depend upon.

The new study builds on that with data and evidence compiled from two years of field research and laboratory experiments at Rice and theMoorea Coral Reef Long-term Ecological Research station on the South Pacific island of Moorea, French Polynesia. The research showed corallivore feces contained many bacteria that are found in healthy corals under normal conditions. Grazer feces were both found to contain pathogenic bacteria and shown to damage or kill living coral fragments in controlled laboratory experiments.

To understand why corals might benefit from their predators' feces, Grupstra said it is important to consider that coral-eating fish do not devour their prey. Ever wary of being eaten themselves, they spend their days repeating a simple two-step process: grab a mouthful, swim to a new location. Because they relieve themselves as they go, they naturally disperse poop -- and any beneficial organisms it contains -- over a wide area.

Grupstra said the new study suggests corallivore feces could be an important source of beneficial microbes for corals.

"It's analogous to fecal microbiota transplantation therapy in humans," Grupstra said.

The study examined bacteria from both corallivore and grazer feces, and compared their effects on live corals. The researchers began by collecting fresh feces from corallivores and grazers during research dives. They both assayed bacteria in the samples and conducted controlled experiments to see how each type of feces affected coral.

They placed pieces of coral in jars with microbe-free seawater. To some jars, they added fresh feces from either corallivores or grazers. To determine whether the physical characteristics of the feces alone might harm the coral, they sterilized some fecal samples and added them to other jars. To the final group of jars, an experimental control, nothing was added. At the end of the experiment, coral fragments from all jars were examined and categorized as apparently healthy, containing lesions or dead.

The experiments showed that some feces could kill or smother corals. In most cases, the effect was localized, producing lesions on the coral fragment. In others, the entire fragment died. Feces from grazers caused lesions or death in all jars. Corallivore feces produced fewer and smaller lesions and rarely led to fragment death, and sterilized feces produced comparable damage.

"The bacterial assays from our field samples helped explain the results from the laboratory experiments," said Correa, an assistant professor of biosciences. "We found coral pathogens were more abundant in grazer feces, and beneficial microbes were more abundant in corallivore feces."

Grupstra said researchers need to further test how fish feces affect corals in the ocean. For example, damaging or beneficial effects could be limited if fecal pellets disintegrate or are eaten or removed by another organism. Better understanding the drivers of fecal effects could allow reef managers to implement treatments that promote beneficial effects or minimize 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," he said. "Both corallivores and grazers have important ecological roles and understanding those roles can help us better manage and conserve these important ecosystems."

The research was supported by 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.