New findings from marine biologists at Rice University have challenged the long-standing belief that certain fish species contribute to the health of coral reefs. According to a recent study, feces from these so-called “grazer” fish have been found to cause damage and, in some instances, even death to coral colonies.Traditionally, grazers—fish that feed on algae and detritus—have been considered beneficial for reef ecosystems, while corallivores, those that consume coral, were seen as potential threats. However, the researchers at Rice University discovered that grazer feces contain alarmingly high levels of coral pathogens, posing a significant risk to reef structures.“Corallivorous fish are generally regarded as harmful,” according to lead author Carsten Grupstra, “because they bite the corals.“But it turns out that this doesn’t tell the whole story.”The latest research study expands upon previous findings by utilizing data and evidence gathered over a two-year period of field research and laboratory experiments conducted at Rice University and the Moorea Coral Reef Long-term Ecological Research station situated on the South Pacific island of Moorea in French Polynesia. The study’s findings reveal that corallivore feces, which were analyzed, contained a multitude of bacteria typically present in healthy corals during normal circumstances. Conversely, the feces of grazers were discovered to contain pathogenic bacteria and were observed to cause harm or even mortality to living coral fragments in controlled laboratory tests.To comprehend the potential advantages of coral reefs from the feces of their predators, Grupstra emphasizes the importance of recognizing that coral-eating fish do not consume their prey entirely. Being constantly cautious of becoming prey themselves, these fish engage in a repetitive two-step process: taking a mouthful of food and swimming to a different location. As they travel, they naturally disperse their excrement, along with any beneficial organisms it may contain, across a wide area.Grupstra further indicates that the findings of this new study propose corallivore feces as a noteworthy source of beneficial microbes for corals.“It’s analogous to fecal microbiota transplantation therapy in humans,” Grupstra adds.In the study, they analyzed the impact of bacteria found in the feces of both corallivores and grazers on live corals. The researchers collected fresh feces from these marine organisms during research dives, and then assayed the bacteria in the samples. They conducted controlled experiments to determine how each type of feces affected coral.To conduct the experiments, the researchers placed pieces of coral in jars filled with microbe-free seawater. They then added fresh feces from either corallivores or grazers to some of the jars. To test whether the physical characteristics of the feces alone could harm the coral, they sterilized some fecal samples and added them to other jars. The final group of jars served as an experimental control, with nothing added.After the experiment, the researchers examined the coral fragments from all jars and classified them as either apparently healthy, containing lesions, or dead. The results showed that some feces could kill or smother corals, with the effect being mostly localized, producing lesions on the coral fragment. In some cases, the entire fragment died. Feces from grazers caused lesions or death in all jars, while corallivore feces produced fewer and smaller lesions and rarely led to fragment death. The sterilized feces produced comparable damage.“The bacterial assays from our field samples helped explain the results from the laboratory experiments,” adds Grupstra’s doctoral adviser, Adrienne 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.”In the realm of oceanic research, scientists are eagerly exploring the impact of fish feces on coral reefs. Dr. Grupstra, a prominent researcher in the field, emphasizes the need for further testing to comprehend the precise effects of fish excrement on corals.The crucial question revolves around whether these effects are detrimental or beneficial, considering variables such as disintegration, ingestion, or removal of fecal pellets by other organisms.By gaining a better understanding of the underlying factors influencing the effects of fish feces, reef managers can implement targeted measures to enhance positive outcomes and mitigate any potential negative impacts.Dr. Grupstra adds, “Together, these findings [published in Frontiers in Marine Science] 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.“Both corallivores and grazers have important ecological roles and understanding those roles can help us better manage and conserve these important ecosystems.”Image Credit: Carsten Grupstra