25 April, 2023

Author: George Beccaloni

Coral eating fish were one regarded as harmful to coral but research suggests their poo could be keeping reefs healthy. The faeces of coral-eating fish may act as “probiotics” for reefs, according to a  recent scientific study in 2023 published in the journal Frontiers in Marine Science.

Previously it was thought that corallivore – fish such as pufferfish, parrotfish and butterfly fish that eat coral – weakened marine surfaces. But new research suggests that by eating some parts of the coral and then pooing in different areas of the reef, they are part of a cycle that redistributes beneficial microbes that can help coral thrive.

“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 Dr Carsten Grupstra, of Rice University, the lead author of the study. “They often get a bad rep, and lots of scientists see them as damaging. But I hope this work shows that they can be a part of a healthy ecosystem.

“Corallivore faeces contain many of the bacterial taxa that associate with healthy corals under normal conditions, potentially resulting in the natural dispersal of ‘coral probiotics’, analogous to faecal microbiota transplantation therapy in humans.”

This means that when some fish faeces interact with coral colonies on the reef, and these microorganisms are transferred from the poo to a new coral colony, they can be beneficial to that new coral colony.

The study looked at the different impacts of grazer fish, which consume algae and detritus. Until now it had been assumed that they contributed to reef health, but the study found faeces from grazers, which contain disease pathogens, can leave dying patches, known as lesions, on corals.


The scientists examined the impact of faeces from corallivore and grazer fish and found that the grazers caused lesions or death in all coral pieces, whereas faeces from corallivores caused fewer and smaller lesions and rarely caused death. Moreover, the corallivore faeces helped disperse helpful microbes, keeping the reefs healthy.

The research was done to determine what types of bacteria were contributing to the impacts noticed on the coral, whether the excrement included specific coral diseases, and whether the results of the faeces addition experiment could be applied to other fish that ate coral, algae and detritus.

Grupstra said more research was needed in more natural conditions, where things such as waves and other wildlife could affect the results.

“More research needs to be done to test how fish faeces affect corals to see how we might use these faeces in management efforts to support coral reef health,” he said. “I think [at the moment] there hasn’t been a lot of research into fish species and how they affect corals, but this might be really important in coral reef ecology. We’ve seen some other labs publishing stuff about microbes in fish faeces, so I really hope more people look into this because it’s interesting and could provide a new angle.