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Who knew fish poo could be a key to restoring coral reef health? In a study by Dr Grupstra’s team from Rice University in the United States, they found that poo from coral-eating fish contain microbes that can help corals thrive.

Fish that eat corals (known as corallivores) have been thought to damage reefs, while fish that feed on algae (known as grazers) have been thought to have lower impacts on reefs.

“But it turns out this doesn’t tell the whole story. Corallivore poo contains many of the bacterial taxa that associate with healthy corals, like ‘coral probiotics’,” Dr Grupstra says.

Under the French Polynesian sea, Dr Grupstra’s team collected corallivore and grazer poo to test their effects on corals. They found that poo from grazers caused damages or deaths in all the coral samples. Surprisingly, they also found that corallivore poo caused less damage and rarely led to coral death.

While fish poo can help distribute nutrients in water, it can also contain harmful pathogens that can cause patches of corals to die. The scientists suspect that grazer poo might contain more disease-causing microbes, while corallivores had more beneficial microbes in their poo.

“More research needs to be done to test how fish poo affect corals to see how we might use them to support coral reef health,” Dr Grupstra says.