Nature is full of surprises! One recent example came from a new American study that suggests that some fish that are coral predators can actually promote the well-being of these reefs… And it’s all down to their excrement!

Coral is essential to the protection of marine ecosystems, but its health is threatened by various factors, including being eaten by other species. However, some corallivorous fish, until now exclusively seen as coral predators that weaken reefs, could actually make a key contribution to their survival.

For example, that’s the case of the butterflyfish and the parrotfish. Curiously, these species do bite the coral and potentially harm them, but they also provide these reefs with substances that promote their health. How? Through their poo! That’s right, the fecal deposits left by these fish are a source of “good” bacteria which become part of a cycle that enables the coral to thrive. Meanwhile, fish that eat algae and detritus spread feces with pathogenic bacteria that can be harmful to the reefs.

This was the conclusion reached by US-based scientists, whose work was published this month in the journal Frontiers in Marine Science.

“Animal waste products are an important component of nutrient cycles and result in the trophic transmission of diverse microorganisms. There is growing recognition that the feces of consumers, such as predators, may impact resource species, their prey, via physical effects and/or microbial activity,” the study outlines.

The researchers conducted laboratory tests using fecal samples from several reef fish species. The scientists also wanted to verify if the “fresh” feces and those sterilized by heat produced the same amount of bacteria.

The analyses showed that in the case of algae and detritus-eating fish, fresh feces caused 4.2 times greater lesions on corals, compared to those from sterilized feces. The microbial activity found in colliavore feces caused fewer and smaller lesions.

“Testing how feces derived from animal species … affect cohabiting organisms can inform agriculture, ecosystem management, and restoration,” the authors conclude.

While the authors emphasize the beneficial effect of corallivorous species, they note that more research is needed to better understand how the feeding behaviors of these fish can affect coral health.