Article image

Poo from fish that eat coral is beneficial for coral reefs

A recent breakthrough in coral reef research has revealed that corallivorous fish, those that consume coral, may not be as harmful to coral reefs as previously believed. Contrary to popular belief, these fish might even play a crucial role in maintaining coral health by distributing beneficial microbes through their feces.

Traditionally, corallivorous fish have been considered harmful to coral reefs due to the physical damage caused by their bites. Conversely, grazing fish that feed on algae and detritus were thought to contribute to a reef’s overall health. However, a new study published in Frontiers in Marine Science and led by Dr. Carsten Grupstra of Rice University is challenging this notion.

“Corallivorous fish are generally regarded as harmful because they bite the corals. But it turns out that this doesn’t tell the whole story. Corallivore feces 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 fecal microbiota transplantation therapy in humans,” explained Dr. Grupstra.

Good bacteria may boost coral reef health

Coral reefs are teeming with various fish species, all of which produce waste in the form of feces. While this waste can provide essential nutrients to support a thriving coral reef ecosystem, it may also contain pathogens and sediments that can smother coral and cause the formation of lesions – patches of dying coral. Understanding this complex cycle of waste and nutrients is critical for the protection of delicate coral reef ecosystems.

In the study, Dr. Grupstra and his team analyzed the effects of feces from both corallivores and grazers on live coral. They placed coral fragments in jars filled with sterile seawater, then added feces from both corallivore and grazer fish to different jars. Some samples were sterilized to determine if the physical characteristics of the feces alone caused the lesions. After the experiment, each coral fragment was examined and categorized as healthy, containing lesions, or dead.

The researchers also sampled the feces of several corallivore and grazer fish species to identify the bacteria present within them. This information helped them understand what types of bacteria might be contributing to the effects observed on the coral, whether the feces contained specific coral pathogens, and whether their results from the feces addition experiment could be generalized to other fish that also consume coral or algae and detritus.

Maintaining healthy coral reefs

The scientists placed coral fragments in jars with sterile seawater and added feces from corallivorous and grazing fish to different jars. They found that adding feces sometimes resulted in the formation of lesions on the coral pieces, and in some cases, even the death of the fragment. 

Coral fragments without any feces remained healthy. Interestingly, feces from grazers caused lesions or death in all coral pieces, while feces from corallivores caused fewer and smaller lesions and rarely led to death.

Sterilized feces from either type of fish caused little harm, comparable to the low levels of damage caused by corallivore feces. The scientists suspected that this was due to the greater abundance of coral pathogens found in the fresh feces of grazers and the higher abundance of beneficial microbes found in the fresh feces of corallivores. Consequently, the fish that were previously assumed to be harmful may actually contribute to important processes that promote coral reef health.

“More research needs to be done to test how fish feces affect corals to see how we might use these feces in management efforts to support coral reef health,” said Dr. Grupstra.

The scientists noted that the effects of lesions caused by feces might not be as severe under real-world conditions and may not be evenly distributed. Factors such as fish territories and behaviors can affect where and when they defecate, and feces could disintegrate in the water, thus limiting lesion formation. Additionally, some feces are consumed by other fish, and organisms living on coral may move remaining feces that fall on corals, potentially diminishing feces’ effects.

“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. Both corallivores and grazers have important ecological roles, and understanding these roles can help us better manage and conserve these important ecosystems,” said Dr. Grupstra.

These findings underscore the importance of continued research into the intricate relationships within coral reef ecosystems. By broadening our understanding of these dynamics, we can develop more effective strategies to protect and conserve the world’s coral reefs, which are vital to the health of our oceans and the planet as a whole.

How climate change is impacting coral reefs

Climate change is having significant impacts on coral reefs, which are among the most sensitive ecosystems on the planet. The main ways climate change affects coral reefs are through ocean warming, ocean acidification, sea level rise, and changes in weather patterns. These factors lead to various negative consequences for coral reefs and the marine life that depends on them:

  1. Coral bleaching: Coral bleaching occurs when coral expel the symbiotic algae, known as zooxanthellae, that live within their tissues. This symbiosis provides corals with nutrients and their vibrant colors. Ocean warming due to climate change causes stress to corals, leading them to expel the algae, resulting in the coral appearing white, or “bleached.” Bleached corals are more susceptible to disease, and if the stress continues for an extended period, the corals can die.
  2. Ocean acidification: The increase in atmospheric CO2 levels due to human activities also affects the chemistry of the ocean. As the ocean absorbs more CO2, it becomes more acidic, reducing the availability of carbonate ions that corals and other marine organisms need to build their skeletons and shells. This process, known as ocean acidification, makes it difficult for corals to grow and repair themselves, and can even cause their skeletons to dissolve.
  3. Sea level rise: Rising sea levels caused by climate change can have both positive and negative impacts on coral reefs. While deeper waters can provide protection against heat stress, excessive sunlight, and wave damage, they can also reduce the amount of light available for photosynthesis, making it harder for corals and their symbiotic algae to survive. Additionally, rising sea levels can lead to increased coastal erosion and sedimentation, which can smother coral reefs and deprive them of the light needed for photosynthesis.
  4. Changes in weather patterns: Climate change can lead to changes in weather patterns, including more frequent and intense storms, as well as altered ocean currents. Strong storms can physically damage coral reefs, breaking apart coral colonies and altering reef structure. Changes in ocean currents can impact the distribution of nutrients, temperature, and salinity in reef ecosystems, potentially causing stress to corals and other marine organisms.

These climate change-driven impacts on coral reefs have far-reaching consequences for marine biodiversity, fisheries, and coastal communities that depend on healthy coral reef ecosystems for their livelihoods and protection against storms and coastal erosion. 

To mitigate these impacts and preserve these vital ecosystems, it is essential to address the root causes of climate change by reducing greenhouse gas emissions and transitioning to more sustainable energy sources. 

Additionally, local conservation efforts, such as reducing pollution and protecting coral habitats, can help support the resilience of coral reefs in the face of climate change.


Check us out on EarthSnap, a free app brought to you by Eric Ralls and

News coming your way
The biggest news about our planet delivered to you each day