Newswise — In the past, it was commonly believed that corallivores, or fish that feed on coral, were detrimental to reef structures, while grazers, or fish that consume algae and detritus, were thought to play a positive role in maintaining reef health. However, recent scientific research has challenged this notion. It has been discovered that fecal matter from grazers can actually cause significant lesions on coral, potentially due to the presence of coral pathogens. On the other hand, feces from corallivores may actually contain beneficial microbes that promote the thriving of coral colonies. This new understanding is reshaping our perception of the ecological dynamics between fish and coral in reef ecosystems.
"Previously, corallivorous fish were commonly viewed as harmful due to their coral-biting behavior," explained Dr. Carsten Grupstra of Rice University, who led the study published in Frontiers in Marine Science. "However, our research reveals that this perception may not capture the full picture. Corallivore feces actually contain a diverse array of bacterial taxa that are known to be associated with healthy corals in normal conditions. This suggests that the fecal matter of corallivores could serve as a natural source of 'coral probiotics', similar to fecal microbiota transplantation therapy used in humans, promoting the dispersal of beneficial microbes to aid in coral health."
Good bacteria for healthy reefs?
Tropical coral reefs are home to a diverse array of fish, whose fecal matter is constantly released into the surrounding waters. While fish feces can contain valuable nutrients that can support a healthy coral reef ecosystem, they can also carry harmful pathogens and sediments that can smother living coral, leading to the formation of lesions or patches of dying coral. To effectively protect these fragile coral reef ecosystems, it is crucial to gain a comprehensive understanding of the intricate dynamics between fish waste and nutrient cycling in order to mitigate potential negative impacts and promote coral health.
In their research, Dr. Grupstra and his team conducted a study to investigate the impacts of fecal matter from both corallivore and grazer fish on live coral. They set up an experiment where they placed fragments of coral in separate jars filled with sterilized seawater, and then applied feces from corallivores and grazers to different jars. In order to isolate the effects of fecal matter itself, some of the samples were sterilized to rule out any influence from other factors. After the experiment, the coral fragments were carefully examined and categorized as either apparently healthy, showing signs of lesions, or completely dead. This experimental approach allowed the researchers to assess the direct impacts of fish feces on the health of the coral samples.
In addition to the feces addition experiment, the researchers also collected fecal samples from various corallivore and grazer species. They analyzed these samples to identify the bacteria present in the feces. This analysis provided valuable insights into the types of bacteria that could be contributing to the observed effects on the coral, such as the presence of specific coral pathogens. It also helped determine whether the results obtained from the feces addition experiment could be extrapolated to other fish species that feed on coral or algae and detritus. By examining the fecal bacterial composition, the researchers gained a better understanding of the potential microbial factors that could be influencing the health of coral reefs and how different fish species may contribute to these dynamics.
Keeping coral healthy
The results of the experiment revealed that the addition of fecal matter to the jars had significant effects on the health of the coral fragments. In some cases, the presence of feces led to the formation of lesions on the coral, and in severe cases, even caused the death of the fragments. Fragments that did not receive any feces remained healthy. Interestingly, feces from grazers caused lesions or death in all coral fragments, while feces from corallivores resulted in fewer and smaller lesions, and rarely led to death.
“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 Grupstra.
Dr. Carsten Grupstra highlighted the need for additional research to thoroughly investigate the impact of fish feces on coral health and explore potential strategies for utilizing feces in coral reef management efforts to promote reef health. This statement emphasizes the importance of further scientific inquiry to better understand the complex interactions between fish feces, coral health, and overall reef ecosystem dynamics. By gaining a deeper understanding of these processes, it may be possible to develop evidence-based management approaches that can effectively support the conservation and preservation of vulnerable coral reef ecosystems.
The researchers noted that the effects of feces-induced lesions on corals may not be as severe in natural, real-world conditions, and that the distribution of feces on corals may not be uniform. The behaviors and territories of fish can influence where and when they defecate, and feces may disintegrate in the water, reducing the formation of lesions on corals. Additionally, some feces may be consumed by other fish, and organisms that inhabit corals may also move feces that fall on corals, potentially mitigating the impact of feces on corals. These factors highlight the complexities of feces dynamics in coral reef ecosystems and the need for further research to better understand the nuances of feces effects on corals in natural settings.
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