by Sage Bauman
— Usually if the topic of fish comes up in a casual conversation it consists of “would you like the swordfish or haddock for dinner?” or maybe you see a clownfish in an aquarium and are reminded of the classic animation “Finding Nemo”. While these are entertaining or necessary topic to discuss, how often might we consider what the fish actually eat themselves? The answer is: not enough!
There are over 33,000 different species of fish on Earth. This accounts for the greatest species diversity among all vertebrates. This means fish are more important than you’d think! Fishes are important parts of our lives for many reasons; they are a major food source as well as responsible for maintaining and indicating the health of our oceans. Since there are many different types of fish, there are many different diets they can have. Just like people, some fish are vegetarians, some fish eat both plants and meat, and there are some fish that eat only other fish. The common saying “You are what you eat” is all too true when you consider the diet of some fish species and how their diet applies directly to their health.
One important factor to consider when thinking about the health of fishes is the condition of their gut microbiome. A microbiome is a community of microorganisms in a particular area. The health of this community is determined by how many ‘good’ or ‘bad’ microorganisms are present. A microbiome can be thought of as a garden: if you provide the right nutrients and care it will flourish, but if there are too many weeds, the garden can become overrun and deteriorate. The weeds represent the bad bacteria that can grow in the gut microbiome of a fish and lead to negative health impacts and leave fishes more susceptible to disease.
Territorial reef fish Stegastes nigricans also known as the damselfish.
A study was done on damselfish in which the effects of polluted food on the fish’s gut microbiome were tested. In the study, pollutants that stood in for fertilizer runoff and other harmful nutrients that could enter the water as a result of human activities were added to damselfish territories to simulate the effects of large-scale human polluton. The added nutrients then were observed to alter the habitat and the food of the vegetarian damselfish. The study showed that fish that ate the polluted food had accumulated significantly larger populations of harmful bacteria and microorganisms in their gut. The imbalance of harmful microorganisms in the gut of these fish can cause the fish to have a damaged immune system which can lead to the contraction of diseases and infections. It also may impact the future reproductive ability for the fish which could potentially lead to population decline.
In another study, biologists examined Atlantic cod that were living in habitats containing crude oil contamination. The gut microbiomes of fish in these contaminated habitats showed reactions to oil pollutant exposure which lead to health issues including heart failure and immunosuppression, increasing likelihood of infections and disease. Overall, the mortality rates in Atlantic cod were increased as a result of oil pollution in the water.
It is important that research will continue examining the impact of pollution on fish diets. Not only does it impact the health of the fish themselves, but it can impact the health of humans that eat polluted fish. It is also important to note that fishes play an important role in the evolutionary history of vertebrates. Understanding how different diets and pollutants impact the gut microbiome of fish can help us further understand the human gut microbiome and factors that might effect it.
The evolutionary history of how land vertebrates evolved from fishes.
References:
Degregori, S. et al. 2021. Nutrient Pollution Alters the Gut Microbiome of a Territorial Reef Fish. Marine pollution bulletin DOI: 10.1016/j.marpolbul.2021.112522
Walter, J. M., et al. 2019. Insights into the Potential of the Atlantic Cod Gut Microbiome as Biomarker of Oil Contamination in the Marine Environment. Microorganisms DOI: doi:10.3390/microorganisms7070209.
Image sources:
Image 1: https://reeflifesurvey.com/species/stegastes-nigricans/
Image 2: https://www.eurekalert.org/news-releases/540459. Image credit: Dr. Guojie Zhang