Mussel Filtration and Ocean Acidification

In Biology 326, we have the unique opportunity to perform an experiment of our own design. This usually doesn’t happen in undergraduate courses and so its a really exciting opportunity. I am very interested in how human activity affects our marine ecosystems and so for this project I wanted to investigate how ocean acidification interacts with predator presence on the feeding of a common mussel on our coasts- Mytilus trossulus. Ocean acidification, through a series of steps, causes not only a decrease in pH, but also a decrease in the calcium carbonate available for calcification. To learn more about this click here. This is a process that many marine animals undergo to build shells or other skeletal elements. This is a huge problem for many organisms but I wanted to focus my study on mussel Mytilus trossulus. These are part of a group of calcifyers that require a large amount of calcium carbonate to make their protective shell and so a decrease in pH resulting from the high levels of carbon dioxide we are expecting to see could be potentially problematic.
Also, mussels are also ecosystem engineers. This means that they shape and maintain the ecosystem around them. Mussels do this by filter feeding which not only affects the nutrient composition of the water, but also affects the amount of particles in the water. This is incredibly important to any organism planning on getting its next meal from sunlight as more particles mean less light and in turn less capacity for photosynthesis, as light is the key ingredient. To learn more about mussels and bivalves click here
A lot of study has been done in the area of how ocean acidification affects calcification so, I wanted to look at how ocean acidification affects a process like sensing the presence of a predator. To do this I set up four treatments: elevated CO2 and predator cue, elevated CO2 and no predator cue, ambient CO2 and predator cue and ambient CO2 and no predator cue. I measured both how many mussels were feeding in each treatment and how many particles they were filtering out of the water assuming that the more mussels were open, the more filtering of particles would occur. I was predicting that because the mussel Mytilus trossulus has this calcium carbonate shell as its only protection from predators and environmental stressors (like heat or heavy waves) in the presence of higher amounts of CO2 its metabolism would increase and so would its feeding rate. I also predicted that the presence of CO2 could interfere with their ability to sense predators and so they would continue to feed in treatments with CO2 and predator cue. This response has been seen in a variety of studies with other marine invertebrates, but I wanted to see if this occurred with mussels, as it is important for them to stay shut in the presence of starfish (their natural enemy and in this casse, my predator) as even opening a small amount could cost them their lives.
I found that less stressed mussels (mussels in my ambient CO2 and no predator cue treatment) tend to have higher amounts of mussels feeding but actually have less filtration of particles. What I found was that the amount of mussels feeding and how much filtration is occurring is not necessarily correlated. Mussels in treatments with more stress tended to have less mussels feeding however filtration in these treatments was relatively high. Through this experiment I learned a lot about how mussels feed, how to measure they’re feeding, how to troubleshoot, and how to collaborate to problem solve. I also learned that to answer my initial question with any certainty, I need to do this many more times as I had trends to support my predicts not but not significantly so. However this was a valuable experience that I will remember next time I plan out an experiment, and hopefully one day will find an answer to my original question- how is mussel filtration affected by ocean acidification and predator presence and what are the future implications of this for the entire ecosystem.

Photo Cred: Paul Naylor

Photo Cred: Paul Naylor

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