The energetic cost of mussel byssal thread attachment

Specialized biological structures are key for organisms’ survival.

September 11, 2020 1:30 am
Mussels are farmed on ropes suspended in the water. Mussels anchor themselves to the rope and other substrates by producing a network of fibers known as byssal threads. Photo taken at Penn Cove Shellfish LLC in Coupeville, Washington. (M. Stone, UW Media/contributed photo)
Mussels are farmed on ropes suspended in the water. Mussels anchor themselves to the rope and other substrates by producing a network of fibers known as byssal threads. Photo taken at Penn Cove Shellfish LLC in Coupeville, Washington. (M. Stone, UW Media/contributed photo)

Originally published by Friday Harbor Labs Tide Bite

Specialized biological structures such as teeth, bone and spider webs are key for organisms’ survival. Animals use the energy they acquire from food to maintain their body tissue, to grow new tissue, and to build specialized structural materials like those above. It’s a constant challenge for organisms to allocate, or distribute, this energy to a range of different biological processes and growth of materials in such a way that maximizes their survival. One way that resource allocation can be conceptualized is through a framework called Scope for Growth. Similar to how we all must balance our financial budgets by calculating our income, cost of rent and net savings, biologists use Scope for Growth to calculate the intake of energy from food, metabolic costs and resulting surplus energy available for the growth of new tissues. What is less well-understood is how much of their ‘energy budget’ animals allocate to building specialized structural materials.

Mussel byssusMussels are bivalve mollusks that live in a wave-swept environment, and the ability to strongly anchor to the habitat is key to their survival. Mussels attach to surfaces by producing a network of protein-based fibers called byssal threads. I was interested in the cost of producing these threads, and the effect of food availability and energetics on their production. I worked with two species, Mytilus trossulus and Mytilus galloprovincialis, a native mussel species and an identical-looking cultured mussel species, both of which end up in stores and restaurants in Washington.

First I wondered whether mussels that have more resources (e.g. with abundant food availability and moderate seawater temperature) produce a stronger network of byssal threads. Mussels feed by filtering tiny algae from seawater. The concentration of algae in the seawater can fluctuate over time, affecting the resources that mussels have available for growth. In addition, higher temperatures cause organisms to metabolize faster, increasing demands and the “cost” of metabolism. I tested the effect of seawater temperature and food availability on growth and byssal thread production for these two Mytilus species in a 10-week lab experiment. I observed that mussels had different rates of growth across a range of temperature and food conditions, but there was no measurable effect of these long-term experimental conditions on byssal thread strength and quantity at the end of the experiment.

This made me wonder, do mussels prioritize making byssal threads above other processes or above growth? I’d found that mussels grew at different rates depending on temperature and food availability, but did not make a different number of byssal threads. An analogy to our own daily lives is that each month we prioritize our financial resources toward our homes or rent (staying anchored), and can only allocate what is left (surplus) toward our savings (growing bigger).

I worked with my advisor Emily Carrington as well as Ken Sebens, Laura Newcomb, and FHL undergrads Katie Harrington, Michelle McCartha, and Sam LaFrambois to quantify the cost of producing threads. Our group performed an experiment where byssal threads were removed either daily, weekly, or just once over the course of a month, and we measured the total number of new threads mussels produced as well as tissue growth. This experiment demonstrated that when we removed threads frequently (e.g. daily), mussels produced a greater number of new threads. Even more interestingly, mussels that produced more byssal threads grew less tissue!

The question that remained was: what does this loss of growth tell us about the energetic cost of producing byssal threads? I calculated the cost of thread production with a bioenergetics model using information about how much energy it takes to grow new tissue and the relationship between growth and the number of threads made (Scope for Growth). I found that for mussels in the control group, about 10 percent of the energy budget was spent on byssal thread production, but that this ranged up to 50 percent of the energy budget for mussels whose byssal threads were severed daily. In other words, mussels being “forced” to make new threads frequently had much less energy available for growth.

Finally, I was interested in whether these ideas also held in a mussel aquaculture setting. I worked with Penn Cove Shellfish in Coupeville, Washington. At this farm, mussels are grown on ropes that hang vertically in the water. The seawater conditions at the farm provided a “natural experiment” to test whether mussels that have greater resources­ – given food availability and seawater temperature­ – produced a greater number of byssal threads. Would fluctuations in temperature and food availability over time end up affecting byssal thread production or growth? To answer this question, I used published relationships between temperature, feeding rate and respiration to calculate mussel energy surplus under different conditions, and measured growth and byssal thread production. I found that the calculated energy surplus did correlate with mussel growth over the two-year period, but this value was not a predictor of the number of byssal threads produced by mussels at the farm. Instead, mussels experiencing longer periods of low oxygen and acidic water produced fewer byssal threads.

So, what have we learned? While remaining anchored to a habitat is key to survival for mussels, it is costly. There’s a trade-off between allocating energy to growth versus byssal thread production. We are left to ponder the evolutionary trade-offs of distributing resources to a range of biological traits and specialized biological structures. Over evolutionary timescales, it may be that natural selection has selected for mussels to prioritize resources to attachment whenever the risk of dislodgement is heightened.

Dr. Molly Roberts completed her Ph.D. with Dr. Emily Carrington in 2019 in the Biology Department at UW. Her research involved both experimental work at FHL and fieldwork at Penn Cove Shellfish in Coupeville, Washington. Prior to her time in graduate school, she was a technician in the FHL Ocean Acidification Environmental Laboratory. She is now working with Dr. Sarah Gilman to predict barnacle growth at a field site at Friday Harbor using an energetics framework.