Molly Roberts: A network of threads and perspectives, energetic cost of mussel attachment

Emily “Molly” Roberts, postdoctoral fellow at Claremont McKenna College, presents her recent work ‘Resource allocation to a structural biomaterial: induced production of byssal threads decreases growth of a marine mussel, Mytilus trossulus’ where she and her colleagues tested the trade-off between survival and growth of mussels.

About the paper

Molly Roberts, pictured at a mussel farm, estimated the cost of byssus from the experimental data using a Scope For Growth framework. (Photo credit: Hilary Hayford)
Molly Roberts, pictured at a mussel farm, estimated the cost of byssus from the experimental data using a Scope For Growth framework. (Photo credit: Hilary Hayford)

This paper is about the energetic ‘investment’ that mussels make to stay anchored to their habitat. Mussels produce a network of ‘byssal’ threads, thin stretchy threads made of collagen-like proteins, to stay anchored in habitats with large waves and currents. In this paper, we quantified how mussels trade off investment in survival (stronger attachment) with growth and reproduction.

First, we devised an experiment to force mussels to produce more byssus than usual.  Mussels sense when they are unstable and respond over the course of several days to produce more byssal threads to establish stability. We manually disrupted this process by severing the threads periodically. We hypothesized that mussels whose byssal threads were severed more frequently (e.g. daily) would produce more threads – and grow less – than mussels whose threads were not severed during the experiment.

Mussels within their mesh enclosure. (photo credit Laura Newcomb)

The month-long experiment was performed in mesh enclosures off the side of a floating dock. We had three different treatments – cutting the byssal threads either (1) every day, (2) every week, or (3) just once at the beginning of the experiment. Throughout the month we measured the number of new byssal threads produced, and estimated the growth of the mussels by measuring their change in length throughout the experiment. We also estimated how much their tissue mass and shell changed by measuring their weight at the end of the experiment. We found that mussels whose threads were severed every day for a month produced many more threads than mussels whose threads were not severed, and did grow less!

Mussels within their mesh enclosure. (photo credit Laura Newcomb)
Mussels within their mesh enclosure. (photo credit Laura Newcomb)

Next, we used these data to estimate the cost of making byssus using a Scope for Growth model.  This framework balances the consumption of food and allocation to metabolic costs, and can be used to evaluate trade-offs of energy allocation. Using this approach, we finally had an answer to our initial question; we estimated a single thread cost 1 joule to manufacture and byssus production can represent up to 50% of a mussel’s energy budget.

About the research

Sam LaFramboise places mussels in mesh enclosures. (Photo credit Laura Newcomb)
Sam LaFramboise places mussels in mesh enclosures. (Photo credit Laura Newcomb)

We performed this experiment because we were interested in connections between organism energetics, physiology, and the mechanical properties of organisms, a discipline known as “integrative ecomechanics.” We were interested in whether organisms prioritize investing energy into structures that increase survival, and the extent to which this allocation would result in less energy for other processes such as growth. In our case study, we were able to show conclusively that mussels prioritize putting their resources into mechanical attachment rather than growing larger. We wonder if this resource allocation rule might be evolutionarily advantageous for mussels – avoiding getting swept off of a rock, away from potential mates or from suitable habitat, might be a higher priority than growing larger. Many organisms produce specialized materials such as bone, tendons, shell, scales, teeth, and spider silk that have unique mechanical properties that allow the organism to perform a range of ecological functions. Organisms producing these materials may be ‘more fit’ if they are able to use these mechanical structures to efficiently acquire resources, protect from predators, disperse, or remain attached to their habitat. We are left to ponder when and why other organisms prioritize resources towards making and strengthening these specialized mechanical structures vs. growth.

About the authors

This collaborative project was sparked initially by a conversation between three of the authors (doctoral student Laura Newcomb and faculty advisors Emily Carrington and Ken Sebens), who wondered how much it cost a mussel to produce a byssal thread. They conceived the manipulative experiment to demonstrate a trade-off between byssus production and growth, and engaged the three undergraduate student authors (Michelle McCartha, Katie Harrington, and Sam Framboise) whose combined brainpower and creativity designed and carried out the procedure of manipulating mussel byssal thread production with daily trips to the dock to cut byssal threads.

Michelle – Working with this team at Friday Harbor Laboratories was an amazing experience which, as a non-traditional student and mother, was once in a lifetime. As I gained more exposure to experimental design, collaborative research, and scientific communication, I fulfilled the requirements of my Marine Biology Minor at The University of Washington and afterward was incredibly inspired and determined in my pursuit of graduate studies. The tools which were developed through this research, and the collaborations and friendships that I‘ve made, will follow me through the remainder of my academic career and beyond.

Michelle McCartha counting byssal threads on the floating dock. (Photo credit Katie Harrington)
Michelle McCartha counting byssal threads on the floating dock. (Photo credit Katie Harrington)

Katie – I hadn’t realized this at the time, but working on this project turned out to be a pivotal experience in my research career. I learned that co-authors can also be mentors and that there’s power in collaborating, idea sharing, and most of all listening to our colleagues’ research journeys. Moreover, in terms of understanding what it meant to conduct experimental studies, it was liberating to be shown a shed of materials and encouraged to try building different cage designs to see what worked. It expanded my view of science; beyond asking and answering questions, it suddenly involved design thinking and returning to the drawing board multiple times. It was thrilling to realize what’s possible when you get creative.

Katie Harrington and Michelle McCartha ready to deploy mesh enclosures and mussels. (photo credit Laura Newcomb)
Katie Harrington and Michelle McCartha ready to deploy mesh enclosures and mussels. (photo credit Laura Newcomb)

Laura – My involvement in this project taught me about the wonderful and creative ways student researchers can open the door to a new aspect of a research question, as well as challenge my own preconceived notions and norm. I continued to collaborate with students in my current work to explore new tools and directions, and I hope they get as much out of it as I do!

Molly – I picked up the modeling part of this project when I was a grad student investigating a different question – if mussels have greater energetic resources with which to grow – greater food availability, lower metabolic cost, greater SFG, etc – do they also produce a greater number of byssal threads? My experiments in the lab and field both suggested that no, this was not the case. This manipulative study provided a better foundation for incorporating byssal thread production into an energetic framework, because it evaluated a trade-off between byssus vs. growth. This different energetics model methodology was sparked by conversations with faculty advisors Ken Sebens and Emily Carrington.

Read the article in full here.

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