Hanging by a Thread
Biologist Emily Carrington probes the secrets of the humble mussel’s powerful attachment, and how mussels will fare as sea chemistry changes.
By Elizabeth Cooney, WSG Communications Fellow
By Elizabeth Cooney, WSG Communications Fellow
Hundreds of invertebrates along Washington’s shores have evolved ways of clinging, sticking, and anchoring themselves against crashing waves. One of the most successful is the humble mussel, which dominates turbulent rocky intertidal zones in temperate seas worldwide. But will the remarkable structure that enables mussels to stay put continue to work as the seas become warmer and more acidic? That’s one question University of Washington biology professor Emily Carrington is trying to answer. The answers she’s finding aren’t simple, but they’re sometimes surprising.
The unassuming but commercially valuable mussel encrusts rocks, docks, and pilings by producing a cluster of thread-like anchors called the byssus. The unique protein matrix that makes up byssal threads gives each strand surprising strength and stretch, even in salt water. Carrington ventured into the world of mussel attachment during her postdoctoral days when she discovered that the byssus was incredibly understudied. Since then, she has remained at the leading edge of efforts to characterize it, seeking to discover how byssal production and strength might differ between species or depend on a mussel’s environment. Her current research, supported by Washington Sea Grant, investigates several questions regarding mussel attachment: Do lower pH and higher temperatures affect the byssus? Does low food supply or spawning drain resources from byssal production? Will different mussel species respond differently to changing environmental conditions? To answer these questions, Carrington has embarked on a data-collecting journey in the lab and in the field.
Laura Newcomb, a graduate student working with Carrington, has conducted laboratory experiments and field assessments of the effects of seawater conditions on mussels since 2013. In the lab, Newcomb and previous student researchers observed that when seawater’s pH (a standard measure of relative acidity) drops below about 7.6, the strength and elasticity of byssal threads decline. Since pH can range from just above 7.0 to well above 8.0 in the bays where mussels grow, they do encounter this threshold in the field. Likewise, 19°C is the “magic” temperature above which byssal strength drops off, and rising temperature seems to have a more dramatic impact on mussels than falling pH. It is common in research comparing environmental factors to see synergistic effects when more than one condition changes. But in this case, “one really seems to dominate over the other,” says Newcomb.
The results reveal themselves to a simple tug test: When conditions are less than ideal, mussels can be pulled loose more easily. The Carrington lab uses quantitative approaches to measure attachment strength, like yanking mussels from the rocks with a force gauge and stretching individual threads in a tensometer to measure extension and breaking point. But the implications of byssal thread quality are simple for a mussel: hold fast or die. “It’s a binary thing,” explains Newcomb. Sometimes she revels in the simplicity of looking at “just one thing that really determines survival.”
While the possible outcomes from environmental trends are relatively straightforward, impacts vary
depending on scenario and species. When temperatures rise, the native bay or Pacific blue mussel (Mytilus trossulus) grows fewer threads but the naturalized Mediterranean mussel (M. galloprovincialis) grows more. Another native species, the California mussel (M. californianus), shows no change as waters warm but is more sensitive to low pH and less resilient in low salinity. These results suggest that as the climate warms and water temperatures rise, Mediterranean mussels may outcompete the native species. As it happens, the less-adaptable M. trossulus is also called “the foolish mussel.”
Another factor that Carrington suspects plays a role in byssus production is changing energy demand over the course of the reproductive cycle. M. trossulus spawns in spring, M. galloprovincialis in winter, and M. californianus year-round. Laura Newcomb’s lab experiments found that the negative effects of reduced pH and high temperature were less obvious during spawning. Carrington suspects this is because there is little additional damage pH and temperature can do to the already-weakened byssal threads produced by mussels allocating their energy to eggs and sperm. In addition to observing these trends in the lab, Carrington and Newcomb are conducting field studies to see if the same responses occur in the mussel-encrusted bays and coves of Whidbey Island.
The mussel byssus has long been a subject of fascination for Carrington. In recent years, however, she has discovered that as climate and ocean conditions change, her research could also provide valuable insights to the aquaculture industry. She began examining the ways environmental conditions affect mussel attachment in the bays and inlets where mussel farmers cultivate their stock. Along the way, she has developed an unexpectedly close working relationship with a resident team of mussel farmers, whose input and collaboration have become an integral part of her lab’s research.
Early on, Carrington tapped Ian Jefferds, the general manager of Whidbey Island’s Penn Cove Shellfish, as her go-to field contact. Penn Cove, founded in 1975, is the oldest and largest mussel farm in the United States. Its operation depends on wild mussels to seed each year’s harvest, making it a natural laboratory for studying mussel attachment, both in the wild and in aquaculture operations.
The collaboration started small but grew when Penn Cove helped Carrington install monitoring equipment on one of its mussel rafts. Carrington’s sensors supplemented a system already in place at the farm and set the stage for joint data collection. During setup, Jim Nagel, Penn Cove’s resident engineer, worked closely with Newcomb to make sure the telemetry equipment would serve her needs as well as those of the company. Over time, and as the research began to entail regular trips into the field, Jefferds and his team’s input became even more deeply implanted in the research effort. “They are taking these skiffs out to the harvesting barge,” Carrington explains, “and sometimes they might notice the water’s a little greener or the mussels look a little different. A really important part of our research is communication with the people who are actually on the water every day.”
For Newcomb, the anecdotal information Penn Cove’s employees provide is extremely useful for interpreting data. Carrington and Newcomb are also quick to point out that the farmers were the first to observe changes in mussel attachment. “This research is really a way to quantify what they’ve been observing all along!” says Newcomb.
For their part, mussel farmers appreciate having access to data from Carrington’s monitoring systems. With some skillful maneuvering by Nagel and help from the UW Applied Physics Lab’s Emilio Mayorga, information collected at the perimeter of the mussel rafts is sent remotely to a computer in the company warehouse, then uploaded to the visualization system on the Northwest Association of Networked Ocean Observing Systems (NANOOS) website. Here, data can be viewed by Penn Cove employees, members of the Carrington lab, and the public at large. “It’s interesting for our employees, because they have questions too,” says Jefferds. “It’s one thing to read about a study that happened in a lab, but allowing our crew to assist and watch Laura and Emily has provided opportunities to ask more questions and learn what’s going on.”
Jefferds sees other reasons to be involved. Although his operation remains healthy and seems sustainable, he doesn’t take things for granted. “We don’t want a situation. Why wait for one to happen?”
This proactive spirit makes the mussel attachment project unusual. Much aquaculture-related research, such as recent work on oyster settlement and fish farming, has mobilized in response to challenges that the industry already faced. But, as Newcomb has learned while working with Carrington and the Penn Cove team, talking about farming in the face of climate change does not have to mean doom and gloom. “I’ve changed the way I talk about it,” she says, emphasizing the preventive rather than reparative role her research could play. “As a student, I am learning the perspective of the farmers who have to ask, ‘At what point do you start switching your practices’?”
For any future problems, there are many potential solutions. For example, Penn Cove grows both the native M. trossulus and the warm-water-friendly M. galloprovincialis. Jefferds and his team could use environmental data to determine which species will fare better and manage accordingly. Carrington’s lab also has plans to investigate whether altering the food supply could strengthen mussel attachment during times of high energy demand such as spawning season.
Although Carrington and her lab have made progress in discerning patterns in mussel attachment, many questions remain. “I often give these seminars,” she explains, “and I show mussel attachment over the course of the year, and there’s such a strong pattern. They’re twice as strong in winter as they are in summertime. After 15 years I still don’t know why.” By working together, however, the farmers and researchers have a better chance of uncovering answers.
Many other countries, including China, Canada, and Spain, have mussel-growing industries larger than the United States’. “What we’re learning here in Washington will be transferable to all these other industries,” says Carrington. She muses on the way her offbeat research vocation has taken on global significance: “It is really cool that the problem I happen to be passionate about, how mussels attach underwater, has important implications for a major global industry.”
A flotilla of large wooden platforms bobs in neat rows just off the Whidbey Island shore. These unassuming rafts, together with a smaller collection in Quilcene Bay on the Hood Canal, support the entire crop of Penn Cove Shellfish, the largest mussel farmer in the United States. “That and a couple bucks will get you a cup of coffee,” jokes Ian Jefferds, Penn Cove’s general manager. He’s spent four decades working to sustain and grow the business his family created in 1975. “We’ve gone from just a few employees to around 84 people,” he says proudly.
Compared with large-scale farming operations on land, Penn Cove Shellfish is unobtrusive. Its office and warehouse sit atop a hill overlooking its namesake bay. Half a mile away, a small boat launch points toward six rows of 130-foot rafts. The Mytilus, one of two custom-built 64-foot harvesting vessels, cozies up to a raft as its crew hauls in lines encrusted with mussels ready to eat. From a distance it’s impossible to imagine the crop that the Penn Cove team works five days a week to extract from this suspended garden. Peering through the top of a raft, however, you get an idea: hundreds of mussel lines disappear into the murk. A single raft may anchor as many as 2,500 lines, each bearing up to 50 pounds of mussels. Harvesting this bounty and preparing it for sale require special equipment; Penn Cove makes its own unique tools at its own mainland fabrication shop in Burlington.
One contraption, the “de-bysser,” is custom-designed to remove the byssal “beard” from each mussel prior to bagging. The byssus may be unappetizing, but it plays a vital role, anchoring the growing mussel to the line for more than a year until it reaches market size.
Jefferds recalls one year when some of his Quilcene Bay crop showed alarmingly poor byssal strength and he could not find a cause. Then, a few years later, an unimpressive larval set mysteriously grew into a healthy crop. In the intervening period, UW biologist Emily Carrington (see “Hanging by a Thread”) deployed sensors on his rafts, and Jefferds’ team was able to pull up pertinent water-quality data. “We saw that we’d had a huge spike in our chlorophyll levels in the past week or so,” indicating a phytoplankton feast for the mussel larvae. “By having water quality data, maybe we can begin to better understand why some of these things are occurring, both good and bad.”
Unlike some other recent aquaculture-related research, Penn Cove’s collaboration with the Carrington Lab was not motivated by poor growth, disease, or larval die-offs. In fact, Penn Cove farm manager Tim Jones says excess mussel weight is a more immediate threat than any of those ills. “It’s no fun when they sink,” muses Jones, eyeing a slumping raft. “You have to involve diving and cranes…. Better to avoid that.” Jefferds would rather his team trim their farming strategy to avoid such losses.
Jefferds has come to appreciate his partnership with Carrington on several levels. Beyond ensuring the continued resilience of his yield, he values exploration and the acquisition of knowledge in its own right. “I’m sure we’ll observe things that will provide more questions than answers,” he muses. Meanwhile, shellfish enthusiasts around the country will continue happily consuming his mussels. Elizabeth Cooney