UW researchers explore the shifting environment of mussels

UW researchers in the San Juan Islands have been studying mussels and their environment, researching their changing world as different climates and human activity take effect.

Mussels play a surprisingly large role in marine ecosystems. They provide habitats for smaller organisms and species, as well as shelter and protection from harmful factors such as heat, desiccation, and predators. Stabilizing soft sediment of the ocean floor by attaching to roots, transferring energy by converting algae into tissue, and filtering water are other valuable mussel occupations, in addition to their status as a decent entree for fish, crabs, birds, and people.

“They are often referred to as a foundation species,” said Emily Carrington, UW professor of biology. “They provide several ecosystem services; mussels filter water very effectively, a liter of water per hour.”

Environmental factors affecting changes in mussel habitats throughout the Puget Sound and beyond could generate a ripple of response up the oceanic food chain; increasing fluctuation in ocean temperature, acidity, and weather conditions may begin affecting mussels negatively, even accounting for the variations in mussel types. Mytilus californianus, for example, thrives among crashing waves and dominates on the outer coast, while in bays, the more delicate Mytilus trossulus serves as a local delicacy.

“We are trying to identify what environmental conditions promote strong mussel attachment, for mussel farms and natural habitats,” Carrington said.

What enables mussels to serve so many functions is the byssus, a collection of extracellular threads formed by the mussel’s foot, a muscular organ the mussel uses to move. Between 40 and 100 threads are employed by the mussel in tethering to various substrates. The byssal threads, which are roughly three to 10 times the width of a human hair, have a lifespan of one to two months and are replaced by the mussel as needed.

“We think mussels are weaker in summer because they make poor quality threads that don’t last as long,” Carrington said. “In winter they seem to be stronger and more durable.”

However, through research at the UW Friday Harbor labs, Carrington and colleagues have established that byssal threads become 60 percent weaker in water that is 15 degrees Fahrenheit above the mussel’s native water temperature. Warming summer water weakens the threads already, and further heat renders the mussels even more susceptible to looser holds.

“What we’re trying to do is isolate a few variables and how mussels make threads at those temperatures so we can see how it differentiates by temperature,” said Laura Newcomb, a graduate student working on the project.

In the Friday Harbor labs, researchers expose the mussels to one environmental variable at a time in water tanks. For example, they’ll acidify the water for a set amount of time, then remove the mussels and test the strength of the byssal threads by pulling them apart with a machine that records the force necessary to tear the threads. After the tearing, the mussel can regrow the byssus.

“We want the field studies linked back to the environment, so we can try to pinpoint specific factors and see what’s actually producing this change of strength,” Newcomb said.

Other long-term studies in the Friday Harbor labs have also begun to show significant thread weakness resulting from increased ocean acidification. At high levels of carbon dioxide concentration in the ocean that are expected to be the norm soon, wave forces 40 percent lower than average dislodge the weakened mussels.

According to a summary published March 10 by Carrington and Michael O’Donnell, a colleague and fellow biology professor, mussels with a weakened ability to attach could cause significant ecological shifts in rocky intertidal communities and a frustrating economic loss to an aquaculture industry at $1.5 billion annually.

“Understanding the effects on different species, since we focused on trossulus and can work on californianus next, is important,” O’Donnell said. 

Varying byssal thread strengths may have further implications for the aquaculture industry. For example, O’Donnell cited an oyster farm in Oregon that discovered the typical practice of shifting water levels in oyster tanks to keep the water fresh preserved more oysters, since photosynthesis makes water become more acidic overnight, which can damage oysters. Such examples may prove illuminating in further studies of mussels.

“We see mussels naturally disappear in the absence of predators,” Carrington said. “But there is more to it than that … we realized they just get weak, they literally loosen their grip.”

Such changes in response to environmental factors may seem obvious conclusions, yet the slowly decreasing pH of the marine environment and the ever-increasing shifts in ocean temperature prompt such miniscule causes and surprisingly large results.

“It shows even if something is still growing that doesn’t mean it’s fine,” O’Donnell said.

Reach reporter Garrett Black at science@dailyuw.com Twitter: @garrettjblack

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