Hanging by a thread – secrets of mussel’s attachment

By Elizabeth Cooney

WSG Communications Fellow, Washington Sea Grant

Though they’re less glamorous than oysters, mussels dominate rocky coastlines and support aquaculture worldwide. Now Washington Sea Grant-supported researchers at University of Washington’s Friday Harbor Labs are investigating potential climate-related threats to the amazingly tough mussel threads that anchor them to wave-pounded rocks and docks.

The unassuming but commercially valuable mussel dominates temperate seas worldwide, clinging to rocks and docks by a cluster of thread-like anchors called the byssus or “the beard.” The byssus’s unique protein matrix gives each thread extraordinary strength, even in salt water. But will byssal threads still hold fast as the seas become warmer and more acidic? Supported by Washington Sea Grant, UW biology professor Emily Carrington is trying to answer that question and determine whether food supply and spawning may also affect byssal strength. The answers she’s finding aren’t simple, but they’re sometimes surprising.

Carrington and her colleagues have found that impacts vary depending on mussel species. When temperature rises, the West Coast’s native Pacific blue mussel (Mytilus trossulus) grows fewer threads, but the naturalized Mediterranean mussel (M. galloprovincialis) grows more. The native 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 Mediterranean mussels may outcompete the native species as the ocean becomes warmer and more acidic.

Measuring byssal strength is a straightforward process; the researchers yank mussels from the rocks with a force gauge or stretch individual threads in a tensometer to determine extension and breaking point. Working with Carrington, graduate student Laura Newcomb has investigated byssal strength in laboratory experiments and field assessments. She found that byssal strength and elasticity decline when seawater pH drops below 7.6. Since pH ranges from just above 7.0 to well above 8.0 in the bays where the mussels grow, this threshold presents a real danger. Fortunately, 66°C, the temperature at which byssal strength drops off, is still well above the average temperatures in Puget Sound.

The picture gets more complicated: pH and temperature seem to make less difference when mussels spawn, apparently because they’re already shifting energy to producing sperm and eggs rather than byssus. But for the mussels themselves the imperative is always simple: hold tight or die. That’s gotten the attention of Ian Jefferds, the general manager of the Penn Cove Shellfish Company on Washington’s Whidbey Island, where Carrington and Newcomb conduct their fieldwork. Jefferd’s operation remains healthy and seems sustainable, but he doesn’t take things for granted: “We don’t want a situation. Why wait for one to happen?”

The effects of warming, acidifying waters on byssal strength have “important implications for a major global industry,” notes Carrington. Many countries, including China, Canada and Spain, grow even more mussels than the United States, and Carrington hopes her lab’s work will also benefit them. “What we’re learning here in Washington will be transferable to other industries.”