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Abstract
Not everyone thinks of mussels as heroes. But, to hear some researchers tell it, these sedentary animals, which have about as much charisma as a rock, deserve the title - especially at this time of year.
| Mussels live in a very challenging habitat. |
Marine mussels occupy the high intertidal zone on rocky coasts, and they pay for their residence on such real estate by being alternately pounded by waves and fried by the sun. "This is one of the most physically demanding habitats on earth," says Emily Carrington, who studies mussel biomechanics in the Department of Biology at the University of Rhode Island at Kingston.
Humans can't tolerate such forces, says Carrington. "We'd get smooshed to smithereens if we sat on the rocks and let breaking waves crash on us." And yet these bivalves, which escape the notice of most humans (except those contemplating dinner), have evolved special mechanisms to withstand such forces without being crushed or pulled off the rock.
| Byssal threads are "nature's bungee cords." |
It is this latter trait - the ability to cling to a rock under extreme tidal forces - that attracts the attention of Carrington and other researchers in the field of biomechanics. Mussels attach themselves to a rock, a piling, or whatever they reside on by proteinaceous strands called byssal threads, which Carrington calls "nature's Bungee cords" because of their stretchiness. (These threads are sometimes collectively called the beard or byssus.) Remarkably, the animals alter the byssal threads to compensate for changes in weather and waves over short time spans, as well as over seasonal ones.
To figure out how they adapt to varying conditions, Carrington treks out to her field sites on the Rhode Island shore once a month to measure how much force is required to dislodge the mussels from their rocks. She says it takes about twice as much effort to pull the animals off a rock in the winter as in the summer, about seven newtons versus three.
| Attachment strength varies seasonally. |
She speculates that the animals have less attachment strength in the summer because they're devoting more energy toward reproduction and that allocation of limited resources makes them vulnerable this time of year. "They are really weakly attached in August, September, and October" after spending months producing gametes.
But September is also hurricane season, and a big wave could knock them off their exposed perch, says Carrington. Even though the waves in a December or January storm are just as big as hurricane waves in September, they don't do as much damage to mussel beds as the unexpected fall storms do. She estimates that around 10 percent of the animals are pulled off the rocks during a big winter storm but during a hurricane it can be as many as 30 percent.
| They produce more threads in winter. |
Now she's trying to figure out what is different about the winter and summer byssal threads. Is there a difference in the number of threads laid down by the mussel, or are the fibers themselves changed? Thus far she's not entirely sure, but she is doing the tests to find out. Not only is she counting threads when she's at the beach, but, back in the lab, she uses a tensiometer to measure the strength of individual threads. "I'm not really there to watch them every time they put one down," she says, but "it does look like they produce more threads in the winter and fewer in the summer."
Carrington isn't the only one fascinated by these Bungee cords. Herbert Waite in the Department of Molecular, Cellular, and Developmental Biology at the University of California at Santa Barbara is working to understand the day-to-day and tide-to-tide variations the threads display. Noting that "many of the greatest challenges [for a mussel] come in the short term," Waite says that his group focuses on "the correlation between mechanical forces and molecular structures."
| Most materials are deformed after yielding to a force. |
Normally when a material is stretched - be it the plastic that holds a six pack of soda cans together or a steel girder - it gives at a steady rate for a while and then yields to the force, becoming deformed. Think of what the plastic six-pack holder looks like after one of the cans has been removed: The empty ring is larger than the other ones and the plastic is stretched in a nonuniform manner. Once a material is stretched to the point of yielding, it can't go back to its previous unstretched state.
Except byssal threads, says Waite. They are the only material, natural or manmade, that he knows of that can recover from a stretch and yield cycle. And Waite finds that this stretch, yield, and recovery cycle plays an integral part of a mussel's ability to remain attached to the substrate.
| Byssal threads are arranged like spokes on a bicycle wheel. |
Generally a mussel sets down its byssal threads in a radial arrangement like spokes in a bicycle wheel, thus allowing the animal to resist forces coming from any direction. But what is really remarkable, says Waite, is how the force gets distributed among the threads.
If all of the threads are the same length, like lines tying down a hot air balloon, then when a wave comes along and pushes the mussel in a given direction only one or a few threads would be pulled taut, while the rest would be slack. And that's what happens when the first wave hits. One thread bears the brunt of the strain and is stretched until it yields, which causes it to lengthen. Thus, when the next wave hits, the already stretched thread is longer than the rest, which allows the neighboring threads to join in the effort of resisting the wave. Therefore, the stretching, or softening as it is sometimes called, of one byssal thread allows other threads to take on some of the workload, and the mussel doesn't have to rely on a finite number of threads.
| Threads recover from stretching by the next tide cycle. |
Waite points out, excitedly, that if byssal threads were like other known materials, this yield would be "the biscuit for that thread." But instead, the byssal threads can recover from this stretching and return to their previous, unstretched state. The recovery takes somewhere between 20 and 50 hours, but by the next tide cycle, some of the threads will be ready to go again.
What gives the byssal threads their unique quality? According to Waite, it's an unusual combination of proteins. Byssal threads are composed of collagen "with the peculiar added property of having silk-like or rubbery domains. Pull on byssal threads to [the point they] yield and the stiff fibers made from collagen and silk give. This passes the load on to the softer fibers made from collagen and the rubbery protein, which will extend another 50 percent [of its length]," says Waite. "On relaxation, the rubbery fibers will return the silky fibers to a point at which they can reanneal." Together these protein fibers behave in an entirely singular manner.
| Byssal threads are heroic tendons. |
So while many casual observers might continue to overlook mussels, Waite and Carrington appreciate their largely unique approach to life in a tough environment and think materials scientists might learn something from them. As Waite observes, "Much of biomimetics research nowadays is focused not on the ordinary, but on the heroic. Lots of people know about the heroic enzymes from [hydrothermal] vent animals because they withstand temperatures around 95°C. Byssal threads are heroic tendons. They withstand sustained and punishing loads and yet are ready to serve another day."
Rabiya S. Tuma is a freelance science writer based in Oregon and New York.
Susan Wolsborn is Web designer of HMS Beagle.
Mussel Byssus and Biomolecular Materials - reviews recent advances in protein isolation and localization, for both adhesive and fibrous byssus proteins. From Current Opinion in Chemical Biology, 1999, 3:100-105. Full text available from BioMedNet.
Environmental Bioadhesion: Themes and Applications - reviews the adhesive strategies used by some organisms and their applications. From Current Opinion in Biotechnology, 1997, 8:309-312. Full text available from BioMedNet.
Elastomeric Proteins: Biological Roles, Structures and Mechanisms - discusses recent and future directions for research. From Trends in Biochemical Sciences, 2000, 25:11:567-571. Full text available from BioMedNet.
Extensible Collagen in Mussel Byssus: A Natural Block Copolymer and Versatile Collagens in Invertebrates - two recent articles from Science. Full text access requires registration.
Mussels' Stick-on Protein Isolated - highlights research conducted by Herbert Waite. From Access Excellence.
Mussel Adhesive Proteins: Sticky Business - a report from the September 2000 issue of Protein Spotlight. Requires Adobe Acrobat Reader.
Mussel Strength - discusses the role of mussels in the development of a new super glue.
Mytilus edulis - provides environmental requirements and physical attributes of one of the most widely studied mussels. From the UK Marine Special Areas of Conservation.