The main form of protection for bivalves is the hard pair of shells that envelop and seal closed the soft body inside. But having this defense isn't quite enough to protect them from the many predators that will hound a slow or non-moving animal. As we saw in previous diaries, some species dig underground (clams) and others have even evolved limited mobility (scallops).
Another defensive mechanism they can use is to become firmly attached to a rock or other hard substrate so that they can't be picked up and carried away. An extreme example of this is found in oysters, which create a powerful cement to permanently attach themselves to rocks. An equally effective means of attachment, but one that also allows some degree of movement, can be found in mussels, and these are known as byssal threads.
Like most bivalves, mussels don't have much use for mobility. They have no need to actively search for food because, being filter feeders, they simply let their food (plankton) come to them. Unlike burrowing bivalves, mussels (and I'll be referring mainly to the blue mussel, Mytilus edulis) are epifaunal, meaning they live on the surface of the ocean bottom, not under it.
Although nearly all bivalves are capable of secreting byssal threads when young, mussels are nearly unique in carrying this ability into adulthood. In the seafood industry, this mass of threads is called a "beard" and is torn off and discarded before the animal is sold.
At the base of the foot is a byssal gland which secretes the byssal threads. It starts out as a fluid that, when it comes in contact with seawater, solidifies. The foot can also secrete an enzyme that dissolves the base of the thread. In this way it can create and destroy these threads, allowing the mussel to move across a rock, much like a mountain climber uses ropes and anchors to move up a rock face or rappel down one. This process is normally restricted to the young mussels. Adults tend to become sedentary, secreting a mass of threads and staying put for the rest of their lives.
The byssus itself is made out of a collagen protein, which explains it's elasticity (collagen is what gives our skin its elasticity as well. Pinch your arm and notice the skin springs back to its regular position when you let go.) The only part that is different, on a molecular level, is the very tip of the the thread where there is a small plaque which glues the thread to the surface of the substrate. When the thread is broken this adhesive tip remains in place and the used byssal threads are left behind, dangling by the plaque.
So, just how strong are the byssal threads these animals produce as anchors? Take the achilles tendon in your foot, the strength and flexibility of which allows you to walk, run and jump and still allow your feet to balance your weight. The byssus of a mussel is about five times as strong and up to fifteen times as flexible as that tendon of yours. In fact, researchers have been studying the mussel's byssus to try to find an artificial substitute for replacing damaged ligaments and tendons.
Interestingly, the strength (known as the tenacity) of the byssus of any one individual mussel changes seasonally, being strongest in the spring and progressively weaker throughout the summer. Although I haven't found any explanation for this, I'm assuming it is diet related since both the quantity and quality (diversity) of plankton peaks in the spring.
Mussels live in huge colonies, called "beds". Hundreds, or even thousands, of individuals live connected to one another and the rocky substrate that forms its base. This proves to be very useful for aquaculturists. Mussel farms can grow large numbers of these animals using suspended ropes hanging from bouys. The initial colonizing animals will attach to the ropes and in turn serve as substrates for more mussels. To harvest the crop, the mussel farmer simply hauls up the ropes and pulls off the mature bivalves as needed. Blue mussels are so easily farmed this way that few are actually collected wild anymore.
Living in large colonies benefits the mussels in several ways. It aids in reproduction, for one. Since the gametes of these mollusks are fertilized by chance in open water, the presence of thousands of others close by increase the odds of egg fertilization. And protection is afforded against predators that can penetrate the shells, like starfish and oyster drill snails, as well as those that could carry the animal away, like sea gulls.
In the sea gull's case, the bird is unable to remove one single animal away from the colony because of the combined byssus strength of the mussel and its surrounding members. And in the case of starfish, the predator may feed on the outer ring of the colony, but the empty shells left behind protect the other inner individuals from attack.
One often overlooked method animals have of avoiding predators is to live in a habitat that is hostile to their enemies. For example, oysters are so vulnerable to predators because they are unable to either dig underground or move. However, they are extremely tolerant of salinity variations. They can live in nearly freshwater, thus escaping predators that require higher salt levels to survive. Mussels, thanks to the strength of the byssal threads, thrive in the wave-pounded intertidal zone. An environment that would wash away or crush potential predators.
And, unlike most marine predators, mussels can survive hours out of water by closing their shells tightly until the high tide returns.
Above is a photo of a huge bed of mussels. Reviewing what was stated above, predators feeding on these animals will have to overcome the surf, the shell, the byssus strength and, as you can see here, being exposed to air during low tide. In coastal areas around the world, this is what allows this group of animals to become the dominant macro-organism in the intertidal zone.
And one last note that can't be dismissed is the importance these beds are to the young of many other species. The piles of live mussels and their maze of byssal threads provide vital shelter for small snails, crabs and even fish that live their early lives in what is essentially an impenetrable fortress protecting them from their predators.
[This essay was originally posted to Daily Kos on September 22, 2006. Top image has been changed, via Wikipedia Commons.]
Other diaries in this series can be found here.