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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.
