Do you see the slender whitish "string" coming from this mussel? Those are called byssal filaments and they are why it can be difficult to pull a clam or mussel off of rocks. The mussel excretes these filaments in order to anchor itself to rocks in the ocean. It can take them from one day to a week to excrete the filaments. Other marine and freshwater bivalve mollusks also have these filaments.
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