Well, it is important to mention that not all mussels are found on pilings. Mussels attach themselves to any type of hard substrate in the intertidal regions, including pilings. On pilings the top most mussels indicate the high tide line.
To go off on a random tangent, here is a yummy mussel recipe: http://allrecipes.com/HowTo/Cooking-Mussels/Detail.aspx
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