barnacle
US /ˈbɑːr.nə.kəl/
UK /ˈbɑːr.nə.kəl/
1.
a marine crustacean with an external shell, which attaches itself permanently to a variety of surfaces including ship bottoms, rocks, and whales.
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The ship's hull was covered in barnacles.
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We saw many barnacles clinging to the rocks at low tide.