All crabs must to be cleared on a single pirate. Crabs are special pieces in the Bilging puzzle, and will only appear once you have reached the "five star" level in the puzzle. Your current star rating will appear at the lower left corner. You can skip earlier levels, depending on your standing in Bilging. For example, as "Distinguished" rank you will start at "four star" level and at "Respected" rank you will start at "five star" level. Once the crabs begin appearing on the puzzle, raise them above the water line for them to clear. Avoid using pufferfish near the crabs as they will destroy them and it will not count as a clear. Clear as many pieces as possible and hope that a few crabs appear. The longer you bilge above "five star" level, the better.