Picking wild mussels off the rocks is about touch. Go down when the tide is out and stick your hands under the water. Feel around the sides of the rocks crowned by strands of yellow seaweed. Think raccoon. As you rummage around you'll be able distinguish the sharp, curved edge of the shell from the smooth rock it's clinging to.
When you think you found one give it an exploratory tug; if it holds on to the rocks you've got one. If it's small just leave it to keep growing, but if it's a good size, tug it until it comes loose. Give it a quick look and if it isn't dead throw it in the bucket. Later, steam them and serve with melted butter, nothing else.
It takes a bit of courage to crouch down in the ocean and stick your hands somewhere you can't see them, to work by touch alone, unaided by sight. But after you find a few mussels you'll get into the calming rhythm of gathering. Your hands will feel the contours of the rocks and the cold weight of water while you gaze at the glittering shoreline and the birds in the sky.
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(photo of mussel picking by Spike)