Swansea beach is crunchy with shells, and that means that the waters are pulsating with life. Other beaches, less so. Shellfish eat stuff. Other stuff eats shellfish. Yet other stuff eats the stuff that eats shellfish, and so on.
But it’s all out of sight, beneath the water. The shells are the marine population’s equivalent of pizza boxes discarded after a Friday night of revelry, every night being Friday night down there.