There is a belief that seashells contain the spirits of the sea. These spirits should be in the ocean and they do not belong in the home. The seashells do not belong in any enclosed space.
It is believed that if they are in a small space then the spirit within the shell is locked up. People often collect seashells so they can make memories of their vacation. While they are taking these shells home they are locking in spirits and they are bringing bad luck to their home. It is better to admire the beauty of the shells at the beach than to take them home and trap the spirits.