The gooseneck barnacle (a 'stalked' barnacle) has an edible foot, and is especially valued along along the coasts of Portugal, Spain and Morocco. They are now being farm raised in several areas, including Washington State.
During the Middle Ages, gooseneck barnacles were thought to be developing geese, and when they reached a certain size they would fall to the water and become full grown geese.
Barnacle geese breed in the summer north of the Arctic Circle, and spend their winters on the northern European coasts, seeming to appear from nowhere, hence the myth. Since these geese (the barnacle goose and brant goose) came from shellfish, they were considered to be fish and could be eaten on days that meat was forbidden.