Originally the barnacle goose — transferred to the shellfish through a medieval legend that geese hatched from them. Remarkable folk biology.
A marine crustacean with a hard calcareous shell that attaches permanently to rocks, ship hulls, piers, and other submerged surfaces.
From Middle English 'bernacle' or 'bernake,' originally denoting the barnacle goose (a species of wild goose), later transferred to the crustacean. The connection lies in a medieval legend: people observed that barnacle geese appeared in northern Europe without anyone seeing their nests, and simultaneously noticed goose-shaped shellfish (goose barnacles) attached to driftwood. They concluded that the geese hatched from the shellfish — the barnacle produced the bird
The medieval belief that barnacle geese hatched from barnacle shellfish had a practical consequence: since the geese were 'born from the sea,' some clergy argued they were fish rather than fowl and could therefore be eaten during Lent and other fast days when meat was forbidden. Pope Innocent III had to issue a specific papal decree in 1215 prohibiting this creative loophole.