Sea silk is an extremely fine, rare, and valuable fabric that is made from the long silky filaments or byssus secreted by a gland in the foot of pen shells. The byssus is used by the clam to attach itself to the sea bed.
Sea silk was produced in the Mediterranean region from the large marine bivalve mollusc Pinna nobilis until early in the 20th century. The shell, which is sometimes almost a meter long, adheres itself to rocks with a tuft of very strong thin fibers, pointed end down, in the intertidal zone. These byssus or filaments are spun and, when treated with lemon juice, turn a golden color, which never fades. The cloth produced from these filaments can be woven even finer than silk, and is extremely light and warm.