Atrina rigida (Lightfoot, 1786) is commonly found on the beaches of Sanibel and Captiva during the winter months. Storm winds and waves remove the live, dark-shelled bivalves from the bottom, depositing them in large quantities on the beaches. Pen shells anchor themselves to the sandy bottom using a bundle of silky fibers called a byssus. The large shells are buried with the narrower side of “triangle” oriented upward.
The Stiff Pen Shell, with detail showing opening of the live animal on the left.
(This entry was originally published with an illustration of the live animal of Atrina seminuda.)