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.
![Atrina rigida](https://static.wixstatic.com/media/13e525_41847ed1c12d44bf8475b1401fdf17a0~mv2_d_2814_1500_s_2.jpg/v1/fill/w_147,h_78,al_c,q_80,usm_0.66_1.00_0.01,blur_2,enc_auto/13e525_41847ed1c12d44bf8475b1401fdf17a0~mv2_d_2814_1500_s_2.jpg)
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.)