United States, Florida, Lee, W. Gulf Drive, Sanibel, Lee Co., FL
26.437068 -82.127505 +-500m.
Medium-sized red-striped barnacles like these were found to be a regular colonist of the abundant stiff pen shells (Atrina rigida) thrown up on the beaches at Sanibel. In almost every example I found, neither the pen shell nor the barnacles (nor the remanant oyster half-shells under them) were alive when they got to the beach. I hope there's a barnacle expert out there who might narrow this ID a little more.