Stiff Pen Shell Atrina rigid Stiff Pen Shell Atrina rigid This bivalve has a dark brown shell, with 15 to 20 radiating ribs and short tubular spines. Their posterior muscle scar is outside the shiny inner shell area. These large bivalves grow to 12 inches. They can be found in and around seagrass beds, buried and anchored to hard objects by their byssal threads which protrude from the beak of the shell. With the upper shell half exposed, they filter plankton from the seawater. Small crabs often live within the shell's mantle cavity, and snails take up residence on the exterior of the shell. The pen shell is preyed upon by sea stars and the horse conch. It has a large adductor muscle which is edible. The interior of these shells can be very iridescent and a broken piece of shell often catches the eye of beachcombers. There are 3 species of penshells on the west coast of Florida. Live shells should never be taken from any Florida State Park.