The Half-naked Pen Shell, Atrina seminuda (Lamarck, 1819), is the rarest among the three species of pen shells found in Southwest Florida. It differs from the more common Stiff Pen Shell, Atrina rigida, by having its large adductor muscle scar well within the boundaries of the iridescent (nacreous) area inside shell valve (arrow on shell image). The Stiff Pen Shell has that scar “protruding” beyond those boundaries. In addition, the color of the mantle in the living Half-naked Pen Shell is orange, whereas the mantle in the Stiff Pen Shell is light grayish-green. Other than that, the two species share similar sizes, reaching about 240 mm (slightly less than 10 inches), and similar shades of dark-brown shell color.
The Half-naked Pen Shell, Atrina seminuda. Views of shell and living mollusk. Arrow on top left shows adductor muscle scar on internal surface. Shell photos by José H. Leal, photo of living animal by Amy Tripp.