Stiff Pen Shell

Shell size to 300 mm; shell fan-shaped, triangular. Hinge area straight, representing larger side of triangle. Surface sculpture of about 15-25 narrow ribs separated by larger interspaces; ribs bearing regularly spaced, fluted spines. Large muscle scar crosses above border of nacreous area (pallial line). Byssus at pointed extremity anchors penshell into seagrass bottom. Gaping, narrower side of triangle oriented upward. Color dark-olive brown. Mantle grayish- or greenish-brown, mottled with flecks of white and darker color. The additional photos show how the animal can "zip-up" the inner mantle lobe to help regulate the incoming flow of water. The other supplementary image (by Dr. Peter Bush, SUNY/Buffalo) shows the regular arrangement of the platelets (crystals) that comprise the internal, nacreous shell layer. The platelets are very thin, translucent, which imparts the iridescence typical of that kind of shell layer. Compare this species with Atrina seminuda (Lamarck, 1819), which has ribs limited to upper half of fully grown shells, a muscle scar well within the nacreous area, and orange mantle. The small shell (about 57 mm) in the supplementary image on the extreme right was found by Kimberly Nealon on Captiva in December 2015. It illustrates the change in proportions as the shell grows in this genus, with younger individuals being relatively longer.