The Unequal Spoon Clam, Periploma margaritaceum (Lamarck, 1801), is a rare find from the muddy, back-bay environments of Sanibel, Captiva, and neighboring areas. The shell is small (rarely reaching more than ½ inch in length), oval, thin, with a posterior keel that runs from the beak to ventral (“bottom”) of the shell valve. The area behind the keel is usually covered with a thick, dark periostracum. The shell valves are unequal, with the left valve more inflated, larger than and the overlapping right valve. The vernacular name of the species derives from the spoon-shaped chondrophore found under the beak, inside the shell. The chondrophore holds the ligament, a proteinaceous, flexible structure that naturally acts to keep the valves slightly separated, opening the shell when the clam relaxes its adductor muscles. The shell color is white to translucent-white. The shell in the photo was collected in shallow water at Roosevelt Channel, Captiva Island, by Alice and Ken Piech.
The Unequal Spoon Clam, Periploma margaritaceum, from Roosevelt Channel, Captiva. Photo by José H. Leal.