Changeable Ark Shell, Lamarcka mutabilis

Changeable Ark Shell, Lamarcka mutabilis. Size: 3.9 cm (1.5 inches) x 2.3 cm (0.9 inches). Shell collected off the beach at Km 26, Cero Colorado, Baja California Sur, March 2010. Identification courtesy of Bob Hillis, Ivins, Utah.

The Changeable Ark, Lamarcka mutabilis (G.B. Sowerby I, 1833), is a bivalve mollusk that is a member of the Arcidae Family of Arks. They are known in Mexico as arca variable. As their common name suggests, they can vary in shape between specimens. The shell has an oblong profile, with a long, straight, hinge. The beaks are well separated and wide. The exterior is marked with fine radiating ribs and the posterior ends have five or six heavy ribs. The shell is a brownish and the interior is white with brown or purple blotches and is covered by a thick scale-like yellowish brown periostracum. The interior is white with brown or purple blotches. The Changeable Ark reach a maximum of  4.4 cm (1.7 inches) in length and 2.6 cm (1.0 inches) in height.

Changeable Arks are found under rocks and in sand, intertidally to depths of 61 m (200 feet). They range from Scammon’s Lagoon, Baja California to Peru, including Clipperton, Cocos, Galapagos, and Revillagigedo Islands. They are found throughout the Sea of Cortez.

Synonyms include Arca constricta, Arca crossei, Arca mutabilis and Byssoarca mutabilis.