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.

Phylogeny: The Changeable Ark, Lamarcka mutabilis (G.B. Sowerby I, 1833), is a bivalve mollusk that is a member of the Arcidae Family of Ark Shells.  The genus Lamarcka is one of thirty-five genera in this family, and there are twelve species in this genus. They are known in Mexico as Arca Variable.

Description: As their common name suggests, Changeable Arks 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 brownish, and covered by a thick scale-like yellowish brown periostracum. The interior is white with brown or purple blotches. Changeable Arks reach a maximum of  4.4 cm (1.7 inches) in length and 2.6 cm (1.0 inches) in height.

Habitat and Distribution: Changeable Arks are found under rocks and in sand. They live intertidally, and to depths of 61 m (200 feet). They are a subtropical to tropical Eastern Pacific species. In Mexican waters they range from Scammon’s Lagoon, Baja California to Guatemala, including the Revillagigedos Islands. They are also found throughout the Gulf of California.

Ecology and Behavior:  Changeable Ark Shells are suspension feeders, which feed on plankton and suspended organic matter. They are prey for fish, crabs, sea stars, and gastropods. Changeable Ark Shells are gonochoric and reproduce sexually, through broadcast spawning. There is no mention in the available literature of Changeable Ark Shells engaging in any types of parasitic, commensal, or symbiotic relationships. From a conservation perspective they have not been formally evaluated however they are fairly common with a relatively wide distribution and should be consider to be of Least Concern.

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