Alternata Ark Shell, Calloarca alternata

Alternata Ark Shell, Calloarca alternata. Size: 3.3 cm (1.3 inches) x 1.5 cm (0.6 inches). Shell collected off the beach at Km 26, Cero Colorado, Baja California Sur, March 2010. Identification courtesy of Bob Hillis, Ivins, Utah.

The Alternating Ark, Calloarca alternata (G.B. Sowerby I, 1833), is a bivalve mollusk that is a member of the Arcidae Family of Arks. They obtain their name from their shell that has alternating teeth along the hinge that allow the shell to close securely. The shell has a rectangular shape with a rounded anterior and a sloped posterior end. The beak is low. There are radiating weak lines across most of the shell, but the posterior end is strongly ridged, forming a scalloped margin. The shell is grayish white in color.  Alternating Ark Shells reach a maximum length of 4.1 cm (1.6 inches) in length and 1.9 cm (0.7 inches) in height.

Alternating Ark Shells attach themselves to rocks with fine silky byssus, giving rise to their former genus name – Barbatia (bearded one). This species lives at depths between 10 m (35 feet) and 100 m (330 feet) and ranges from the northern end of the Sea of Cortez to Ecuador.

Synonyms include Barbatia alternata and Byssoarca alternata.