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.

Phylogeny: The Alternating Ark, Calloarca alternata (G.B. Sowerby I, 1833), is a bivalve mollusk that is a member of the Arcidae Family of Ark Shells. The genus Calloarca is one of thirty-five genera in this family, and there are ten species in this genus. These shells obtain their name from their shell, which has alternating teeth along the hinge that allow the shell to close securely.

Description: Alternating Ark Shells have 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.

Habitat and Distribution: Alternating Arks 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). They are a tropical Eastern Pacific species that are found in all Mexican waters of the Pacific Ocean with the exception that they are absent from the northern portions of the Sea of Cortez.

Ecology and Behavior: Alternating Arks are suspension feeders, which feed on plankton and suspended organic matter. They are preyed upon by fish and gastropods. Alternating Ark Shells are gonochoric and reproduce sexually, through broadcast spawning. Their engagement in any type of commensal, parasitic, or symbiotic relationship has not been formally documented. From a conservation perspective the Alternating Ark has not been formally evaluated. However, they are fairly common with a relatively wide distribution and should be considered to be of Least Concern.

Synonyms:  Barbatia alternata and Byssoarca alternata.