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 Arks. 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 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). This is a tropical Eastern Pacific species. In Mexican waters it ranges from the northern end of the Gulf of California, along the mainland coast, to Guatemala.

Ecology and Behavior: Alternating Ark Shells are suspension feeders, which feed on plankton and suspended organic matter. They are prey for fish and gastropods. Alternating Ark Shells are gonochoric and reproduce sexually, through broadcast spawning. There is no mention in the available literature of  Alternating 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:  Barbatia alternata and Byssoarca alternata.