Ribbed Ark Shell, Acar gradata

Ribbed Ark Shell, Acar gradata. Size: 2.5 cm (1.0 inch) x 1.3 cm (0.5 inches). Shell collected off the beach El Tule, Km 17, Cero Colorado, Baja California Sur, October 2018. Identification courtesy of Bob Hillis, Ivins, Utah.

Phylogeny: The Ribbed Ark Shell, Acar gradata (Broderip and G.B. Sowerby I, 1828), is a bivalve mollusk that is a member of the Arcidae Family of Ark Shells. The genus Acar is one of thirty-five genera in this family, and there are twenty-six species in this genus. In Mexico they are known as Arca Acanalada. The genus name Acar comes from the Greek word Acares, meaning small. This refers to the small size of these shells compared to other ark shells.

Description: The Ribbed Ark Shell is thick, strong, and small in stature. Both valves are of equal shape (equivalve) with a quadrate outline. The anterior is well rounded and the posterior is sloped and folded. They have a low beak, and a straight, serrate margin. The outside is sculpted with numerous, coarse, radial, ribs, which are crossed by concentric ridges. The hinge plate is narrow and has 14-17 anterior teeth and 18-21 posterior teeth. These shells are white. Ribbed Ark Shells reach a maximum of 4.1 cm (1.5 inches) in length.

Habitat and Distribution: Ribbed Ark Shells are found attached to rocks and mangroves. They live in the intertidal zone, and to depths of 37 m (120 feet). They are a subtropical to tropical Eastern Pacific species. In Mexican waters they range from Laguna Ojo de Liebre, Baja California Sur to Guatemala, including the Revillagigedo Islands. In the Gulf of California they are found as far north as Bahía San Marté, Baja California Sur.

Ecology and Behavior:  Ribbed Ark Shells are a poorly studied and understood species. There is very limited documentation of their diet, predation, reproduction or ecosystem interactions. Other species in Family Arcidae are suspension feeders that feed primarily on planktonic algae and suspended organic matter. They are gonochoric and reproduce sexually, through broadcast spawning, with external fertilization. There is no mention in the available literature of them 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 considered to be of Least Concern.

Synonyms: Acar panamensis, Arca gradata, Arca panamensis, and Barbatia gradata.