Pustulose Ark Shell, Anadara tuberculosa

Pustulose Ark Shell, Anadara tuberculosa. Size: 7.5 cm (3.0 inches) x 4.7 cm (1.9 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 Pustulose Ark Shell, Anadara tuberculosa (G.B. Sowerby I, 1833), is a bivalve mollusk that is a member of the Arcidae Family of Ark Shells. The genus Anadara is one of thirty-five genera in this family, and there are ninety-seven species in this genus. They are also known as the Black Ark Shell and the Mangrove Cockle Shell and in Mexico  as Pata de Mula and Arca Negra. Ark shells obtain their name from their shell, which has alternating teeth along the hinge that allow the shell to close securely.

Description: Pustulose Ark Shells are solidly built, inflated, and equivalve. They have a quadrate outline, with a straight margin, a broadly rounded anterior end, and a truncate posterior end. The posterior end is longer than the anterior. The outer surface is sculpted with 32-40 rounded radiating ribs, which lead to a slightly scalloped margin. Some of the ribs have tubercles. The hinge plate is wide and has 24-27 anterior teeth and 25-30 posterior teeth. These shells are white, but are usually covered by a black periostracum.  Pustulose Ark Shells reach a maximum of 9.7 cm (3.7 inches) in length. They are similar in appearance to the Grand Ark Shell, Larkinia grandis, which is larger and has 24 to 26 ribs.

Habitat and Distribution: Pustulose Arks attach to mangroves with byssus and bury in soft mud. They live in the intertidal zone, and to depths of 20 m (65 feet). They are a subtropical to tropical Eastern Pacific species that are found in all Mexican waters of the Pacific Ocean with the exception that they are absent from north of Laguna San Ignacio, Baja California Sur, along the central and northwest coasts of the Baja Peninsula and north of Bahía Kino, Sonora. in the Sea of Cortez. Temporary populations are found as far north as Southern California following El Niño events.

Ecology and Behavior: Pustulose Arks are suspension feeders, which feed on phytoplankton and suspended organic matter. They are prey for fish, crabs, sea stars, and gastropods. Pustulose 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 Pustulose Ark Shell has 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 tuberculosa.