Many-ribbed Ark Shell, Larkinia multicostata

Many-ribbed Ark Shell, Larkinia multicostatus. Shell collected off the beach at Punta Chivato, Baja California Sur, March 2022. Size: 6.4 cm (2.5 inches) x 7.3 cm (2.9 inches) x 3.1 cm (1.3 inches). Rib Count: 34. Photographs and Identification courtesy of Colin Campbell, DVM, Punta Chivato, Baja California Sur.

Many-ribbed Ark Shell, Larkinia multicostatus. Shell collected off the beach at Punta Chivato, Baja California Sur, December 2021. Size: 9.1 cm (3.6 inches) x 8.4 cm (3.3 inches) x 3.7 cm (1.5 inches). Rib Count: 34. Photographs and Identification courtesy of Colin Campbell, DVM, Punta Chivato, Baja California Sur.

Phylogeny: The Many-ribbed Ark Shell, Larkinia multicostata (G.B. Sowerby I, 1833), is a member of the Arcidae Family of Arc Shells. The genus Larkinia is one of thirty-five genera in this family, and there are only two species in this genus.  They are known in Mexico as Arca Piangua and Arca Costillosa.

Description: The Many-ribbed Ark Shell is large, heavy, and inflated. It is subquadrate in outline, with the posterior end being slightly longer than the anterior. The dorsal margin is squared on both ends and the beak is prominent. The outside is sculpted with 31 to 36 smooth radial ribs. The shell is white, but the outside is covered by a thick dark brown periostracum.  Many-ribbed Ark shells reach a maximum of 12.5 cm (4.8 inches) in length.

Habitat and Distribution: Many-ribbed Ark Shells live in sand and mud habitats and are reside intertidally and to depths up to 79 m (260 feet). They are a temperate to tropical Eastern Pacific species that are found in all Mexican waters of the Pacific Ocean. They are found throughout the Sea of Cortez. They are uncommon north of Guerrero Negro, Baja California Sur.

Ecology and Behavior:  Many-ribbed Ark Shells 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: Anadara multicostata, Arca multicostata, Arca brandtii, and Arca comoluensis.