Western Spoonclam Shell, Periploma planiusculum

Western Spoonclam Shell, Periploma planiusculum. Shell collected off the beach of Punta Chivato, Baja California Sur, December 2025. Size: 3.8 cm (1.5 inches) x 5.5 cm (2.2 inches) x 1.2 cm (0.4 inches). Collection, photographs and identification courtesy of Colin Campbell, DVM, Punta Chivato.

Western Spoonclam Shell, Periploma planiusculum. Shell collected off the beach of Punta Chivato, Baja California Sur, December 2025. Size: 3.9 cm (1.5 inches) x 5.6 cm (2.2 inches) x 1.1 cm (0.4 inches). Collection, photographs and identification courtesy of Colin Campbell, DVM, Punta Chivato.

Western Spoonclam Shell, Periploma planiusculum. Large chrondophore, projecting anteroventrally. A key to the identification. Photographs and identification courtesy of Colin Campbell, DVM, Punta Chivato.

Phylogeny: The Western Spoonclam Shell, Periploma planiusculum (G. B. Sowerby I, 1834), is a bivalve mollusk in the Periplomatidae Family of Spoonclam Shells. The Periploma Genus is one of six genera in the Periplomatidae Family and the are twenty-nine species in the Periploma Genus. Spoonclams get their common name from their spoon-shaped condrophore. They are also known as the Flat Spoonclam and in Mexico as Almeja Cuchara.

Description: Western Spoonclam Shells are ovate to quadrate in outline, and inflated in profile. The right valve is more convex than the left. The left valve fits slightly within the right valve. These shells are thinly constructed. The anterior end is much longer than the posterior end. The anterior is broadly rounded. The posterior is truncate (appearing chopped off). A radial fissure offsets the posterior end. These shells are sculpted with commarginal striae. The pallial sinus is short and blunt. The interior is nacreous. They have a spoon-shaped chondrophore (attachment point for hinge ligament) in both valves, which points anteroventrally (forward and down). Western Spoonclam Shells are white and reach a maximum of 7.6 cm (3.0 inches) in lenght.

Habitat and Distribution:  Western Spoonclam Shells are found buried in mud. They live in the lower intertidal zone and to depths of 26 m (85 feet). They are a subtropical to tropical Eastern Pacific species that are found in all Mexican waters of the Pacific Ocean.

Ecology and Behavior:  Western Spoonclam Shells are suspension feeders that filter plankton and fine detritus from the surrounding water. Their predators include shorebirds, crabs, carnivorous mollusks, and rays. They are simultaneous hermaphrodites (having both male and female reproductive organs)  and reproduce sexually, through broadcast spawning, with external fertilization. The eggs develop into free-swimming trochophore larvae, which transition into bivalve veliger larvae resembling miniature clams. Their engagement in any type of commensal, parasitic, or symbiotic relationship has not been formally documented.

Conservation: From a conservation perspective the Western Spoonclam has not been formally evaluated.

Synonyms: Periploma (Periploma) planiusculum, Periploma argentaria, Periploma obtusa, and Periploma papyracea.