Panamic Cardiid Shell, Americardia planicostata

Panamic Cardiid Shell, Americardia planicostata. Shell collected off the beach of Punta Chivato, Baja California Sur, June 2025. Size: 3.7 cm (1.5 inches) x 2.1 cm (0.8 inches) x 1.6 cm (0.6 inches). Collection, photographs and identification courtesy of Colin Campbell, DVM, Punta Chivato.

Panamic Cardiid Shell, Americardia planicostata. Size: 2.0 cm (0.8 inches) x 2.1 cm (0.8 inches). Shell collected from within the estuary of the Magdalena Bay complex, Baja California Sur, October 2018.  Identification courtesy of Bob Hillis, Ivins, Utah.

Phylogeny: The Panamic Cardiid Shell, Americardia planicostata (Broderip and G.B. Sowerby I, 1833), is a bivalve mollusk that is a member of the Cardiidae Family of Cockles and True Cockle Shells. The genus Americardia is one of forty-nine genera in this family, and there are eight species in this genus. They are also known as the Flat-ribbed Cardiid Shell.

Description: Panamic Cardiid Shells are sturdy with a trigonal outline.  They are higher than they are long.  The posterior is truncated (appears chopped-off). The exterior of the shell is sculpted with 30 to 35 radial ribs. The interior margins are marked with short grooves. The exterior of the shells may be cream or tan in color, usually with brown blotches. The interior is smooth and white, with some having a purple wash. Panamic Cardiid Shells reach a maximum of 5.6 cm (2.5 inches) in length and 4.4 cm (1.7 inches) in height. The similar looking Western Strawberry Cockle Shell, Americardia biangulata differs from this species in that it has wider and fewer ribs (26-28), the interior is reddish or purple,  and the angle between the central and posterior slopes is more rounded.

Habitat and Distribution: Panamic Cardiids are found within sand substrate in the intertidal zone, and to depths up to 24 m (80 feet). They are a tropical Eastern Pacific species that are found in all Mexican waters of the Pacific Ocean with the exceptions that they are absent from the west coast of the Baja Peninsula and north of La Paz, Baja California Sur, in the Sea of Cortez.

Ecology and Behavior: Panamic Cardiids are suspension feeders that feed primarily on plankton and other suspended organic matter. They are gonochoric and reproduce sexually, through broadcast spawning, with external fertilization. Their commensal, parasitic, or symbiotic relationships have not been formally documented. 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: Cardium guanacastense, Cardium planicostatum, Ctenocardia planicostata, and Trigonicardia guanacastensis.