Panamic Cardiid Shell, Americardia planicostata

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.

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 Cardiid Shells 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 Mexican waters of the Pacific Ocean from La Paz, Baja California Sur, south to Guatemala. They have not been documented from the west coast of the Baja Peninsula.

Ecology and Behavior: Panamic Cardiid Shells 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.  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 consider to be of Least Concern.

Synonyms: Cardium guanacastense, Cardium planicostatum, Ctenocardia planicostata, and Trigonicardia guanacastensis.