Mexican Chocolate Clam Shell, Megapitaria squalida, Juvenile. Shell collected off the beach at Punta Chivato, Baja California Sur, January 2023. Size: 2.5 cm (1.0 inches) x 1.8 cm (0.7 inches). Collection and, photographs courtesy of Colin Campbell, DVM, Punta Chivato, Baja California Sur. Identification courtesy of Dr. Paul Valentich-Scott, Santa Barbara, California.

Mexican Chocolate Clam Shell, Megapitaria squalida. Shell collected off the beach at Punta Chivato, Baja California Sur, January 2023. Size: 3.7 cm (1.5 inches) x 2.7 cm (1.1 inches) x 1.8 cm (0.7 inches). Collection, photographs and identifications courtesy of Colin Campbell, DVM, Punta Chivato, Baja California Sur.

Mexican Chocolate Clam Shell, Megapitaria squalida. Shell collected off the beach at Punta Chivato, Baja California Sur, January 2023. Size: 6.6 cm (2.6 inches) x 6.7 cm (2.6 inches) x 3.4 cm (1.3 inches). Collection, photographs and identifications courtesy of Colin Campbell, DVM, Punta Chivato, Baja California Sur.

Mexican Chocolate Clam Shell, Megapitaria squalida. Shell collected from within the estuary of the Magdalena Bay complex, Baja California Sur, October 2018. Size: 10.5 cm (4.1 inches) x 8.2 cm (3.2 inches).

Phylogeny: The Mexican Chocolate Clam, Megapitaria squalida (G.B. Sowerby I, 1835), is a bivalve mollusk that is a member of the Veneridae Family of Venus Clam Shells. The genus Megapitaria is one of one hundred seven genera in this family, and there are three species in this genus. This species is also known as the Chocolate Clam and in Mexico as Almeja Chocolate, Almega Negra and Chocolate.

Description: The Mexican Chocolate Clam Shell is thick and equivalve (both valves equal in size and shape), and has  an ovate profile. The margin and anterior end of the shell are round. The posterior end slopes to a weak point. The external surface is smooth. The exterior of the shell is cream to purple-brown, sometimes with darker brown bands, chevrons or maculations. The interior is white, sometimes with orange or purple blush. Mexican Chocolate Clams reach 13 cm (5.1 inches) in length and 10 cm (4.0 inches) in height.

Habitat and Distribution: Mexican Chocolate Clams are found buried in mud and sand substrates from the intertidal zone to depths of 200 m (650 feet). They move to deeper waters as they mature. They are a subtropical to tropical Eastern Pacific species that are found in Mexican waters of the Pacific Ocean and range from Guerrero Negro, Baja California south to Guatemala. They are found throughout the Sea of Cortez.

Ecology and Behavior:  Mexican Chocolate Clams 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. Mexican Chocolate Clams are known to host endo parasitic trematodes, which can damage their reproductive organs. There is no mention in the available literature of them engaging in any types of commensal or symbiotic relationships. Mexican Chocolate Clams are an important food source for humans and sold commercially throughout Mexico, though they can cause paralytic shellfish poisoning. They are also used as a cut bait for sportfishing.  From a conservation perspective they are considered to be Data Deficient, meaning there is insufficient information to assess their risk of extinction.

Synonyms: Callista squalida, Cytherea biradiata, Cytherea chionea, Cytherea squalida, Macrocallista orcutti, Macrocallisa squalida, and Venus (Cytheria) guttata.