Hatchet Surfclam Shell, Simomactra dolabriformis

Hatchet Surfclam Shell, Simomactra dolabriformis. Shell collected off the beach of Punta Chivato, Baja California Sur, April 2026. Size: 7.6 cm (3.0 inches) x 5.2 cm (2.0 inches) x 1.1 cm (0.4 inches). Collection, photograph and identification courtesy of Colin Campbell, DVM, Punta Chivato.

Hatchet Surfclam Shell, Simomactra dolabriformis, Resilifer. A key to the identification. Photograph and identification courtesy of Colin Campbell, DVM, Punta Chivato.

Hatchet Surfclam Shell, Simomactra dolabriformis. Shell collected off the beach of Punta Chivato, Baja California Sur, April 2026. Size: 8.1 cm (3.2 inches) x 6.3 cm (2.5 inches) x 1.2 cm (0.5 inches). Collection, photograph and identification courtesy of Colin Campbell, DVM, Punta Chivato.

Hatchet Surfclam Shell, Simomactra dolabriformis. Shell collected off the beach of Punta Chivato, Baja California Sur, April 2026. Size: 9.4 cm (3.7 inches) x 6.3 cm (2.5 inches) x 1.5 cm (0.6 inches). Collection, photograph and identification courtesy of Colin Campbell, DVM, Punta Chivato.

Phylogeny: The Hatchet Surfclams, Simomactra dolabriformis (Conrad, 1867), is a bivalve mollusk in the Mactridae Family of Surfclams and the Mactrinae Subfamily. The Simomactra Genus is one twenty-four genera in this subfamily, and the Simomactra Genus contains five species. The species name dolabriformis and the common name Hatchet Surfclam both refer to the flattened hatchet blade-like shape of these shells. They are known in Mexico as Almeja Zapapico.

Description: Hatchet Surfclam Shells are ovate to elongate in outline. They are less inflated and thicker-shelled than most species in this family. The anterior end is longer and tapers to a sharply rounded margin. The posterior is broadly rounded and offset by a radial keel. There is a long, narrow gap between the valves at the posterior end. These shells are sculpted with commarginal striae. The interior has a smooth ventral margin. The resilifer (internal ligament attachment process) does not protrude ventrally from the hinge plate. Hatchet Surfclam Shells are white with a dull brown periostracum.  They reach a maximum of 9.4 cm (3.7 inches)  in length.

Habitat and Distribution: Hatchet Surfclams are found buried in mud and muddy sand substrates in the intertidal zone, and to a depth of 28 m (92 feet). They a a subtropical to tropical Eastern Pacific species that are found in all Mexican waters of the Pacific Ocean with the exception that they are absent from north of Magdalena Bay, Baja California Sur,  along the central and northwest coast of the Baja Peninsula.

Ecology and Behavior: Hatchet Surfclams are suspension feeders that feed primarily on plankton and other suspended organic matter. Their predators include birds, crabs, carnivorous mollusks, sea stars, and fish. They are gonochoric and reproduce sexually, through broadcast spawning, with external fertilization. The fertilized eggs hatch into planktonic larvae. Their commensal, parasitic, or symbiotic relationships have not been formally documented. From a conservation perspective Hatchet Surfclam Shells 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: Mactra dolabriformis and Spisula dolabriformis.