Hatchet Surfclam Shell, Simomactra dolabriformis

Hatchet Surfclam Shell, Simomactra dolabriformis. Shell collected off the beach of Punta Chivato, Baja California Sur, April 2025. 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 2025. 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.

Phylogeny: The Hatchet Surfclam Shell Simomactra dolabriformis (Conrad, 1867) is a bivalve mollusk in the Mactridae Family of Surfclams. It is in the Mactrinae subfamily. The genus Simomactra is one twenty-four genera in this subfamily, and this 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. In Mexico these shells are called 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 length of 9.4 cm (3.7 inches).

Habitat and Distribution: Hatchet Surfclam Shells 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. In Mexican waters they range from Magdalena Bay, Baja California Sur,  to Guatemala. They are found throughout the Gulf of California.

Ecology and Behavior:  Hatchet Surfclam Shells 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