Two-row Rock Shell, Stramonita biserialis

Two-row Rock Shell, Stramonita biserialis. Shell collected off the beach in the greater Loreto area, Baja California Sur, March 2025. Size: 4.3 cm (1.7 inches) x 2.7 cm (1.1 inches). Identification courtesy of Bob Hillis, Ivins, Utah.

Two-row Rock Shell, Stramonita biserialis. Shell collected off the beach in the greater Los Cabos area, Baja California Sur. Size: 4.7 cm (1.9 inches) x 3.1 cm (1.2 inches). Identification courtesy of Bob Hillis, Ivins, Utah.

Two-row Rock Shells, Stramonita biserialis. Shell collected off the beach for consumption  in the greater Bahía Santa Rosalillita area, Baja California, March 2016. Collections and photographs courtesy of Barry Mastro, Escondido, California.

Phylogeny: The Two-row Rock Shell, Stramonita biserialis (Blainville, 1832), is a gastropod mollusk that is a member of the Muricidae Family of Rock Shells. The genus Stramonita is one of one hundred ninety-eight genera in this family, and there are ten species in this genus. They are known in Mexico as Púrpura de Blainville. The genus name Stramonita is derived from the Latin words meaning “small straw-like thing”. This is possibly a reference to the color or texture of these shells.

Description: Two-row Rock Shells derive their name from the double row of knobs that are most prominent at the shoulder. The shell is large, sturdy, and consists of six whorls. They have a moderately high and pointed spire. They are sculpted with numerous fine, dark, spiraling lines.  The aperture is large and the lips are crenulated (sculpted with fine grooves), which may extend well into the interior of the shell. The exterior of the shell is white, with heavy gray to dark brown mottling. The interior is white, with varying amounts of apricot, light brown, orange, and yellow. The Two-row Rock Shells reach a maximum of 8.1 cm (3.2 inches) in length and 5.3 cm (2.1 inches) in height.

Habitat and Distribution: Two-row Rocks are found attached to rocks in the intertidal zone, and to depths up to 3 m (10 feet).  They are 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 Cedros Island, Baja California along the central and northwest coasts of the Baja Peninsula. Following El Niño events they can range north to Southern California.

Ecology and Behavior: Two-row Rocks are predators that feed primarily on bivalves and gastropods including the Fluctuating Turban Shell, Turbo fluctuosus, the Palmate Oyster Shell, Saccostrea palmula, and the Frilled Venus Clam Shell, Chione undutella. They are gonochoric and reproduce sexually, with internal fertilization. The eggs are laid in protective capsules. They are known to host the parasitic Shell-boring Barnacle, Cryptophyalus wainwrighti. Their engagement in any type of commensal or symbiotic relationships has not been formally documented. Their empty shells are known to be used by hermit crabs for housing. 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: Purpura bicostalis, Purpura haematura, Purpura peruviana, Thais biserialis, and Thais peruvensis.