Triangular Rock Shell, Penethais triangularis

Triangular Rock Shell, Penethais triangularis. Size: 2.2 cm (0.9 inches) x 2.0 cm (0.8 inches). Shell courtesy of the commercial fishermen of the greater Los Cabos area, Baja California Sur, May 2013.

Phylogeny: The Triangular Rock Shell, Penethais triangularis (Blainville, 1832), is a gastropod mollusk that is a member of the Muricidae Family of Rock Shells. The genus Penethais is one of one hundred ninety-eight genera in this family, and there are only two species in this genus. This shell was described by the French zoologist Henri Marie Ducrotay de Blainville. Blaiville wrote the Manuel de Malacologie et Conchyliogie in 1827. Copies of this two volume work are still sold online for the bargain price of $4,750.

Description: Triangular Rock Shells are solid, medium-sized shells that consists of four to five whorls. The spire is low and blunt. They are sculpted with a series of knobs that begin at the shoulder and spiral along the body whorl.  Two equal sized knobs project as blunt spines at the outer lip of the aperture. The aperture is wide, smooth, and ovate. The exterior of the shell is white, often with some rust to light brown blotches. The interior is white. Triangular Rock Shells reach a maximum of 3.0 cm (1.2 inches) in length and 2.7 cm (1.1 inches) in height.

Habitat and Distribution: Triangular Rock Shells are found attached to rocks in the intertidal zone, and at depths up to 5 m (15 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 along the west coast of the Baja Peninsula.

Ecology and Behavior: Triangular Rock Shells are a poorly studied and understood species. There is very limited documentation of their diet, predation, reproduction or ecosystem interactions. Other species in the Muricidae  Family are predators that feed on shelled mollusks and barnacles. They are gonochoric and reproduce sexually, with internal fertilization. The eggs are laid in protective capsules. Their engagement in any type of commensal, parasitic, or symbiotic relationships has not been formally documemded. From a conservation perspective they have not been formally evaluated. However, they are fairly common with a relatively wide distribution and should be considered to be of Least Concern.  

Synonyms: Acanthais triangularis, Purpura carolensis, Purpura triangularis, and Thais (Thais) triangularis.