One-horn Rock Shell, Ceratosoma monoceros

One-horn Rock Shell, Ceratosoma monoceros. Shell provided by the commercial fishermen of the greater Los Cabos area, May 2016. Size: 2.8 cm (1.1 inches) x 1.5 cm (0.6 inches).

Phylogeny: The One-horn Rock Shell, Ceratostoma monoceros (G.B. Sowerby II, 1841), is a gastropod mollusk that is a member of the Muricidae Family of Rock Shells. The genus Ceratostoma is one of one hundred ninety-eight genera in this family, and there are six species in this genus. The genus name Ceratostoma means horn-mouth, and the word monoceros comes from a Greek word meaning single horned.

Description: One-horn Rock Shells derive their name from the single, tooth-like, projection on the outer lip of the aperture. These shells consist of six whorls and a tall spire. They are sculpted with three varices, with nodes in between, and numerous spiral ridges.  They have a moderately sized aperture, which has fine nodes inside the outer lip. Both the inner and outer lips of the aperture are flared. The siphon canal is short and open. The exterior of the shell is gray to reddish brown in color and the interior is white. The One-horn Rock Shells reach a maximum of 6.0 cm (2.4 inches) in length and 3.2 cm (1.3 inches) in height.

Habitat and Distribution: One-horn Rock Shells are found attached to rocks. Their depth range has not been determined. They are a subtropical Eastern Pacific species that have a very limited range in Mexican waters of the Pacific Ocean. They are endemic to and limited  from San Ignacio Lagoon, Baja California to Cabo San Lucas, Baja California Sur along the west coast of Baja. They have not been documented from the Sea of Cortez.

Ecology and Behavior: One-horn Rock Shells are a poorly studied and understood species. There is very limited documentation of their range, diet, predation, reproduction or ecosystem interactions. Other species in Family Muricidae are predators that feed primarily on other mollusks. They are gonochoric and reproduce sexually, with internal fertilization. The eggs are laid in protective capsules. There is no mention in the available literature of them engaging in any types of commensal, parasitic, or symbiotic relationships. From a conservation perspective they have not been formally evaluated.

Synonyms:  Murex aciculiger and Murex monoceros.