Dark Spiny File Shell, Lima tetrica

Dark Spiny File Shell, Lima tetrica. Size: 4.2 cm (1.7 inches) x 3.1 cm (1.2 inches). Shell collected off the beach at Km 26, Cero Colorado, Baja California Sur, March 2010.

Dark Spiny File Shell, Lima tetrica. Size: 5.0 cm (2.0 inches) x 3.7 cm (1.5 inches). Shell collected off the beach in Loreto, Baja California Sur, March 2006. Collection, photograph and identification courtesy of Bob Hillis, Ivins, Utah.

Phylogeny: The Dark Spiny File Shell, Lima tetrica (Gould, 1851), is a bivalve mollusk that is a member of the File Shells of the Limidae Family. The genus Lima is one of ten genera in this family, and there are twenty-five species in this genus. This species is also called the Delicate File Shell. They are known in Mexico as Lima Delicada. The species name “tetrica” comes from the Latin word meaning “gloomy” or “harsh, probably referring to the rasp-like exterior.

Description: The Dark Spiny File Shell is delicate and trigonal. Both valves are similar in size and shape (equivalve). The posterior auricle (ear) at the hinge is much more pronounced than the anterior auricle. The shell has between 24 and 26 deep ribs. The ribs becoming increasingly scaly toward the margins. The shell is white in color. They have a translucent to tan periostracum. Dark Spiny File Shells reach a maximum of 7.3 cm (2.9 inches) in length and 5.5 cm (2.2 inches) in height. The Dark Spiny File Shell is very similar to the Pacific File Shell, Lima pacifica, but is smaller size and has less pronounced ribs.

Habitat and Distribution: Dark Spiny File Shells are found attached to rock. They use byssal threads to hold to the rock. They live in the intertidal zone, to depths of 110 m (360 feet).  They are a subtropical to tropical Eastern Pacific species. In Mexican waters they range from Bahia Willard, Baja California, to Guatemala. They appear to be absent from the west coast of the Baja Peninsula.

Ecology and Behavior: Unlike most file shells, Dark Spiny File Shells are sedentary and unable to swim. They are suspension feeders that feed primarily on planktonic algae and suspended organic matter. They  reproduce sexually, through broadcast spawning. There is no mention in the available literature of them engaging in any types of parasitic, commensal, or symbiotic relationships. 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: None