Rugose Slit Keyhole Limpet Shell, Fissurella microtrema

Rugose Slit Keyhole Limpet Shell, Fissurella microtrema. Shell collected off the beach in the greater Los Cabos area, Baja California Sur, May 2019. Size: 1.8 cm (0.7 inches) x 1.4 cm (0.6 inches).

Rugose Slit Keyhole Limpet Shell, Fissurella microtrema. Shell collected off the beach of Punta Chivato, Baja California Sur, March 2025. Size: 2.5 cm (1.0 inches) x 1.6 cm (0.6 inches) x 0.4 cm (0.2 inches). Collection, photographs and identification courtesy of Colin Campbell, DVM, Punta Chivato.

Rugose Slit Keyhole Limpet Shell, Fissurella microtrema. Shell collected off the beach of Punta Chivato, Baja California Sur, March 2025. Size: 3.1 cm (1.2 inches) x 1.8 cm (0.7 inches) x 0.7 cm (0.3 inches). Collection, photographs and identification courtesy of Colin Campbell, DVM, Punta Chivato.

Phylogeny: The Rugose Slit Keyhole Limpet Shell, Fissurella microtrema (G.B. Sowerby, 1835), is a gastropod mollusk that is a member of the Fissurellidae Family of Keyhole Limpets. The genus Fissurella is one of forty-five genera in this family, and there are fifty-five species in this genus. They are known in Mexico as Lapas de Cerradura. The species name microtrema comes from the Greek words, meaning small hole, referring to this shell’s small orifice.

Description: Rugose Slit Keyhole Limpet Shells have an irregular outline, which narrows at the anterior end. They are sculpted with three strong ribs at the anterior end that often project onto the margin. Otherwise, the exterior surface is rough and irregularly ribbed. They have a small ovate orifice. The exterior of the shell is variegated, with red or greenish rays, divided by lighter colored rays. The interior is white with a greenish or bluish tinge. The callus line is gray. Rugose Slit Keyhole Limpet Shells reach a maximum of 3.2 cm (1.3 inches) in length and 2.5 cm (1.0 inches) in height.

Habitat and Distribution: Rugose Slit Keyhole Limpet Shells are found attached to rocks in the intertidal zone, and to depths up to 30 m (100 feet).  They are a subtropical to tropical Eastern Pacific species. In Mexican waters they range from La Manga, Sonora to Guatemala. They have not been documented from the west coast of the Baja Peninsula.

Ecology and Behavior: Rugose Slit Keyhole Limpet Shells are herbivorous grazers  that feed primarily on algae. They are gonochoric and reproduce sexually, through broadcast spawning, with external fertilization.  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 considered to be of Least Concern.

Synonyms: Fissurella chlorotrema and Fissurella humilis.