Onyx Slipper Shell, Crepidula onyx

Onyx Slipper Shell, Crepidula onyx. Shell collected off the beach of Punta Chivato, Baja California Sur, May 2025. Size: 3.4 cm (1.3 inches) x 2.4 cm (0.9 inches) x 1.1 cm (0.4 inches). Shell covered with brown shaggy periostracum. Collection, photographs and identification courtesy of Colin Campbell, DVM, Punta Chivato.

Onyx Slipper Shell, Crepidula onyx. Shell collected off the beach of Punta Chivato, Baja California Sur, May 2025. Size: 4.5 cm (1.8 inches) x 3.1 cm (1.2 inches) x 0.9 cm (0.4 inches). Shell covered with brown shaggy periostracum. Collection, photographs and identification courtesy of Colin Campbell, DVM, Punta Chivato.

Onyx Slipper Shell, Crepidula onyx. Shell collected off the beach of Punta Chivato, Baja California Sur, May 2025. Size: 4.5 cm (1.8 inches) x 3.1 cm (1.2 inches) x 0.9 cm (0.4 inches). Collection, photographs and identification courtesy of Colin Campbell, DVM, Punta Chivato.

Onyx Slipper Shell, Crepidula onyx. Shell collected off the beach of Punta Chivato, Baja California Sur, February 2023. Size: 5.1 cm (2.0 inches) x 3.0 cm (1.2 inches) x 1.0 cm (0.4 inches). Collection, photographs and identification courtesy of Colin Campbell, DVM, Punta Chivato.

Onyx Slipper Shell, Crepidula onyx. Shell collected within the greater Magdalena Bay complex, Baja California Sur, May 2019. Size: 5.7 cm (2.2 inches) x 3.5 cm (1.4 inches). Collection, photograph and identification courtesy of Bob Hillis, Ivins, Utah.

Onyx Slipper Shell, Crepidula onyx. Shell collected off the beach of Punta Chivato, Baja California Sur, May 2024. Size: 6.1 cm (2.4 inches) x 5.2 cm (2.0 inches) x 1.5 cm (0.6 inches). Collection, photographs and identification courtesy of Colin Campbell, DVM, Punta Chivato.

Phylogeny: The Onyx Slipper Shell, Crepidula onyx (G.B. Sowerby I, 1824), is a gastropod mollusk that is a member of the Calyptraeidae Family of Cup-and-Saucer and Slipper Shells. The Crepidula genus is one of eleven genera in the Calyptraeidae Family, and there are forty-eight species in the Crepidula genus. They are known in Mexico as Pique Onyx.

Description: The Onyx Slipper Shell is thick and has a variable outline, from circular to comma-shaped. Specimens found on rocks have a more flattened profile than specimens found on mud. The apex may be pointed or rounded and the exterior of the shell is sculpted with moderate radial ridges or fine concentric lines.  The exterior of the shell is covered with a brown thick periostracum. The interior is mostly brown (for which it is named), with a white shelf that covers about half of the shell’s underside. Onyx Slipper Shells reach  a maximum of 6.8 cm (2.7 inches) in length and 4.2 cm (1.6 inches) in height.

Habitat and Description: Onyx Slipper Shells are found attached to rocks and other shells or on mud substrate. They are sometimes found in mating stacks, up to sixteen shells high. They live in the intertidal zone, and to depths up to 79 m (260 feet). Onyx Slipper Shells are a subtropical to tropical Eastern Pacific species. They are found in all Mexican waters of the Pacific Ocean with the exception that they are absent from north of Santa Rosalia, Baja California Sur, in the Gulf of California.  They have also been introduced in both Japan and the Puget Sound, in Washington State, where they are now considered to be invasive.

Ecology and Behavior:  Onyx Slippers are suspension feeders that feed primarily on planktonic algae, which they filter from the water by use of mucous strings on their gills. They are protandrous hermaphrodites, beginning life as males and later becoming females. They reproduce sexually, with internal fertilization. During mating they form stacks of individuals, with a female at the bottom of the stack and several males on top of her. The females brood egg capsules under their foot. Other than being epibionts on other gastropods, Their engagement in any type of commensal, parasitic, or symbiotic relationship has not been formally documented. 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: Calyptrea amygdalus and Crepidula rugosa.