Slate Pencil Urchin, Eucidaris thouarsii

Slate Pencil Urchin, Eucidaris thouarsii. Urchin collected off the beach in the greater Los Cabos area, Baja California Sur, May 2020. Pictured urchin: diameter 4.7 cm (1.85 inches); height: 3.3 cm (1.3 inches); spine length 2.2 cm (0.9 inches) to 3.8 cm (1.5 inches).

Slate Pencil Urchin, Eucidaris thouarsii. Underwater photographs taken in Zihuatanejo Bay, Guerrero, March 2018. Photographs courtesy of Ron Woheau, Zihuatanejo.

Slate Pencil Urchin, Eucidaris thouarsii. Underwater photograph taken in Zihuatanejo Bay, Guerrero, December 2019. Photograph and identification courtesy of Maude Jette, Dive Zihuatanejo, www.Divezihuatanejo.com.

Phylogeny.  The Slate Pencil Urchin, Eucidaris thouarsii (L. Agassiz & Desor, 1846), is a member of the Cidaridae Family of Pencil Urchins. The genus Eucidaris is one of twenty-four genera in this family, and there are five species in this genus. They are also known as the Eastern Pacific Pencil Urchin and the Gulf Slate Pencil Urchin and in Mexico as Erizo Lápiz.

Morphology:  Slate Pencil Urchins are covered with ten columns of five to eight heavy spines. These spines are surrounded by a ring of very small flat spines. The large spines, with blunt tips, reach 5.1 cm (2.0 inches) in length.  There are tiny pincers (pedicellariae) located between the spines. Their mouth is located on the underside. Inside the mouth is a complex chewing apparatus, made of five jaws, which is known as an Aristotle’s Lantern. The test is brown to black, with a white star shape on the arboral (top) surface.  The test can a maximum of 7.0 cm (1.75 inches) in diameter. Their spines are reddish to orange brown in color and often encrusted with marine life including bryozoans and sponges.

Habitat and Distribution:  Slate Pencil Urchins are found under rocks and in small pockets and crevasses of the reef and are found intertidally, and to depths of up to 150 m (492 feet). They are a tropical Eastern Pacific species that are found in Mexican waters of the Pacific Ocean from from Guadalupe Island, Baja California south to Guatemala, including the Tres Marias, and Revillagigedos Islands. They are found throughout the Sea of Cortez. During El Niño events, on rare occasions, they are found as far north as Santa Catalina Island in Southern California.

Diet:  Slate Pencil Urchins are omnivores, feeding on sea grass, calcareous algae, coral polyps, and small invertebrates.

Predators: Slate Pencil Urchins are primarily preyed upon by fish including Guineafowl Puffer, Arothron meleagristhe, Mexican Hogfish, Bodianus diplotaenia, Orangeside Triggerfish, Sufflamen verres and Porcupinefish, Diodon hystrix.

Reproduction:  Slate Pencil Urchins are gonochoric (male or female for life).  Reproduction is sexual, with fertilization taking place externally. Reproduction occurs via broadcast spawning where the females release hundreds of thousands of eggs into the surrounding water concurrently with the release of sperm by males. After fertilization, the larvae will attach to substrate and develop into their adult form.

Ecosystem Interactions:  Slate Pencil Urchins host a large number of colonies of different animals such as bryozoans, Suberites sponges, Cirripede barnacles,  the Commensal Brittle Star, Ophiothela mirabilis, and other organisms that live as epibionts. It is generally thought that the urchin’s spines provide protection to these epibionts. It is unknown if these relationships are commensal or parasitic.

Human Interactions:  Slate Pencil Urchins have no direct impact on human activities. 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:  Idaris (Eucidaris) thouarsii, Cidaris (Gymnocidaris) thouarsii, Cidaris callao, Cidaris danae, Cidaris thouarsii, Eucidarus thouarsii, Leiocidaris danae, and Leiocidaris thouarsii.