Identification Photos: Slate Pencil Urchin, Eucidaris thouarsii: The Slate Pencil Urchin is easy to recognize and one of the most common echinoids as it is found in all coastal Mexican waters.

The Slate Pencil Urchin has ten vertical rows of five to eight clubbed, thick, blunt-tipped, pencil-like spines that are slightly shorter than the diameter of the body which is also called a "test," see below.

The spines of the Slate Pencil Urchin are brownish-purple being white tipped with one or two white bands below the tip and they serve as homes for bryozoan colonies, sponges, and calcareous algae which attach to the surfaces. The spines are omnipresent around the midsection of the body but absent from the top and bottom.

The cleaned body has a unique pattern of ten uniform sized rows of bumps (making it easy to identify) with a red-brown tinge. The Slate Pencil Urchin reaches a maximum of 2.75 inches in diameter. It is found primarily in crevices on shallow reefs in the mid and low tidal zones often exposed to violent surf. However, it also can be found up to depths of 450 feet.

The Slate Pencil Urchin ranges from California to Ecuador and the Galápagos Islands. The Slate Pencil Urchin feeds primarily on algae and the Pocillopora and Pavona corals. It is a favorite food of triggerfish, large wrasses, and other well-toothed fishes.

The Slate Pencil Urchin is one of the Echinoderms and a member of the Cidaridae Family. The Echinoderms are a phylum of marine animals which include brittle stars, sea cucumbers, sea lilies, starfish, and urchins that are of great scientific interest because, via fossil records, they date to the Cambrian Age (over 500 million years ago) with 7,000 living species and 13,000 extinct ones. Globally there are 700 known species of sea urchins. They all are spiny, hard-shelled animals that live on the sea floor normally attached to rocks.

They have the ability to move and do so very slowly. They have a five sided radial symmetry and their skin has hard, chalky plates that form a shell called the "test." The body is a globular shape from which spines radiate outwards. These spines are used for protection, locomotion, and food collection. They also have five paired rows of tiny tube feet with suckers that are utilized for locomotion and attachment to the sea floor. The mouth is claw-like with five tooth-like plates that point inwards (called Aristotle’s Lantern) located in the middle of the under or ventral side. They do not have a brain. The anus is on the top or dorsal side of the shell.

Sea Urchins feed on animal and plant materials including algae, barnacles, and decaying matter (fish, kelp, mussels, and sponges). They are preyed on by birds, crabs, fish, sea otters, snails, starfish, and humans.

Sea Urchins reproduce via external fertilization with a litter consisting of several million eggs. Juvenile development takes several months and adulthood requires two to five years.

In California, Sea Urchins were consumed by Native Americans for thousands of years, and today are harvested commercially along the Pacific coast by divers by hand, in waters between five and one hundred feet deep. The roe is the focused end-product sold primarily in Japan for human consumption. The annual harvest of Sea Urchins reached 52 million pounds in 1988 before declining rapidly due to over fishing. Subsequently restrictions are now in place.

Being "gaffed" by a Sea Urchin is a fairly common and with some species a very painful experience.

Slate Pencil Urchin, Eucidaris thouarsii: Beach collection at Km. 21, Cabo Real, San Jose del Cabo, Baja California Sur, Mexico, in July 2006. Size approximately 4.5 cm; spine length 3.6 cm. First picture, dorsal view as collected. Second picture, dorsal view with spines removed. Third picture, ventral view with spines removed. Description and photos courtesy of John Snow.