Echinoderms of Zihuatanejo

PHYLUM ECHINODERMATA

-Sea Urchins and Sea Cucumbers (Subphylum Echinozoa)

-Sea Urchins, Sand Dollars, and Heart Urchins (Class Echinoidea)

-Pencil Urchins (Subclass Cidaroidea)

-Pencil Urchins (Order Cidaroida)

-Superfamily Cidaroidea

-Family Cidaridae

-Subfamily Cidarinae

-Genus Eucidaris

-Slate Pencil Urchin (Eucidaris thouarsii)

The Slate Pencil Urchin (Eucidaris thouarsii) is species of sea urchin belonging to the Cidaridae family. It is primarily found in the eastern Pacific Ocean. Characterized by thick, blunt, pencil-like spines that are often shorter and more irregularly arranged. The spines are usually brown, gray, or reddish.

Diet

Eucidaris thouarsii is primarily a grazing sea urchin, feeding on algae and other organic material growing on rocks and reef surfaces. Its strong jaw apparatus allows it to scrape food from hard substrates.
Its grazing contributes to the removal of algae from rocky and reef surfaces, while material associated with the seafloor can also form part of its diet.

Range

The Slate Pencil Urchin is a tropical eastern Pacific species ranging from the Gulf of California and mainland Mexico southward along the Pacific coast to Ecuador. It occurs on the Mexican Pacific mainland and offshore islands, including the Revillagigedo Archipelago.
It occurs in the Zihuatanejo area, including Zihuatanejo Bay, where it has been observed on the local rocky seafloor.

Habitat

Eucidaris thouarsii is a benthic species found from the low intertidal zone into deeper water. It commonly shelters between or beneath rocks and is associated with rocky reefs and other hard-bottom habitats. Depth records extend to approximately 135 m, although it is particularly characteristic of shallow rocky environments.
Around Zihuatanejo, it occurs on rocky reef and hard-bottom habitat, including areas where crevices and spaces beneath rocks provide shelter.
During daylight it may remain partly concealed among rocks, becoming more active while grazing over the surrounding substrate.

Description

The Slate Pencil Urchin has a thick, rounded to somewhat turban-shaped test that can reach about 15 cm in diameter. The test is relatively flattened on both its upper and lower surfaces.
It is named for its distinctive primary spines, which are unusually thick, bluntly tapered and somewhat pencil-like rather than extremely slender. The spines are commonly reddish-brown, purplish or dark brown, often with contrasting bands or lighter tips.
Numerous much smaller secondary spines cover the test between the large primary spines. These secondary spines are laterally compressed, longitudinally ridged and have characteristically truncated tips.
The combination of a robust rounded test and relatively few, thick primary spines makes E. thouarsii readily distinguishable from the much more finely spined Diadema mexicanum found on the same eastern Pacific reefs.