Eucidaris thouarsii, the slate pencil urchin (named after slate pencil), is a species of cidaroid sea urchins that inhabits littoral regions of the East Pacific Ocean.

[3][4][5] The Galápagos, Clipperton and Cocos populations are now often recognized as a separate species, E. galapagensis, instead of a subspecies of E.

[6] Like all urchins these are primarily herbivores, but feed on a wide range of invertebrates.

[7] This species has a high nutrient absorption efficiency when it comes to the coral Pocillopora damicornis, but would require a large intake to make nutrient requirements.

[8] Due to its high intake of coral, it reduces reef growth.