Heterocentrotus mamillatus, commonly known as the slate pencil urchin, red slate pencil urchin, or red pencil urchin, is a species of tropical sea urchin from the Indo-Pacific region.

It inhabits the coastal Pacific waters of North America. Heterocentrotus mamillatus, the slate pencil urchin, lives in tropical Indo-Pacific oceans. It has stubby spines with rounded, striped ends that can bore into rock.

Its name comes from its thorns thick and robust. They used to be pencils for writing on slates. Today, we make jewels or souvenirs with these big rigid quills. The Slate pencil urchin is known for its strength and resistance out of the ordinary. It can sometimes be confused with Heterocentrotus trigonarius of which it is very close.

Heterocentrotus mamillatus (Brown Pencil Urchin) is a species of echinoderms in the family Echinometridae. Dead Brown Pencil Urchin form shallow marine sediment s. They are omnivores. They have sexual reproduction.