Red Slate Pencil Urchin

The red slate pencil urchin (Heterocentrotus mammillatus) is a sea urchin of Indo-Pacific reefs with a few thick, blunt spines that look like slate pencils, surrounded by short, shorter spines. Its spines are among the best studied examples of biomineralization.

🧬 Field Guide Specimen: Open Red Slate Pencil Urchin in the CRV Science Field Guide to log the specimen, see its place among the Deuterostomia lineages in the Cladogram Explorer, and test your recall in the Red List conservation game.

Each spine is a single crystal of magnesium-rich calcite organized into a porous, ribbed structure that is both light and tough. Materials scientists study spines for ideas to make lightweight, damage-tolerant ceramics.

The urchin wedges itself into crevices by day using its strong spines and emerges at night to graze algae from the reef surface, which keeps coral free of overgrowth.

The dramatic spines are collected for ornaments, jewelry and the aquarium trade. Because a sea urchin's skeleton is a single mineral structure, studies of how cells control crystal growth in spines also inform biomineralization more broadly.

• Purple Sea Urchin — a temperate urchin
• Coral bleaching — reef grazers and coral health