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Animal profile

Slate Pencil Urchin

Heterocentrotus mamillatus

EchinoideaShallow reef flats and surge zones of the Indo-Pacific

Field profile

An urchin with thick, blunt, cigar-shaped spines rather than the usual needles. The spines are heavy solid calcite and were historically used as writing implements on slate. It wedges itself into reef crevices by day and grazes algae at night.

Scope note

This is an editorial profile, not a diagnostic field-identification guide or a substitute for current conservation assessments. Follow the cited sources on each fact for the evidence trail.

Published field notes

2025-08-06

Slate Pencil Urchin

The pencil urchin's spines are a fracture-resistant natural ceramic

Each thick spine is a single crystal of calcite, a mineral that normally cleaves along flat planes. Its internal architecture of struts and layers arrests cracks, so the spine crumbles locally rather than splitting apart.

Read with evidence →

Slate Pencil Urchin | Daily Animal Facts