Heterocentrotus trigonarius
Heterocentrotus trigonarius, commonly known as the slate pencil urchin or red slate pencil urchin, is found in the tropical waters of the Indo-Pacific region. Heterocentrotus mamillatus is a similar, related species.

Heterocentrotus trigonarius can grow to  in diameter (spine-to-spine). The spines are potentially harmful to humans if trodden on.

References

Heterocentrotus
Fauna of the Indian Ocean
Fauna of the Pacific Ocean
Animals described in 1816
Taxa named by Jean-Baptiste Lamarck