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 30cm (10inches) in diameter (spine-to-spine). The spines are potentially harmful to humans if trodden on.