Heterocentrotus mamillatus, commonly known as the slate pencil urchin, red slate pencil urchin, or red pencil urchin, is found in tropical waters.  According to the World Register of Marine Species it occurs in Aqaba, Eastern Africa & Madagascar, Kenya, Madagascar, Marsa Shagra, Mascarene Basin, Mauritius, Red Sea, Seychelles and the West Indian Ocean.  According to Wikipedia it occurs more broadly in the Indo-Pacific region and is most abundant in Hawai‘i.