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This species is a large sea urchin, with some specimens reaching over 8 cm in diameter, with spikes up to 10 cm. Most specimens are bright red, but brown and purple colorations are also seen. The spines may have a different color from the body. Spines have a white ring at their stem and have alternating light and dark rings. The spines are rounded to triangular in cross-section and taper towards the tip. Surprisingly during the night the red spines turn into a chalky pink. Specimens from Hawai‘i tend to have bright red spines, while specimens from other parts of the Pacific may have yellowish or brown spines. Other variations of H. mammillatus, such as those living on the Ogasawara Islands, have slimmer spines instead of the species' characteristic thick, broad spines. It has been debated whether this specific variant truly belongs within the classification of H. mammillatus and not Heterocentrotus trigonarius, another type of pencil urchin. However, it has been concluded that the Ogasawara type is indeed a form of H. mammillatus. Heterocentrotus trigonarius is a similar species that has longer and darker spines, more angular, and is always monochromic.
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