[4] 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.

This species can be found throughout the tropical waters of the Indo-Pacific region (from the east coast of Africa to the Pacific archipelagos), but is especially abundant in Hawai‘i.

It roams the subtidal zones of these areas and appears to prefer burrowing into hard sediments, like limestone, coral, and basalt.

Sea urchins are primarily marine grazers and tend to eat the algae in closest proximity to them.

For example, fish, such as those belonging to the families Balistidae, Labridae, and Lethrinidae, are known to prey on sea urchins.

[3] Heterocentrotus mammillatus has thick, rounded spines that allow it to bore into hard substrates and defend itself against predators, wave drag, and pressure.

H. mammillatus from a reef near where the Gulf of Aqaba meets the Red Sea displayed an annual reproductive cycle.

Its resting period is from October to January, accumulating nutrients in its gonads in preparation for gametogenesis.

Spawning usually occurs between May and June, but sometimes can take until October for all the urchins to finish releasing their eggs and sperm.

[9] Given that spawning did not always happen during the same phase of moon, other factors may contribute to the timing of H. mammillatus’ reproductive stages (such as the tidal cycle, for example).