This urchin is usually seen wedged into holes during the day, but emerges on shallow reefs at night to graze. It is very similar to the intertidal Heterocentrotus trigonarius, but has a more reddish maroon color and the spines are more smoothly rounded, without the one sharp edge seen in the other species. They tend not to overlap in habitat.
The Heterocentrotus mamillatus is on the left below; the one on the right is Phyllacanthus imperialis.
Sometimes you can see this commensal crab nestled among the spines of this urchin, particularly among the small spines on the underside around the mouth.
Another crab, or perhaps a color form, has also been seen once on Heterocentrotus mamillatus.
Created 16 June 2018