This urchin is very common on the atoll's interisland intertidal reef flat, but only out near the outer edge near the algal ridge where the waves break. They wedge themselves in with their tough spines during high tides when the waves are crashing right over them. They look very similar to Heterocentrotus mamillatus, but tend to be darker in color, more of a dark purple maroon, and the spines have a sharp edge rather than being rounded all the way around.
The wave crashing habitat occupied by this urchin.
I have to say I'm not sure about this one. It was in typical H. trigonarius habitat but has the more the appearance of H. mamillatus.
Created 16 June 2018