The urchin that eats rock. It settles on limestone as a juvenile and starts grinding - with its spines and with the five teeth of its jaw apparatus - until it sits in a socket that fits its own body, and it can spend years in that one hole, leaving only to graze the algae within reach and returning to the same spot.
The effect at scale is enormous. Dense populations of boring urchins remove kilograms of limestone per square meter per year, and on shallow reefs they are among the main agents of bioerosion - the process that turns reef rock back into sand. Where predators that eat urchins are fished out, the population explodes and the reef erodes faster than it grows.
The jaw apparatus is worth knowing on its own: five teeth arranged in a lantern-shaped frame, self-sharpening, and strong enough to work stone. Aristotle described it, and the structure still carries his name.
Spines are sharp and break off in the wound. The sting is minor by urchin standards, but a hand pushed into a shallow crevice usually finds one of these before it finds anything else.
- short stout spines, usually purple-brown with pale tips
- oval rather than round outline
- sits in a socket in the rock that fits it exactly
- test up to 8 cm across
