Banded black and white spines up to twenty-five centimeters long, with a second set of much finer, darker ones packed between them - and that second set is the dangerous one. The long spines are blunt and mostly structural; the short fine ones are barbed and venomous, and they are what a hand actually meets.
The pale sac visible at the top between the spines is the anal sac, and it is often the easiest way to identify the species from above.
What lives in there is the other half of the card. A single urchin can shelter cardinalfish, shrimps, small crabs and juvenile fish, all of them keeping the spines between themselves and everything else, and rotating around the animal as a diver circles it. Some of those species live nowhere else. An urchin is not just an animal, it is an address.
By day it wedges under a ledge; at night it moves out to graze algae, and a reef flat that looked empty at sunset is covered in them by full dark.
- two kinds of spines: long banded black-and-white ones, and short fine dark ones between them
- dark test often visible through the spines, with a pale anal sac at the top
- shrimps and cardinalfish sheltering among the spines
- spines to 25 cm
