I've written the biology and resistance of these neat sea urchins before in one of my earliest posts (go here!)
Long story short:
1. SEA URCHINS with PLATE-LIKE spines!
2. NAME of the most commonly encountered species is Colobocentrotus atratus, called kaupali in Hawaii
3. HOLD ON TO BOTTOMS in shallow but rough seas at the edge of wave-swept rock like this...
4. LIVE IN CENTRAL/SOUTH INDO-PACIFIC From Hawaii, Japan to the Indian Ocean
Don't believe me that these are actually sea urchins?? here's a nice video that shows the underside with tube feet and everything! Note this species is the Asian C. mertensi.
Here are some NICE close ups of Colobocentrotus atratus showing the plates (photo by Ken-ichi)
Another excellent close photo up by "Fishandfungi"
and yet another by Alan Cressler
A nice shot by tAkOyaKI
This species often occurs in these clusters.. presumably to conserve moisture..
Here are some from Christmas Island in the Indian Ocean (that's Northwest of Australia)
Here's a nice little video about Colobocentrotus from Reunion Island
Colobocentrotus mertenseni also called ジンガサウニ or Jingasauni
Here..we see one that is partly denuded (i.e., spines removed) showing the urchin beneath...