This urchin could be another Echinometra sp. (lucunter or viridis) or Arbacia sp. (punctulata). It too is an effective algal and detrital consumer, but can also be a pain to other inhabitants. Other possibilities in the urchin department include the various species of slate and pencil urchins, Eucidaris or Heterocentrotus species (Photo #1by Paul Auger, Photo #2 by FFE). These urchins are mostly predatory scavengers and should be watched closely. With that said, most urchins also prey on sponges periodically and should not be included when you plan on culturing sponges.