Pin cushion sea urchin, Lytechinus variegatus, also commonly called a variegated sea urchin. Size will vary from 1\" test diameter to about 2\" test diameter, and color will range from purple to white to green. These urchins are agressive algae eaters and will quickly eat any algae they can get to. They also have an interesting habit of collecting bits of stuff from the aquarium and carrying it around on their backs.