Pin cushion sea urchin, Lytechinus variegatus, also commonly called a variegated sea urchin. 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.
Small Size: These little guys are around the size of a Fifty cent piece.
Regular Size will vary from about 2″ diameter to about 3″ diameter, and color will range from purple to white to green.