Sea urchins have been around for hundreds of millions of years and have spent most of that time being stepped on by surprised swimmers. The Chartreuse Sea Urchin has retired from that line of work. It is a glazed ceramic urchin for the wall, 4.5 inches wide and 3 inches deep, mounted on a single screw. The spines are short, dense and bright yellow-green, a color that sits somewhere between a lime and a highlighter. It needs no water and no tank, and it will not move to a different rock overnight. It stays where it is put.