A coral reef is built by tiny animals called polyps, each one a soft tube with a ring of tentacles on top. Most of them are smaller than a fingernail. The Chartreuse Sea Polyp is 6.5 inches across, which makes it unusually ambitious for its species. It is a glazed ceramic polyp for the wall, caught with every tentacle fully open, in a yellow-green that looks lit from inside. A single screw does the work of the reef, and it needs no water or plankton. It has stopped feeding entirely and seems fine about it.