I've been fascinated by these things for years.  They grow in colonies, and they're often crammed together so tightly that when you look down at them, they look like candies in a box.  When you get close, and they aren't so thick, you can see that the disk grows on the end of a long stalk, like a parasol.  I'd been working on them the whole trip, and I made this picture on my last dive.

Nikon F4, 105mm f/2.8 Auto Micro Nikkor,  TC-14A teleconverter, PK-27, PK-14 tubes, Aquatica Housing, 2 Ikelite 200 strobes, f/32, Kodak Royal Gold 100.