Giant Clam spin
These clams don't close up, but will begin to when they sense your shadow.. then they relax and stay open. If you set your shutter speed to very slow- from 1/10th to 1/20th second and spin your housing the edges become blurred like this. Since you are leaving the shutter open for so long, close your Aperture down all the way (F-20-22 or so) and shoot a test image to adjust. I find leaving the strobes on TTL works for for these.