Nautilus pompilius. This ancient mollusk is a cephalopod, related to octopus and squid. They live in deep water, 800-1000ft. We hung a trap baited with fish at a depth of about 800ft over night, then brought the trap up to ~80ft, released the nautilus and let them swim back to depth. The chambers in the shell are gas filled and allow the nautilus to control its buoyancy. Swimming is via jet propulsion like a squid. Had we brought the animals up to the surface, the gas bladder would likely have ruptured resulting in death.