Nautilus shells, sectioned and whole. The sectioned shell is at left. The nautilus is a marine mollusc related to the octopus and squid (all are examples of cephalopods). It has a spiral shell divided into many chambers, of which the animal inhabits only the outermost. The rest are filled with gas and fluid. The nautilus can regulate its buoyancy by varying the ratio of gas to fluid in these chambers. The nautilus is a nocturnal bottom-feeder, hunting crustaceans and scavenging. It is found in the Indo-Pacific region, in the warm seas around South-East Asia.