The fossil record shows that hard external shells were common among early cephalopods, but today the six living species of nautilus are the last cephalopods to retain the external shells of their ancestors, limiting their agility and flexibility but providing them with a highly effective defence against predators (primarily octopuses and large fish) and granting them a high degree of control over their buoyancy: within the shell of a nautilus there are several chambers with a tube-like structure across which gas and water can be transferred, known as a siphuncle, running between them - by filling a chamber with gas the nautilus can increase its buoyancy, by filling a chamber with water it can decrease its buoyancy, and by balancing the number of gas-filled chambers and water-filled chambers it can maintain neutral buoyancy. The Chambered Nautilus is the largest living nautilus (with exceptionally large individuals having a shell diameter of up to 25cm/10 inches, although smaller sizes are more typical), and can be found in the Indian and western Pacific oceans where it typically inhabits shallow coastal waters and coral reefs. It swims by forcing water out of a flexible tube-like structure near its head called a siphon (which can be aimed to alter its direction of movement), and feeds largely on carrion and detritus which it locates using a pair of chemical-sensitive structures that protrude from above its eyes, known as rhinophores. Although they lack true tentacles, Chambered Nautiluses possess as many as 90 small limbs beneath their eyes (called cirri), which lack the strength and adhesive suction cups of the limbs of squids and octopuses but are capable of grasping carrion (as well as the occasional live animal, such as small crabs) and transporting it to the small, parrot-like beak hidden beneath them. When food is abundant Chambered Nautiluses often gorge themselves, storing excess food in a muscular pouch-like organ called a crop where it can be slowly released into the stomach when it is needed. Chambered Nautiluses reproduce by laying eggs (which are typically hidden in cracks in rocks or between lobes of coral, and hatch after about a year into small but fully-developed young instead of the larvae seen in most cephalopods), and while most cephalopods reach maturity, mate and die within just a few years nautiluses are relatively long-lived, reaching maturity at around 5 years old and potentially living for 15-20 years. The interior of a Chambered Nautilus’ shell is lined with a beautiful silvery substance known as nacre or mother-of-pearl which helps to protect the internal organs if the outer shell is damaged, and the demand for nacre (as well as full nautilus shells) for use in decorations and jewellery has led to a high rate of both legal and illegal hunting of this species, resulting in the Chambered Nautilus being classified as a CITES Appendix II species in 2016 (essentially meaning that they are not currently threatened with extinction, but are now legally protected so as to prevent them from being put at risk of extinction in the future.)
