Nautilus

The nautilus is a unique marine animal belonging to the family Nautilidae. This family includes nine living species divided into two genera. The most well-known genus is Nautilus, which includes the species Nautilus pompilius. These animals are often called "living fossils" because they have remained relatively unchanged for hundreds of millions of years. They are the only living members of the subclass Nautiloidea. All species within this family are currently protected under CITES Appendix II.

The most striking feature of the nautilus is its external planispiral shell. This coiled shell is composed of two distinct layers. The outer layer is a matte white color with dark orange stripes. The inner layer is a striking, iridescent, pearlescent blue-gray material. This shell is divided into internal chambers called camerae. These chambers are separated by walls known as septa. A tube called the siphuncle pierces the center of each septum. As the nautilus grows, it builds new, larger camerae and moves into them. It then seals off the old, vacated space with a new septum.

Managing buoyancy is a complex process involving the siphuncle and the camerae. To stay at certain depths, the nautilus must control its density. The siphuncle extracts salt from the liquid inside the chambers. This salt then diffuses into the animal's blood. Through osmosis, the animal can either remove liquid from its chambers or allow liquid from its blood to refill them. This allows for long-term changes in buoyancy. However, the nautilus cannot manage sudden changes in pressure perfectly. If it goes too deep, the shell may implode at approximately 800 meters.

To move through the water, the nautilus uses a method called jet propulsion. It uses a muscular organ called the hyponome to draw water in and out of its living chamber. This method is considered less efficient than the fins used by some other marine animals. However, research suggests the nautilus is quite efficient compared to other jet-propelled animals like jellyfish. This efficiency might help the animal save energy while foraging at great depths.

The anatomy of a nautilus is very different from other cephalopods like octopuses. Instead of eight or ten arms, a nautilus can have between 50 and over 90 tentacles, known as cirri. These tentacles fall into three categories: ocular, digital, and labial. Ocular tentacles are located near the eyes and act as sensory organs. Digital tentacles form a ring around the mouth and provide adhesion. Unlike an octopus, the nautilus has no suckers. It uses a neutral mucopolysaccharide, a type of sticky liquid, to grip surfaces.

Sensory perception in the nautilus relies heavily on smell rather than sight. Their eyes are quite simple and function like a pinhole camera. Because they lack a solid lens, they cannot form highly focused or detailed images. Instead, they use olfaction, or smelling, to find food and mates. They also have structures called otocysts behind their nerve ring to help with balance. Their nervous system is centered around an oesophageal nerve ring. This ring lacks the complex, differentiated lobes found in the brains of squids or octopuses.

Nautiluses exhibit a form of camouflage called countershading. The dark, striped top of the shell helps the animal blend into the dark water when viewed from above. The white underside helps it blend into the bright surface water when viewed from below. This helps the animal remain hidden from predators. Sizes among species vary significantly. Nautilus pompilius is the largest, while Nautilus macromphalus is much smaller. Some populations, like those in the Sulu Sea, are even smaller, with a mean diameter of only 30 millimeters.