Nautilus

Quick Take

• The nautilus has survived virtually unchanged for 500 million years, yet one very specific depth kills it instantly. See the fatal depth →
• Unlike other deep-sea creatures, the nautilus can be brought to the surface without dying. But that resilience hasn't saved it from disappearing. Explore the population decline →
• The nautilus has over 90 tentacles but a glaring sensory weakness that forces it to hunt in a way most people wouldn't expect. See how they hunt →
• Males dramatically outnumber females in nautilus populations, and the mating behavior that results is far stranger than it sounds. Discover the mating behavior →

Named after the Ancient Greek word for sailor, the nautilus is a marine mollusk in the family Nautilidae. A close relative of the octopus, squid, and cuttlefish, the nautilus is a living fossil whose ancestors evolved nearly 500 million years ago. These deep-dwelling scavengers navigate using jet propulsion and a complicated system of chambers that adjust their buoyancy in the water. Highly prized for their decorative spiral shells that resemble pearls on the inside, nautiluses are threatened by overfishing and are some of the rarest marine animals. 

5 Nautilus Facts

• The first nautiluses evolved nearly 500 million years ago and remain relatively unchanged to this day. 
• Although nautiluses can live for 15 to 20 years, they typically don’t reach sexual maturity until around 10 or 15 years old. 
• Nautiluses have more than 90 tentacles covered in a sticky adhesive and tiny hairs that they use to ensnare prey.
• To get around, nautiluses rely on jet propulsion by blowing water through a siphon below the eyes. 
• Unlike other cephalopods, nautiluses have poor vision, but they make up for their limited eyesight with an excellent sense of smell. 

Classification and Scientific Name

The nautilus is a marine mollusk and a member of the cephalopod class, related to squid, cuttlefish, and octopus. Several living species of nautilus belong to the same family, Nautilidae. Most species belong to the type genus, Nautilus, while others belong to the genus Allonautilus. The word nautilus derives from the Ancient Greek word nautilos or “sailor.” The best-known nautilus species, the chambered nautilus, also goes by the name the pearly nautilus. It gets its name from both its iridescent appearance and the multiple chambers that it uses to adjust its buoyancy. Its scientific name is Nautilus pompilius, after Numa Pompilius, the second King of Rome.   

Appearance 

Most nautiluses look broadly similar to one another. All six species have rolled, smooth shells. The shell’s shape is often used as an example of a golden spiral, but in reality, the ratio is only a nearly perfect logarithmic spiral. Nautiluses rely on their shell for protection and can completely pull themselves into the shell and close the opening using specially folded tentacles. The outside of the shell looks matte white, while the inside is iridescent white or blue-gray. Moreover, the shell often exhibits countershading. With countershading, the top of the shell looks darker, while the bottom appears lighter. Inside the shell are several chambers connected in the middle by a duct known as the siphuncle. At birth, a nautilus shell contains around 4 or 5 chambers, while an adult’s shell can contain up to 30 chambers. 

Each nautilus possesses more than 90 tentacles, depending on the species. These tentacles, or cirri, are made of soft, flexible material and are completely retractable. Each arm is coated in a sticky adhesive substance and tiny hairs known as cilia near the tip of each appendage, which they use to smell for food.   

The chambered nautilus is the largest living nautilus species. At their largest, chambered nautilus shells can measure nearly 10 inches in diameter. However, some dwarf populations measure as little as 4 inches in diameter when fully grown. Meanwhile, the bellybutton nautilus (Nautilus macromphalus) ranks as the smallest standard species, with a shell diameter of up to around 16 cm (roughly 6.3 inches). Most nautiluses measure around 8 inches in diameter. 

Behavior

The nautilus relies on jet propulsion to move through the water. It sucks water into a siphon right below its eyes and then expels the water in a stream, pushing itself forward in a straight line away from the direction its siphon faces. To aid itself in traversing the ocean depths, the nautilus adjusts its buoyancy by controlling the amount of water it lets into the chambers located in its shell. Using osmosis, it can add water — causing it to sink — or expel water, which causes it to rise gradually toward the surface. 

Nautiluses are rarely found near the surface and tend to spend most of their time at depths below 500 feet. Nautiluses can withstand great pressures and are capable of living at depths of up to 2,300 feet below sea level. While they can live at lower depths, they normally rise at night to either feed, mate, or lay eggs. At around 2,600 feet, their shells implode, killing them instantly. Unlike many deep-dwelling fish, which promptly die if brought to the surface too quickly, nautiluses do not suffer physical effects caused by a rapid change in pressure. 

Compared to other cephalopods, the nautilus displays a relatively low level of intelligence. Its brain is rather simple, and although it does not lack intelligence, its memory skills are extremely limited. While it displays some level of short- and long-term memory in response to external conditioning, the nautilus quickly forgets information that more advanced cephalopods can remember for several weeks, retaining it for only a few hours. 

Habitat

Today, nautiluses are confined to the Indo-Pacific, with most located near coastal areas of the Indian Ocean or the western Pacific Ocean. They tend to congregate along the slopes of coral reefs and spend most of their time at depths between 500 and 2,300 feet. That said, you can sometimes find them in very shallow water. They undergo vertical migrations, rising at night primarily to feed, mate, and lay eggs, and descending during the day.  Nautiluses avoid warmer water, preferring temperatures below 75 degrees Fahrenheit. 

Diet

As opportunistic carnivores, nautiluses will eat almost anything they can catch. Their diet consists mainly of crustaceans and shellfish such as crabs and lobsters. Moreover, they are willing scavengers and will not hesitate to eat carrion. They use their long arms to ensnare prey and guide food toward their beaks. Given their poor eyesight, they must rely on other senses to help them find food. For example, the tiny hairs at the end of their arms are likely used to help them “smell” their surroundings and detect detritus and other edible matter by scent. 

Predators and Threats

The nautilus’s main predators include other cephalopods, such as the octopus, as well as sharks, groupers, sea turtles, and triggerfish. During the day, nautiluses dive deep below the surface to avoid predators. The countershading on their shells aids them in avoiding detection. For example, the top of the shell appears darker, while the bottom appears lighter. This makes them harder to detect from above, as they blend into the dark water below, and also difficult to spot from below, as they blend into the lighter water above. When threatened, nautiluses will retract into their shells and cover the opening with a leathery hood created by a specialized pair of folded arms. 

Aside from natural predators, the main threat to nautiluses comes from human activity. Nautiluses are collected both for food and for use as souvenirs and collectibles. People prize their shells for their shape and color, as the inside serves as a suitable pearl substitute. Given their slow sexual maturity, low birth rates, and long gestation periods, nautiluses are extremely susceptible to over-exploitation. 

Reproduction and Life Cycle

Nautiluses don’t have specific mating seasons. They can breed any time of the year and typically do so closer to the surface. Strangely, males vastly outnumber females, who make up only around 20 to 30 percent of the total nautilus population. As a result, a single female often must contend with the affections of multiple male suitors. Nautiluses mate face-to-face, during which time the male will transfer his sperm to the female’s mantle using four specialized tentacles that form an organ known as a spadix. Mating sessions can last for hours, and males sometimes leave bite marks on the females from where they latched on during the coupling. 

Like other cephalopods, nautiluses reproduce by laying eggs. However, unlike most cephalopods, which produce notably small eggs, nautilus eggs are relatively large, measuring approximately 1 to 1.5 inches long. Moreover, they lay relatively few eggs, with most females laying between 10 and 18 eggs per year. Females attach their eggs to rocks in warm water either singly or in small groups. The eggs gestate for around 12 months before hatching. 

Nautiluses can live up to 20 years in the wild, significantly longer than other cephalopods. However, while they can live for a comparatively long time, they mature very slowly. On average, nautiluses reach sexual maturity between 10 and 15 years old. 

Population

Over the past few decades, nautilus populations have declined considerably due to overfishing. Given their low birth rate, delayed sexual maturity, and long gestation period, nautilus populations are extremely vulnerable to even mild reductions in numbers. Each year, fewer nautiluses remain, and sightings become increasingly infrequent. Some species are so rare that years can go by without divers or fishers encountering them in the wild. While regulations exist to protect nautiluses, they remain threatened and on the decline. 

Nautilus Locations

Nautiluses are found primarily in the western Pacific Ocean and coastal regions of the Indian Ocean. They prefer warm temperate waters and are most heavily distributed around Southeast Asia and Australia. Nautiluses are particularly dense in the waters around the Great Barrier Reef, the Philippines, southern Japan, and Indonesia. 

Nautilus Conservation Status

All nautilus species are threatened due to over-exploitation. Their value mainly comes from their shells, which is prized as an ornament, decoration, and souvenir. As of 2016, nautiluses are classified under CITES Appendix II, a multilateral treaty restricting international trade in nautiluses. Additionally, the Endangered Species Act in the United States recognizes the nautilus as a threatened species.