A nautilus is a type of sea creature in the family Nautilidae. This family is the only one still alive today in the larger group called Nautilaceae. It belongs to the suborder Nautilina, order Nautilida, and subclass Nautiloidea.

There are nine living species of nautilus in two groups, with the main group being the genus Nautilus. The name "chambered nautilus" is often used for any species in the family Nautilidae. All nautiluses are protected by an international agreement called CITES Appendix II. The size of an adult nautilus shell ranges from 10 to 25 centimeters (4 to 10 inches).

Nautiluses, both living and extinct, have shells that are tightly coiled or have a more open shape. These shells are usually smooth, with sections that are flat or rounded. The lines where the shell sections meet are straight or wavy, and there is a tube-like structure in the center of the shell. Nautiluses have remained almost the same for hundreds of millions of years and are the only living members of the subclass Nautiloidea. They are often called "living fossils" because they have changed very little over time.

Anatomy

The arm crown of modern nautiluses (genera Nautilus and Allonautilus) is very different from that of coleoids. Unlike the ten-armed Decapodiformes or the eight-armed Octopodiformes, nautiluses can have between 50 and over 90 tentacles (called cirri), depending on the individual's sex. These tentacles are divided into three groups: ocular, digital, and labial (buccal). There are two sets of ocular tentacles: one in front of the eye (pre-ocular) and one behind the eye (post-ocular). Digital and labial tentacles surround the mouth. Digital tentacles form the outermost ring, while labial tentacles are between the digital tentacles and the mouth. There are 19 pairs of digital tentacles, which, along with the ocular tentacles, make up 42 visible appendages (not counting the modified tentacles that form the hood). Labial tentacles are usually not visible because they are smaller and vary in number and shape. Males change four of their labial tentacles into a structure called the spadix, which delivers sperm to females during mating.

Each tentacle has two parts: a fleshy sheath that holds an extendable cirrus. The sheaths of digital tentacles are fused at their base to form a single mass called the cephalic sheath. Digital cirri can fully retract into the sheath and extend more than double their retracted length. They are flexible and can bend and twist easily. Even though digital tentacles lack suckers, they stick to surfaces using a neutral mucopolysaccharide secreted by cells on the ridges of the cirri. The tentacles release their grip by contracting muscles, similar to the system in Euprymna, though it is unclear if these adhesives are related. Ocular tentacles do not stick to surfaces but act as sensory organs. Both ocular tentacles and eight lateral digital tentacles can detect chemicals: preocular tentacles sense distant smells, and lateral digital tentacles detect nearby smells.

The radula (a feeding structure) is wide and has nine teeth. The mouth has a parrot-like beak made of two interlocking jaws that tear food, mainly crustaceans, from rocks. Males can be distinguished from females by the arrangement of tentacles around the buccal cone: males have a spadix on the left side of the cone, making it look irregular, while females have a symmetrical buccal cone.

The crop is the largest part of the digestive system and can stretch greatly. Food moves from the crop to a muscular stomach for crushing, then to a digestive caecum, and finally to a short intestine.

Like all cephalopods, nautilus blood contains hemocyanin, a protein that turns blue when carrying oxygen. Two pairs of gills are the only visible remnants of ancestral body segments in modern cephalopods. Oxygenated blood reaches the heart through four ventricles and flows to organs via aortas. Deoxygenated blood returns through small, varied veins, except for the vena cava, a large vein under the crop where most veins empty. Blood passes through filtering organs (made of one pericardial appendage and two renal appendages) before reaching the gills for re-oxygenation. Waste is released into the pallial cavity through pores.

The central part of the nautilus nervous system is the oesophageal nerve ring, a group of ganglia, commissures, and connectives forming a ring around the oesophagus. Nerves from this ring extend to the mouth, tentacles, and funnel; to the eyes and rhinophores; and to other organs. The nerve ring is not a typical cephalopod "brain" because it lacks differentiated lobes and focuses mainly on feeding. Nautiluses have short memory and no brain case to protect the nerve ring.

Nautiluses are the only living cephalopods with an external bony structure: a planispiral shell. They can retreat completely into the shell and close it with a leathery hood made from folded tentacles. The shell is coiled, made of aragonite, and nacreous, with layers that are matte white with orange stripes and a pearlescent inner layer. The innermost part is blue-gray. The osmeña pearl is not a true pearl but a jewelry item made from the shell.

Inside the shell are chambers called camerae, grouped into a section called the phragmocone. Septa divide the chambers, each with a duct called the siphuncle. As nautiluses grow, they create larger chambers and move into them, sealing old chambers with new septa. The number of chambers increases from about 4 at hatching to 30 or more in adults.

The shell’s color helps the animal blend into its environment. From above, the shell is dark with irregular stripes, matching the dark water below. The underside is almost white, helping it blend with bright surface waters. This camouflage is called countershading.

The nautilus shell is a natural example of a logarithmic spiral, though not a golden spiral. The use of nautilus shells in art and literature is discussed in other sources.

Nautilus pompilius is the largest species in the genus. A form from Indonesia and northern Australia, once called N. repertus, can grow up to 25.4 cm (10.0 in) in diameter. Most nautilus species are smaller than 20 cm (8 in). Nautilus macromphalus is the smallest, usually measuring about 16 cm (6.5 in). A dwarf population from the Sulu Sea (Nautilus pompilius suluensis) has an average shell diameter of 11.56 cm (4.55 in).

Physiology

To swim, the nautilus uses a structure called the hyponome to move water into and out of its living chamber. This movement creates a type of motion called jet propulsion. Jet propulsion is usually less efficient than moving with fins or wavelike movements, but the nautilus is more efficient than other jet-propelled animals like squid and jellyfish. Scientists believe this is because the nautilus uses special, uneven muscle movements that help save energy and avoid problems with low oxygen levels when it dives deep. While water is inside the chamber, a part of the nautilus called the siphuncle removes salt from the water and sends it into the blood.

The nautilus changes its buoyancy over time through a process called osmosis. It either removes liquid from its chambers or allows water from the siphuncle to slowly refill the chambers. This happens when sudden changes in buoyancy occur, such as when fish attack and break parts of the shell. Because of this, nautiluses cannot survive at depths greater than about 800 meters (2,600 feet), where the pressure causes their shells to collapse, leading to death. The gas inside the chambers has slightly less pressure than air at sea level. Scientists do not know the deepest depth at which the nautilus can adjust its buoyancy by removing liquid from its chambers.

The nautilus has a rare ability to survive being brought to the surface from deep water without harm. Unlike fish or crustaceans, which die when brought up from such depths, the nautilus remains unaffected even when exposed to pressure changes as high as 80 standard atmospheres (1,200 psi). Scientists believe this ability may be accidental rather than a specific adaptation. The structure of the nautilus's vena cava, which has small openings, may help it survive these pressure changes.

The nautilus does not have strong vision like many other animals. Its eyes are highly developed but lack a solid lens. A solid lens helps create clear, detailed images, but the nautilus's eyes are simple pinhole structures that only form basic images. Instead of using sight, the nautilus likely relies on its sense of smell to find food and identify mates.

The "ear" of the nautilus includes structures called otocysts, which are oval-shaped and filled with small calcium carbonate crystals. These help the nautilus sense movement and changes in its environment.

Nautiluses are more similar to the earliest cephalopods that lived about 500 million years ago than to modern cephalopods that appeared around 100 million years later, such as ammonoids and coleoids. They have a simple brain compared to the large, complex brains of octopuses, cuttlefish, and squid. For a long time, scientists believed nautiluses lacked intelligence. However, recent studies show they can remember events and change their responses over time.

In a 2008 study, nautiluses (N. pompilius) were trained to associate a bright blue light with food. After the light was shown without food at different times, the nautiluses responded to the light for up to 30 minutes, then stopped reacting after an hour. Between 6 and 12 hours later, they responded again but less strongly. Scientists concluded that nautiluses have memory abilities similar to short-term and long-term memory in more advanced cephalopods, even though their brain structures are different. However, their long-term memory is shorter than that of other cephalopods, and they forgot the training after 24 hours. This may be due to the way the training was conducted. The study showed that scientists had underestimated the memory abilities of nautiluses.

Nautiluses reproduce by laying eggs. Female nautiluses attach fertilized eggs to rocks in warm waters (21–25°C). The eggs take 8 to 12 months to develop, and the 30-millimeter (1.2-inch) juveniles hatch. Females lay eggs once a year and regrow their reproductive organs, making nautiluses the only cephalopods that reproduce multiple times.

Nautiluses are sexually dimorphic, meaning males and females have different physical traits. Males have four modified body parts called cirri that form an organ called the "spadix," used to transfer sperm to females during mating. At sexual maturity, the male's shell becomes slightly larger than the female's. Studies show males outnumber females by 60 to 94% in most populations.

Nautiluses may live more than 20 years, which is very long compared to other cephalopods, many of which live less than three years even in captivity. However, they usually reach sexual maturity at about 15 years old, limiting their reproductive lifespan to less than five years.

Nautiluses have several reproductive organs whose exact functions are not fully understood. In males, this is the Van der Hoeven's organ, and in females, these are the Organ of Valenciennes and Owen's laminated organ.

Ecology

Nautiluses live only in the Indo-Pacific region, between 30° N and 30° S latitude and 90° E to 175° E longitude. They are found on the deep slopes of coral reefs.

Nautiluses usually live at depths of several hundred meters. It was once thought that nautiluses move upward at night to feed, mate, and lay eggs. However, recent studies suggest that some groups of nautiluses have more complex movement patterns. The deepest depth where a nautilus has been seen is 703 meters (2,306 feet) (N. pompilius). The depth at which a nautilus shell would collapse under pressure is estimated to be around 800 meters (2,600 feet). Nautiluses can be seen in very shallow water, as little as 5 meters (15 feet), only in New Caledonia, the Loyalty Islands, and Vanuatu. This is because the surface waters in these southern hemisphere areas are cooler than the waters near the equator, where other nautilus populations typically live in deeper water, usually below 100 meters (300 feet). Nautiluses avoid water temperatures above 25°C (75°F).

Nautiloids were once found worldwide, but their numbers decreased, and their range became limited to their current habitats during the Pleistocene, mainly because of the spread of pinnipeds.

Nautiluses are scavengers and hunt when opportunities arise. They eat lobster shells, hermit crabs, and any dead animal matter.

Evolutionary history

Fossils show that nautiloids have changed very little in their physical appearance over the past 500 million years. Many early nautiloids had straight shells, like the extinct genus Lituites. They first appeared during the Late Cambrian period and became important sea predators during the Ordovician period. Some species grew as large as 2.5 meters (about 8 feet). The other living group of cephalopods, Coleoidea, split from nautiloids long ago, and the nautilus has remained mostly unchanged since. Nautiloids were much more diverse and widespread 200 million years ago. The ancestors of all Coleoidea (cephalopods without shells) once had shells, and many early cephalopod species are only known from their shell remains. After the K-Pg extinction event, most nautiloid species disappeared, leaving only three surviving families, while Coleoidea survived.

The family Nautilidae originated from the Trigonocerataceae (Centroceratina), specifically from the Syringonautilidae in the Late Triassic period. This family continues today with Nautilus, the type genus, and its close relative, Allonautilus.

The fossil record of Nautilidae begins with Cenoceras in the Late Triassic, a diverse genus that forms the Jurassic Cenoceras complex. Cenoceras had shells that were either open or tightly coiled, and their shapes ranged from round to slightly flattened. Their suture lines generally had a shallow ventral and lateral lobe, and the siphuncle (a tube-like structure inside the shell) varied in position but was never extremely low or high. Cenoceras fossils are not found above the Middle Jurassic and were followed by Eutrephoceras in the Upper Jurassic to Miocene periods.

Eutrephoceras had shells that were mostly rounded, with a small or closed umbilicus (the hollow space at the shell’s base), a broad, rounded hyponomic sinus (a groove near the shell’s opening), and only slightly curved suture lines. The siphuncle was small and varied in position.

Next, Strionautilus appeared in the Lower Cretaceous period in India and the European ex-USSR. Named by Shimankiy in 1951, Strionautilus had compressed, tightly coiled shells with fine, lengthwise striations. Its whorl sections were nearly rectangular, suture lines were curved, and the siphuncle was near the center.

Also from the Cretaceous period is Pseudocenoceras, named by Spath in 1927. Pseudocenoceras had compressed, smooth shells with nearly rectangular whorl sections, a flattened lower part, and a deep umbilicus. The suture line crossed the lower part almost straight and had a broad, shallow lateral lobe. The siphuncle was small and near the center. Pseudocenoceras fossils are found in Crimea and Libya.

Carinonautilus is a genus from the Upper Cretaceous of India, named by Spengler in 1919. Carinonautilus had tightly coiled shells with high, narrow whorl sections that sloped inward to form a narrow lower part with a prominent, rounded keel (a raised ridge). The umbilicus was small and shallow, and the suture line was only slightly curved. The siphuncle’s position is unknown.

Obinautilus has been classified into Nautilidae by some scientists, though it may instead belong to the argonautid octopus group.

Taxonomy

The family Nautilidae includes up to nine living species and several extinct species:

• Genus Allonautilus: A. perforatus, A. scrobiculatus
• Genus Nautilus: †N. altifrons, †N. balcombensis, N. belauensis, †N. butonensis, †N. campbelli, †N. cookanus, †N. geelongensis, N. javanus, N. macromphalus, N. pompilius (type species), N. p. pompilius, N. p. suluensis, †N. praepompilius, N. samoaensis, N. stenomphalus, †N. taiwanus, N. vanuatuensis, N. vitiensis

In 2011, genetic research suggested there were only three living species: A. scrobiculatus, N. macromphalus, and N. pompilius. At that time, N. belauensis and N. stenomphalus were grouped under N. pompilius, possibly as subspecies. This conclusion was made before the discovery of three additional living species: N. samoaensis, N. vanuatuensis, and N. vitiensis.

Some species related to the family Nautilidae have unclear classification status.

Conservation status and human use

Nautiluses are collected or fished for sale as live animals or to carve their shells into souvenirs and collectibles. People value the shells not only for their shape but also for their shiny inner layer, which is used as a substitute for pearls. In Samoa, nautilus shells are used to decorate the forehead band of a traditional headdress called tuiga. During the Renaissance and Baroque periods, nautilus shells were popular items in collections of curiosities. Goldsmiths often mounted them on thin stems to create ornate cups.

Nautiluses have few offspring, reach maturity slowly, have long gestation periods, and live for many years. These traits make them vulnerable to overexploitation. The demand for their shells as decorations has caused population declines. To address this, countries like Indonesia have legally protected the chambered nautilus, imposing fines of up to $8,500 and/or 5 years in prison for trading in this species. However, as of 2014, these shells were still openly sold in tourist areas in Bali. Due to continued trade, efforts to increase protection led to all species in the family Nautilidae being added to CITES Appendix II in 2016, which regulates international trade.

• Baroque-style nautilus cup made by Aleksander Kęsowski, abbot of Oliwa, between 1643 and 1667
• Nautilus shell carved and painted with scenes of humans, spiders, dragonflies, dogs, butterflies, sawflies, and flies, with a bronze pendant mount from the 19th century, displayed at the Poldi Pezzoli Museum in Milan
• A nautilus shell shaped like a chicken, created around 1550

In human culture

In Palau, people view nautiluses (called kedarm in Palauan) as a symbol of something weak or easily broken. This belief comes from the idea that nautiluses can die easily from even small bumps against ocean rocks. Because of this, a person who becomes angry quickly after being tricked is sometimes compared to a nautilus (in Palauan: ng ko er a kedarm, el di metirem e metord).