Invertebrates
Chambered Nautilus
Nautilus pompilius · also called pearly nautilus, living fossil
The chambered nautilus is a glimpse of the deep past. While its cousins the octopus, squid, and cuttlefish became fast, soft, clever animals, the nautilus kept the coiled outer shell and simple body of cephalopods that swam over the seafloor half a billion years ago, and it is still doing much the same thing today.
Overview
Chambered nautiluses live on the steep outer slopes of coral reefs in the tropical Indo-Pacific, at depths where the water is cold and dark. By day they rest deep, often below 500 metres; at night they migrate up the slope toward 100 metres or so to feed, then descend again before dawn. They cannot tolerate warm surface water, so populations on separate reefs and seamounts are effectively isolated from one another.
The shell
The nautilus’s shell is a logarithmic spiral divided by internal walls into a series of sealed chambers. The animal lives only in the last and largest one; as it grows it seals off each old chamber behind it and builds a new one. A cord of tissue called the siphuncle runs through all the chambers and slowly regulates the gas and liquid inside them, giving the nautilus almost perfect neutral buoyancy so it can hover and glide with little effort. The shell’s proportions — the same growth ratio maintained at every size — have made it a favourite example of natural geometry.
An old-fashioned body
Compared with modern cephalopods, the nautilus is simple:
- Up to 90 tentacles, retractable, without suckers, gripping by ridges and adhesive secretions.
- Pinhole eyes — open pits of seawater with a light-sensitive lining but no lens — that give only blurry images, so the nautilus relies heavily on smell.
- Four gills (other cephalopods have two) and a relatively small, simple brain.
- Jet propulsion by squirting water from a funnel, moving backward, slowly.
Feeding and reproduction
Nautiluses are mostly scavengers, drawn by scent to dead and moulted crabs, shrimp, and fish, which they tear apart with a hard, parrot-like beak. They also catch some live prey and pick at hermit crabs.
They reproduce slowly for a cephalopod. Females lay only a handful of large eggs per year, attaching them one at a time to rock in warmer shallow water, where each takes 9 to 12 months to develop. Nautiluses do not die after breeding the way octopuses and squid do, and they can live for more than two decades.
Threats and conservation
The main threat is fishing for the shell, sold whole and worked into jewellery and ornaments; tens of millions have been taken in recent decades and some populations have collapsed. The nautilus’s slow growth, late maturity, low reproductive output, and fragmented populations make it highly vulnerable. Since 2017 all nautilus species have been listed on CITES Appendix II, requiring permits for international trade, and some countries have banned the fishery outright.
Chambered Nautilus: common questions
Is the nautilus really a "living fossil"?
More than most animals given that label. Nautiluses are the last survivors of the nautiloids, a group of shelled cephalopods that dominated the seas around 500 million years ago and once numbered thousands of species, including some with shells over 2 metres long. The living nautiluses' external coiled shell, simple eyes, and many-tentacled body are very close to fossil forms from over 100 million years ago, while their relatives the ammonites went extinct with the dinosaurs and the octopuses, squid, and cuttlefish evolved into fast, soft-bodied, big-brained animals.
How is a nautilus different from an octopus or squid?
Several fundamental ways. It keeps an external, chambered shell (octopus and squid have no shell or an internal one). It has up to 90 simple, suckerless tentacles instead of eight or ten sucker-covered arms. Its eyes are pinhole cameras with no lens, giving blurry vision, whereas octopus and squid have sharp, lensed eyes. It has four gills rather than two, a much smaller brain, and it lives for decades rather than a year or two. It is a slow scavenger, not an active hunter.
How does a nautilus float and move?
The coiled shell is divided into gas-filled chambers by walls; only the outermost, largest chamber holds the animal's body. A thin tube of living tissue threads through all the chambers and slowly adjusts the balance of gas and fluid in them, letting the nautilus fine-tune its buoyancy and hang weightless in the water. It swims by drawing water into the body cavity and squirting it out through a funnel, jetting backward, and it steers by aiming the funnel.
Why is the nautilus threatened?
Its beautiful pearly shell. Nautiluses are trapped in baited pots on deep reef slopes and killed for the ornamental shell and jewellery trade, with millions harvested over recent decades. Because they grow slowly, mature late (around 15 years), and lay only a few large eggs a year, populations cannot withstand heavy fishing and have crashed in some regions. In 2017 all nautilus species were placed on CITES Appendix II to regulate the international shell trade.