Invertebrates
Nautilus
Nautilidae · also called chambered nautilus, pearly nautilus
A nautilus is what a cephalopod looked like before octopuses and squid gave up their shells. It has coiled its home the same way for roughly 500 million years, hovering along deep reef slopes in near-perfect balance by adjusting the gas in the sealed chambers trailing behind its body, and feeling its way with dozens of small, sucker-less tentacles and a pair of pinhole eyes.
Overview
Nautiluses are the family Nautilidae, a handful of species in the tropical Indo-Pacific, on the steep outer slopes of coral reefs between about 100 and 500 m deep. They are the only living cephalopods with a full external shell and the closest surviving relatives of the extinct ammonites. They are mostly scavengers, smelling out moulted shells and dead fish and cutting them up with a hard beak.
The shell as a submarine
The animal lives only in the outermost chamber. The older chambers behind it are sealed and gas-filled, making the shell buoyant, and a living tube called the siphuncle pumps fluid in and out to fine-tune balance so the nautilus hovers almost weightless. It adds roughly 30 chambers over a lifetime.
A simpler cephalopod
A nautilus has 60 to 90 sucker-less tentacles, no ink sac, no colour-changing skin, a small simple brain, and pinhole eyes with no lens. Each night it rises several hundred metres up the reef slope to feed, then sinks back by day — and it cannot survive water warmer than about 25 °C or deeper than ~800 m, where the shell implodes.
Status
Not Evaluated as a family, but several species are thought to be declining. Intense fishing for the ornamental shell trade has wiped out local populations in the Philippines and elsewhere, and in 2017 all nautiluses were listed on CITES Appendix II. Maturing at 10–15 years with a few eggs a year, they recover very slowly.
Nautilus: common questions
Is a nautilus the same as an octopus or squid?
They are all cephalopods, the group of shell-less or reduced-shell molluscs that also includes cuttlefish, but the nautilus sits on a branch that separated very early and kept features the others lost. It is the only living cephalopod with a full external coiled shell. It also has far more tentacles, 60 to 90 rather than 8 or 10, and those tentacles have no suckers. It has no ink sac, no colour-changing skin, simple pinhole eyes without a lens, and a much smaller, simpler brain, so it does not show the fast learning and problem-solving that octopuses are famous for. In evolutionary terms the nautilus is closer to the ancient ammonites than to a modern octopus.
How does a nautilus float and sink?
Through the chambers of its shell. The nautilus only occupies the last and largest chamber. Every older chamber is sealed with a wall and mostly filled with gas, which makes the shell buoyant, roughly cancelling out the weight of the shell and body so the animal is close to neutrally buoyant. Running through all the chambers is a living tube called the siphuncle, which can slowly move fluid into or out of the chambers to make small adjustments to buoyancy, for example after the animal grows or as it acclimates to a new depth. Fast up-and-down movement is done by jetting water through the funnel, not by the chambers, which change too slowly for that.
Why do nautiluses live so deep?
They are squeezed between two limits. Above, warm water is the problem. Nautiluses cannot tolerate temperatures much over 25 degrees Celsius, which in the tropics means they cannot spend time in the sunlit shallows and generally stay below about 100 metres even at night. Below, pressure is the problem. The gas-filled shell implodes at around 800 metres, so that is a hard floor. Within that band they favour the cool, food-scented water of the reef slope by day at 300 to 500 metres and rise at night to feed. This narrow habitat, wrapped around tropical island slopes, also means populations are naturally patchy and isolated, which adds to their vulnerability.
Are nautiluses endangered, and why are their shells sold?
The nautilus shell, with its smooth pearly interior and, when cut in half, its elegant spiral of chambers, has been prized as an ornament for centuries, and large numbers are fished specifically for the shell trade to be sold whole or turned into jewellery and buttons. Because nautiluses grow slowly, mature after 10 to 15 years, and produce only a handful of eggs a year, populations that are fished hard collapse and recover very slowly, and local extinctions have been reported. In 2017 all nautilus species were placed on CITES Appendix II, meaning international trade is now regulated and requires permits, and buyers are encouraged to avoid nautilus-shell products altogether.