The nautilus steers its depth with a shell full of sealed chambers
A nautilus lives inside a coiled shell that it keeps extending, sealing off each outgrown room behind a wall. A hatchling has about 4 chambers; an adult can have 30 or more, threaded by a tube that slowly pumps fluid in or out. That shell makes it a living submarine, and sets its fatal depth.
Nine living species in two genera make up the family, the last survivors of a lineage that has changed little over hundreds of millions of years, which is why they are often called living fossils. They sit much closer to the first cephalopods, which appeared around 500 million years ago, than to later squid and octopus relatives. Adult shells measure 10 to 25 centimetres across, and all species are protected under CITES Appendix II.
The shell is a masterpiece of engineering. It spirals in one plane and is one of nature's finest logarithmic spirals, though not the golden spiral it is often claimed to be. Inside, walls called septa divide it, each pierced by a central duct, the siphuncle. By osmosis the animal draws liquid from its chambers or lets them refill, adjusting buoyancy over the long term, for example after a fish bites off part of the shell. The design has a limit: the shell is thought to implode at around 800 metres. Oddly, a nautilus hauled up from depth shrugs off pressure changes of as much as 80 atmospheres that would kill fish, possibly thanks to a perforated main vein.
Its colouring works as countershading: striped and dark on top to vanish against the deep, nearly white below to blend with the bright water overhead. It swims by jet propulsion, pumping water through a funnel, and turns out to be unusually efficient among jet-powered animals. Instead of suckers, it carries 50 to over 90 tentacles that grip with a sticky secretion and let go when the muscles contract.
Its pinhole eyes lack a lens, so it relies mainly on smell to find food and mates. The nerve ring around its gullet has no distinct higher lobes, and nautiluses were long assumed to be dim, but experiments have revealed memory and responses that change with experience.
Source: Nautilus
