The siphuncle of nautiloids controls buoyancy by active transport of ions and osmosis between the siphuncle and shell chamber.

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The nautilus is a free-swimming mollusk related to the squid or octopus, but with a hard, multi-chambered spiraling shell. Reaching through the interior of the shell is a tubular structure called the siphuncle. The nautilus uses this organ to control the volumes of water and gases within each of its shell chambers to regulate its buoyancy. The movement of water into and out of the chambers is driven by osmosis, resulting from changes in the concentration of ions within the chamber fluid. Ions are actively pumped back and forth between chambers to control the movement of the fluids in the chamber. Pumping ions, usually sodium and chloride, out of a chamber makes the fluid within the chamber more dilute (more watery). This causes water to diffuse out of the chamber through the siphuncle in order to equalize the gradient.

The movement of water out of the sealed chamber lowers the gas pressure inside the chamber. At this point, gases–typically nitrogen, oxygen and carbon dioxide–dissolved in the body fluids diffuse into a chamber through the wall of the siphuncle. Initially, the gases are dissolved, but once inside the low-pressure chamber, they start to bubble out. This is much like carbon dioxide bubbling out of a freshly opened soda bottle because the trapped gases escape, reducing the pressure inside the formerly sealed bottle. With more gas in the chamber, the overall density of the Nautilus decreases while its buoyancy increases, enabling it to float upward in the water column.

To sink, the process runs in reverse: ions are pumped into the fluid of the chamber, water flows in under osmosis, pressure increases, gases move out, and density increases. As a result, buoyancy is reduced.

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References

“The body of the mollusc inhabits the very last of a spiralling series of chambers inside the shell. By filling the inner chambers with a mixture of air and water, the nautilus achieves perfect buoyancy, allowing it to rise effortlessly during its nightly migration from the depths of the Pacific Ocean to the surface.” (Downer 2002: 17)

Book
Weird Nature: An Astonishing Exploration of Nature's Strangest BehaviorMarch 2, 2002
John Downer

“It appears plausible that the rate of buoyancy control is dependent on the diameter of the siphuncular tube for circadian and other short period adjustments during feeding and resting periods.” (Westerman  1971: 1)

Book
Form, structure and function of shell and siphuncle in coiled Mesozoic ammonoidsJanuary 20, 1971
Gerd Ernst Gerold Westermann

“. . . a new chamber is formed by the secretion of a body fluid between the animal and the inner wall of the living chamber and that it is only when the septum and the new siphuncular tubes are sufficiently strong to withstand the hydrostatic pressure of the sea that the liquid within the chamber is pumped out.” (Denton and Gilpin-Brown 1966: 723)

Journal article
On the buoyancy of the pearly nautilusJournal of the Marine Biological Association of the United KingdomMay 12, 2009
E. J. Denton, J. B. Gilpin-Brown

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