I'm currently working on an underwater robot and was hoping to use the principle used by the nautilus for buoyancy control. So how do the Chambered nautilus control its buoyancy?
The various species of nautilus use a combination of active transport of salts and passive diffusion of water for buoyancy (Denton and Gilpin-Brown 1966, Ward 1979, Greenwald et al. 1980). The chambers are filled with seawater. Salts from the seawater in the chambers is removed by a structure called the siphuncular epithelium. This process makes the chambered water hypotonic, meaning it has lower solute concentration relative to the animal. The hypotonic water then diffuses from the chambers into the blood, creating air pockets in the chambers. Greenwald and Ward (2010) have a recent summary of what is known about nautilus buoyancy, if you want to know more.
The diffusion of water in is not a rapid process. The maximum ascent and descent rate of Nautilus from depths appears to be about 3.0 m m$^1$ (Dunstan et al. 2011). This does not seem like an efficient process or rate of depth change for an underwater robot.
Denton, E.J. and J.B. Gilpin-Brown. 1966. On the buoyancy of the peraly nautilus. Journal of the Marine Biological Association of the United Kingdown 3: 726-759.
Dunstan, A.J., et al. 2011. Vertical distribution and migration patterns of Nautilus pompilius. PLoS ONE 6: e16311. doi:10.1371/journal.pone.0016311
Greenwald, L., et al. 1980. Cameral liquid transport and buoyancy control in the chambered nautilus (Nautilus macromphalus). Nature 286 55-56.
Greenwald, L. and P.D. Ward. 2010. Buoyancy in NAutilus. Topics in Geobiology 6: 547-560.
Ward, P.D. 1979. Cameral liquid in Nautilus and ammontites. Paleobiology 5: 40-49.
As the siphuncle pumps cameral liquid out of a chamber, the pressure of the chamber falls and nitrogen, oxygen, and some carbon dioxide diffuse into the chamber. Sea water gas pressures are in equilibrium with atmospheric gases (oxygen and nitrogen) and are in equilibrium with nautilus blood. Hence as cameral liquid is pumped out of the chamber, gases enter from the blood. But it would make no difference to the buoyant state of the animal if gases entered or if there were a vacuum. The emptying mechanism relies on sodium pumps transporting salt out of the cameral liquid. There is no way to duplicate this biological mechanism in a mechanical system.