What are the buoyancy control mechanisms of Chambered nautilus?
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.

Literature Cited

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.