What is the mechanism the nautilus uses to increase its buoyancy? (i.e., where does it acquire the gas it fills its shell with in order to rise in the water column?)

Brian,

Part of this previous answer should help to answer this: http://www.askabiologist.org.uk/punbb/v … hp?id=626,

The Nautilus doesn't increase it's buoyancy, it has neutral buoyancy, so any shift within the water column has to be undertaken through muscle power.

Many cephalopods also use a technique called 'dynamic soaring' to shift their position in the water column as well. Soaring seabirds do the same in air and the following website gives a detailed account of how albatrosses use dynamic soaring: http://www.wfu.edu/biology/albatross/at … oaring.htm

The process exploits differences in current velocity at different heights in the column of water (or air). Velocity is usually slower close to the ground because of interaction of the fluid with the ground surface. Higher up there is less interaction, so the current velocity is higher. This method of exploiting physical forces for a 'free ride' allows organisms to travel long distances for relatively little energy expenditure.

"Hope is a duty from which palaeontologists are exempt."
David Quammen

New scientist a couple of weeks ago had an article on nautilii(sp?):
http://www.newscientist.com/channel/lif … crets.html

sadly that's just a preview, but the full article was ery interesting.  they use the siphuncle to drain each chamber of water.

Peter,

When you say drain, is this through osmosis, as I mentioned in the link above or is this an active process??

Hi Dave,
               Nautilus, and we suspect ammonoids, drain the chambers by osmosis. For the sake of clarity, as far as we know they do this only once when they finish constructing a chamber. They do not seem to be able to voluntarily control this after the chamber has been completed. So they do not use this to actively control their buoyancy for the purposes of vertical migration.

"Hope is a duty from which palaeontologists are exempt."
David Quammen