Aha, Nautilus IS able to refill chambers. Refilling rate is from 0.02 -
0.1ml chamber-1 h-1, apparently independent of depth; maximum volume 2-3ml,
even if this leaves the animal positively buoyant, trapped at the surface.
The new cameral fluid is not seawater, but is pumped from the blood.
Nevertheless this mechanism is not used in daily migration. Fluid removal
is more effective, and artificially-filled chambers can be completely
emptied, even though this may take many tens of hours.

The cameral gas IS used for buoyancy; without the gas the shell would sink
like a stone. However the shell does not give positive buoyancy. Normally
the animal is at slightly negative buoyancy overall. Mass submerged is
1-4g. An anesthetized specimen, unable to swim, will sink slowly.

Under normal conditions the animal would not need buoyancy compensation.
However, as often happens, a predator may break a big chunk of shell lip
off, leaving the animal suddenly positively buoyant, possibly even afloat.
In this case the animal would need to increase its mass temporarily in
order to compensate, until the break was repaired. As little as 4g of shell
removed is enough to trap the animal at the surface.

Conversely, the animal may have a large meal, say a lobster molt, which
will leave it in sinking condition.

Experiments have shown that neither cameral fluid movements or chemical
methods of altering specific gravity could account for the necessary
buoyancy changes. It is assumed that these unexplained buoyancy changes are
due to water movement along the pellicle systems, with water moving into or
out of the pellicle layers within the chambers. When added over the entire
30-32 chambers of ate average N. macromphalus, we have at least 3-5ml of
entrained liquid. The pellicle is responsible for removing decoupled liquid
from the chambers; decoupled in that this is the fluid remaining below the
siphuncle but not in contact with it.

I rely heavily here on Ward's "The Natural History of Nautilus" for specifics.

Sculpture of Nautilus belauensis is not nearly as strong as that of
Allonautilus scrobiculatus, but is distinct from all other species of
Nautilus.

Has anyone seen what appears to be a specimen of Allonautilus scrobiculatus
that has faint regular radial ribs? I would very much like to obtain a
specimen(s) of this form; supposedly they are offered in the tourist trade
in Bali. Someone out there may have one which they're willing to
sell/exchange. Hmmm...wouldn't mind specimens of N. belauensis or N.
macromphalus either.

Andrew