Cameral liquid transport and buoyancy control in chambered nautilus (Nautilus macromphalus)

Abstract

Studies of cephalopod buoyancy1 show that transport of cameral liquid by the shallow water cuttlefish (Sepia) can be explained by a reduction of the liquid's osmotic pressure, followed by osmosis out of the chamber. This mechanism seems insufficient to explain chamber emptying in the chambered nautilus (Nautilus macromphalus), which is frequently found below 240 m, the depth beneath which simple osmotic emptying cannot function2,3. At 240 m, the hydrostatic pressure forcing water into the chambers of the nautilus shell exceeds the maximum osmotic emtying pressure. Because deep water experiments were not feasible, it was our aim to determine whether nautilus could transport cameral liquid against osmotic pressure gradients, and to consider mechanisms that could account for transport against both osmotic and hydrostatic gradients. These possible mechanisms include enhanced siphuncular blood concentration (possibly caused by a countercurrent multiplier in the siphuncle4) which could ‘draw’ water from chambers below 240m, and ‘local osmosis’6, resulting from enhanced solute concentrations in siphuncular intercellular spaces. Physiological and ultrastructural evidence for the latter is presented here.