Rupture strength and flow rate of Nautilus siphuncular tube

Abstract

The siphuncular tube is a key component of the buoyancy control and mechanical strength systems in both Nautilus and fossil cephalopods. We measured the rate of hydrostatically induced fluid flow across the tube wall and tube rupture strength of Nautilus pompilius at hydrostatic pressures in the range of 10–85 bars. We found that in fresh, undecayed tubes, rupture occurs at pressures of about 80–85 bars. This is equivalent to the strength of the shell proper and to the depth limit of the live animal. The siphuncular tube is neither markedly stronger, nor weaker, than the shell. Siphuncle rupture strength is constant in the last 20 chambers of the shell despite a strong decrease in the siphuncle strength index (ratio of tube thickness to radius). The notion that strength index gives an accurate indication of tube strength is therefore in error. This suggests that the geometry of the siphuncular tube can not be straightforwardly used as an index of living depth in fossil cephalopods. Rupture occurs at the siphuncle-septum contact. The junction of the tube to its mechanical supports is thus weaker than the tube itself. Measured flow rates are in the range of 1–20 ml/h/chamber. Flow rates increase linearly with applied pressure and in successively larger chambers as a result of size-related variation in surface area and thickness of the tube wall. Rates of osmotic pumping in live animals are up to three orders of magnitude lower than hydrostatically induced flow rates across the siphuncular tubes of empty shells. Pumping capacity of live animals is apparently limited by functional constraints of the osmotic pump rather than by the fluid conductance properties of the tube wall. Living depth in evolving cephalopod lineages may be limited ultimately by physiologic or chemical restrictions of the osmotic pumping mechanism rather than by mechanical strength of the shell or siphuncle.