On the buoyancy of the pearly nautilus

Denton, E.J. and Gilpin-Brown, J.B. (1966) On the buoyancy of the pearly nautilus. Journal of the Marine Biological Association of the United Kingdom, 46 (3). pp. 723-759.

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Abstract

Nautilus macromphalus Sowerby when freshly caught was close to neutral buoyancy having a weight in sea water of about 0'2% of its weight in air. The animals without their shells varied considerably in density but the volume of the shell was an approximately constant fraction of the total volume of the whole animal and whole animals were brought approximately to the same density by having more or less liquid inside the chambers of the shell. About 80% of the gas space in the shell was used to support the weight of the shell itself in sea water. In an adult animal the centre of buoyancy was found to be about 6 mm above the centre of gravity, which made the animal very stable in its natural swimming position, a couple of about 350 g. cm being required to turn it through 90°. The pearly parts of the chamber walls were impermeable to sea water but the chalky and horny siphuncular tubes joining the septal necks were very porous. The most newly formed ten or so chambers were the only ones to contain liquids in appreciable volume and they did this in diminishing amounts from the newest to the oldest. The watery liquids found within the chambers were always hypotonic to sea water and sometimes markedly so; they contained principally sodium and chloride ions. One animal was in the process of forming a new chamber, this incomplete chamber was completely full of liquid with an osmolarity close to that of sea water but differing in composition from sea water. It appears that a new chamber is formed by the secretion of a body fluid between the animal and the inner wall of the living chamber and that it is only when the septum and the new siphuncular tubes are sufficiently strong to withstand the hydrostatic pressure of the sea that the liquid within the chamber is pumped out. We could find no living tissue within the buoyancy chambers and the complete extraction of liquid from a chamber seems to depend upon the physical properties of the pellicle which lines the chambers and the siphuncular tubes; the pellicle makes the walls wettable and the siphuncular tubes act as a wick which draws liquid upwards and into contact with the active siphuncular epithelium. A histological examination of this epithelium has been made. This showed that in a completed chamber, from which liquid has already been taken, there is an epithelial drainage system which can carry liquid into the circulation, whereas in a chamber which is being formed, and which is still full of liquid, this drainage system is very little developed and the epithelium retains many of the characteristics associated with shell secretion. A similar study of the homologous tissue in Sepia showed that, as in Nautilus, a change in the function of the siphuncular epithelium is matched by a change in its structure. The surprising spatial relationship between the porous siphuncular tubes and the liquid within the chambers is probably one by which the main body of this liquid can be decoupled from that small fraction which determines water and salt movements into and out of the chambers. In immature animals the pressures of gases in the newest chambers were much lower than in the older ones.'In the older chambers the total gas pressure tended towards 0·9 atm. irrespective of the depth at which the animal had been living. This supports the hypothesis that liquid is actively extracted from a new chamber and that, into the space so formed, gases slowly diffuse, coming finally into equilibrium with the gases in the body fluids. From the pressures of gases found within the chambers of an immature animal it is calculated on the basis of reasonable assumptions that a new chamber is added about every fortnight. This suggests that after hatching animals take about one year to become fully grown. It seems likely that the oldest four or so chambers are formed whilst the animal is within the egg or using the food in its yolk sac. The shells were sufficiently strong to withstand an external hydrostatic pressure corresponding to a depth in the sea of about 600 m.

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