On the buoyancy of Spirula spirula

Abstract

Spirula spirula (L.) when freshly caught was found to be close to neutral buoyancy. The pressure of gas within the chambers of the shell was always less than atmospheric, being very low in the newest chamber of a growing animal and rising to a value a little below atmospheric in the older chambers. A chamber which was being formed was completely full of liquid. The only connexion between the liquid in a chamber and the living siphuncle is through a small permeable region of the siphuncular wall which lies at the top of a newly formed chamber. These results support the hypothesis that Spirula, Nautilus and Sepia all regulate their buoyancy in the same way. The shell is sufficiently strong to withstand the hydrostatic pressure of the sea at the depths at which the animal lives.