After a particularly bad storm you may find the shell of a paper nautilus Argonauta argo on the beach. This shell is as fragile as the name may suggest, and is secreted by the female paper nautilus, which is in fact an octopus. The delicate shell is white, patterned with wave-like ridges, and has two keels studded with conical knobs. Only the female octopus produces a shell and does so by using large lobes on two of her eight arms, which are also used to hold onto the shell. This shell is not like that of a mollusk which is primarily for protection, it is used as a brood chamber for the nautilus eggs which are 0.6-0.8mm in size. Male paper nautili do not have a shell, and are in fact minute and planktonic – size doesn’t count in this species! Females use their shell to trap air at the sea surface and forcefully dive to a depth where the buoyancy of the compressed air matches the weight of the animal. The octopus is then able to attain neutral buoyancy and float free of the sea floor, yet often near the surface of the sea which is why they may be washed up during storms.
Written by: Minke Witteveen
For more reading:
• Branch, G.M., Griffiths, C.L., Branch, M.L. and Beckley, L.E. 2010. Two Oceans: A guide to the marine life of southern Africa. Pp. 216. Random House Struik Publishers, Cape Town.
• Finn, J.K. and Norman, M.D. 2010. The argonaut shell: gas-mediated buoyancy control in a pelagic octopus. Proceedings of the Royal Society B 277: 2967-2971.
• Guerra, A., González, A.F. and Rocha, F. 2002. Appearance of the common paper nautilus Argonauta argo related to the increase of the sea surface temperature in the north-eastern Atlantic. Journal of Marine Biological Association of the United Kingdom 82: 855-858.