The delicate natural object we call a “paper nautilus” is the paper-thin shell of the aquatic animal known as an argonaut, a free-swimming octopus of the genus Argonauta found throughout the world in warm ocean waters.
The paper nautilus shell is similar in its elegant golden-section proportions to the familiar shell of the chambered nautilus (genus Nautilida). Both the argonauts and the nautiloids are cephalopods—the class of mollusks that also includes squid, octopus, and related free-swimming ocean creatures. But despite certain similarities, the paper nautilus—that is, the argonaut—and the “true” nautilus are actually quite distantly related.
Although the name “paper nautilus” refers to the shell of the female argonaut, it is often applied to the Argonauta genus overall. (It should be mentioned that for Argonauta argo, the largest species of Argonauta, the female can grow to twelve inches in length, while the male is shell-less and considerably smaller.)
The question of how a paper nautilus moves around the ocean is of special interest because, until recently, the locomotion of the genus Argonauta had been long misunderstood. The Greek philosopher Aristotle discusses the paper nautilus in his Natural History of Animals, written around 300 BC. Aristotle suggested that the shell of the paper nautilus functioned as sort of boat. He observed that two of the tentacles of the paper nautilus were flattened and surmised that the animal moved around in its boat by holding these specialized tentacles aloft for use as sails, while employing the additional (non-specialized) tentacles as rudders and paddles.
This fanciful notion lasted for over two millennia, and paper nautilus illustrations produced during recent centuries often show the creature contained in its upright shell vessel, holding its flattened arms in the air to exploit the breezes at the ocean’s surface.
At the end of the 37th section of the ninth book of his Natural History of Animals Aristotle devotes a paragraph to the paper nautilus, which follows a discussion of the more common cephalopods, such as octopus and squid. (In the translated excerpt below, the animal’s tentacles are referred to as “feelers.”)
The nautilus (or argonaut) is a poulpe or octopus, but one peculiar both in its nature and its habits. It rises up from deep water and swims on the surface; it rises with its shell down-turned in order that it may rise the more easily and swim with it empty, but after reaching the surface it shifts the position of the shell. In between its feelers it has a certain amount of web-growth, resembling the substance between the toes of web-footed birds; only that with these latter the substance is thick, while with the nautilus it is thin and like a spider’s web. It uses this structure, when a breeze is blowing, for a sail, and lets down some of its feelers alongside as rudder-oars. If it be frightened it fills its shell with water and sinks. With regard to the mode of generation and the growth of the shell knowledge from observation is not yet satisfactory; the shell, however, does not appear to be there from the beginning, but to grow in their cases as in that of other shell-fish; neither is it ascertained for certain whether the animal can live when stripped of the shell. (Translated by D.W. Thompson)
In Twenty Thousand Leagues Under the Sea, novelist Jules Verne also contributes to the propagation of the misunderstanding of paper nautilus locomotion. A few pages into the second part of the book, there is a scene where the Nautilus—in this case, the name is used for the submarine of the formidable Captain Nemo—travels the waters of the Indian Ocean during late January.
At five o’clock in the evening, before that fleeting twilight which binds night to day in tropical zones, Conseil [the narrator’s manservant] and I were astonished by a curious spectacle.
It was a shoal of argonauts travelling along on the surface of the ocean. We could count several hundreds. They belonged to the tubercle kind which are peculiar to the Indian seas.
These graceful molluscs moved backwards by means of their locomotive tube, through which they propelled the water already drawn in. Of their eight tentacles, six were elongated, and stretched out floating on the water, whilst the other two, rolled up flat, were spread to the wing like a light sail. I saw their spiral-shaped and fluted shells, which Cuvier justly compares to an elegant skiff. A boat indeed! It bears the creature which secretes it without its adhering to it.
For nearly an hour the Nautilus floated in the midst of this shoal of molluscs. Then I know not what sudden fright they took. But as if at a signal every sail was furled, the arms folded, the body drawn in, the shells turned over, changing their centre of gravity, and the whole fleet disappeared under the waves. Never did the ships of a squadron manoeuvre with more unity.
Recent science has produced a better understanding of the physiology of the paper nautilus. It is now known that the female constructs her shell, for use as a brood chamber, with the flattened tentacles that were formerly thought to be sails. It is also known that the paper nautilus is only occasionally at the surface of the ocean, but actually spends most of its time some distance below the surface of the ocean with its arms tucked snugly inside its shell, moving about by means of expelling water through a siphon. Still, the whimsical notion of the “sailing” paper nautilus has produced some of the most unusual natural history illustrations of recent centuries.