Wakayama College Unravels 200-Year Mystery of the Argonaut's 'Pseudo-Shell': Fine Structure Analysis of the Calcareous Egg Case

The calcareous shell (egg case) of Argonauts

A joint research project led by Associate Professor Stearman Defen of National Institute of Technology, Wakayama College (Gobo City, Wakayama Prefecture; Principal: Shion Inoue; hereinafter "Wakayama College") has revealed the secret of the "shell-like structure" possessed by Argonauts (Argonautidae), such as the greater argonaut and the paper nautilus. This research was conducted jointly with researchers from the University Museum, the University of Tokyo, Shimane University, and Nihon University, and the results were published in the online edition of the English journal *Scientific Reports* on Wednesday, April 22, 2026.

■ Points of this research
Argonauts, including the greater argonaut and paper nautilus, possess an "egg case" resembling a calcareous shell. In response to a discovery reported about 200 years ago that "the shell is made by the arms," this study revealed the method of its formation and the characteristics of the shell from the perspective of convergent evolution.

As a result of this research, it was first shown that the argonaut shell is produced by a unique formation mode different from typical molluscan shells and has a fundamentally different structure. Furthermore, it was revealed that there are two methods for repairing the shell: "reusing and reconnecting fragments" and "reconstructing with new secretions," and that they skillfully use these methods depending on the situation.

The ancestors of octopuses lost the shells they once possessed, but argonauts reacquired a calcareous shell-like structure. In other words, the argonaut's shell can be understood as an "extended phenotype" resulting from convergent evolution associated with adaptation to the open ocean.

■ Overview
Pelagic octopuses called argonauts, including the greater argonaut and paper nautilus, have a shell-like "egg case." In the early 19th century, female naturalist Jeanne Villepreux-Power observed through rearing experiments that this "shell" (egg case) is not borrowed from outside but formed by the octopus's own arms.

In this study, the fine structure of argonaut shells was observed and analyzed in detail using a scanning electron microscope (SEM) and an energy-dispersive X-ray spectrometer (EDS), revealing its formation process and evolutionary significance. That is, it was shown that the argonaut shell exhibits a unique formation mode different from typical molluscan shells, and Villepreux-Power's observation that the shell is secreted from the octopus's own first arms was supported after about 200 years.

Furthermore, it was revealed that there are two methods for repairing the shell: "reusing and reconnecting fragments" and "reconstructing with new secretions," and they are skillfully used. It was also found that the fine structure of the argonaut shell exhibits a convergent morphology with the fine structure of rapidly growing biomineral organs, such as chicken eggshells and coral skeletons. This indicates consistency with the results of our gene-level research published recently.

These results indicate that the egg case of the argonaut can be understood as an "extended phenotype" that convergently evolved in association with adaptation to the open ocean.

■ Research Background
Strange pelagic octopuses called argonauts, such as the greater argonaut and paper nautilus, which possess a calcareous shell-like structure called an "egg case," inhabit warm open oceans. The external appearance of this "pseudo-shell" is reminiscent of a nautilus or an ammonite, and has attracted the interest of researchers since ancient times. The oldest record of argonauts dates back to the 4th century BC (about 2000 years ago), in the book "History of Animals" by the ancient Greek philosopher Aristotle. It describes the octopus using its arms like sails or a rudder to manipulate the shell, and at the time, it was thought that they were using the shells of other organisms like hermit crabs.

This understanding was significantly changed by Jeanne Villepreux-Power, a French female naturalist who conducted research in Sicily (Italy) in the mid-19th century (about 200 years ago). She developed an underwater cage, conducted rearing observations of argonauts, and confirmed that they can form their own shells without being given one from the outside. At the same time, she also confirmed that if the first arms are excised, the shell is not formed, and when part of the shell is damaged, the fragments are gathered with the first arms to repair it. From these observational results, she concluded that the argonaut shell is not borrowed but formed by substances secreted from the first arms. The fact that "the octopus itself forms the shell" overturned the common sense of the time and caused a great sensation.

In addition, although the argonaut shell is a structure formed outside the body like a molluscan shell, it is deeply involved in the individual's adaptation through functions such as buoyancy control and reproduction, so it can be considered an "extended phenotype."

However, little research had been done on the structure and formation of the shell since then. Nevertheless, in the seas around Japan, cases of drifting argonauts washing up on beaches or being caught in set nets are frequently reported...