University of Tsukuba reveals mineral nutrition system of pill bugs

A research group led by Associate Professor Atsushi Kyono of the Institute of Life and Environmental Sciences at the University of Tsukuba has shown that the common pill bug (Armadillidium vulgare) builds its exoskeleton (dorsal shell) by changing the structure of minerals it eats. The findings were published online ahead of print in the Journal of Structural Biology.

Many organisms, including crustaceans and shellfish, use minerals such as calcium carbonate to form their hard exoskeletons. This process of mineral production in the body by living organisms is known as biomineralization.

The exoskeleton outer shell (dorsal shell) of the pill bug is made of calcium carbonate (CaCO₃) and serves to protect the body from predators. It is well known that when keeping pill bugs, it helps to place stones in the enclosure to support shell formation. However, what kinds of stones affect shell formation had not previously been studied in detail.

Calcium carbonate exists in different crystal forms, such as calcite and aragonite, which share the same chemical composition but have different crystal structures. The dorsal shell of the common pill bug is known to be also made of calcium carbonate and to have a complex layered structure made up of calcite crystals and a non-crystalline form of calcium carbonate called amorphous calcium carbonate (ACC). ACC in particular is an important substance that helps adjust the hardness and flexibility of the dorsal shell, but how it forms and stays stable had not been fully understood.

The research group conducted experiments in which pill bugs were raised on different minerals and the effects on dorsal shell formation were examined. Specifically, the bugs were fed calcite, aragonite, or quartz (SiO₂), which contains no calcium carbonate, and raised for about 60 days. The shells that formed were then analyzed in detail using electron microscopy, Raman spectroscopy, and synchrotron X-ray diffraction.

Electron microscopy showed that in individuals fed calcite or aragonite, mineralization of the dorsal shell progressed, and a layered structure developed in both the outer layer (exocuticle) and the inner layer (endocuticle). Shell thickness reached about 60 to 80 micrometers.

In contrast, individuals fed quartz, which contains no calcium carbonate, showed much weaker mineralization, and dorsal shell thickness reached only about 30 micrometers. This indicates that calcium carbonate is an important source of calcium for dorsal shell formation.

Furthermore, Raman spectroscopic analysis revealed that calcite-type ACCs are formed in the endocuticle, regardless of the type of mineral given. While aragonite-type ACC also exists as an amorphous form of calcium carbonate, even when the bugs were fed aragonite, all ACC found inside the dorsal shell was of the calcite type. The outer exocuticle was also observed to transition from ACC to calcite.

Synchrotron X-ray diffraction analysis showed that all the crystalline phase detected in the dorsal shell was calcite, and no aragonite crystals formed in the dorsal shell, even when aragonite was fed. These results show that pill bugs do not simply use the crystal structure of the minerals they eat as-is in their dorsal shells. Instead, they rebuild the structure of calcium carbonate inside their bodies as they form their shells.

The research group plans to next investigate how ACC forms inside the shell and what kinds of biological molecules control this process.

These studies are expected to lead to an understanding of the mechanisms of lightweight and strong structural materials made by living organisms, and in the future, to the development of biomimetic materials that mimic the structures and functions of living organisms to develop new materials and technologies.

Journal Information
Publication: Journal of Structural Biology
Title: Effects of mineral nutrition on the cuticle structure of Armadillidium vulgare
DOI: 10.1016/j.jsb.2026.108312