The spines are composed of nanocrystalline hydroxyapatite, protein(collagen), and water—the same materials that scales are made of. But unlike other scales, which originate during development from the mesoderm layer of the skin or dermis, the spines appear to have evolved from specialized cells located under the surface of the skin.

These cells are called skeletal progenitor cells, and they are also the source of other hardened body parts, like the jaws, fins, and skulls. When the skeleton of a puffer fish is inflated, the progenitor cells form spines and other bony appendages in a process known as ossification.

The ossification process is fairly quick—it takes about 24 hours for the spines to fully develop. But when the fish is deflated, the ossification cells quickly revert to their unmodified state—and the spines come tumbling down.

So the spikes get more bone-like when inflated?? There are also “fibers” connecting the spines that rotate the spikes outward when taut. It must be quite difficult to rebuild the skeleton for display!