Starfish grow extraordinary crystals.

The growth and form of crystals in vivo—biomineralization—challenge many ideas about crystalline materials. One typically pictures a crystal as a sterile object with a regular geometric form, but nature frequently challenges this preconception. This is beautifully illustrated by the calcite plates (stereom) of echinoderms, which exhibit complex, sponge-like morphologies and curved surfaces and yet behave as single crystals. Extraordinarily, the stereom of certain species is highly ordered. Observations of the sea urchin Cidaris rugosa more than six decades ago (1) identified a stereom reminiscent of one of the three simplest triply periodic minimal surfaces (TPMS), the so-called P surface, with a lattice parameter four orders of magnitude larger than that of calcite. On page 647 of this issue, Yang et al. (2) provide a new example of a TPMS at this extreme length scale. Detected in the knobbly starfish Protoreaster nodosus, the stereom resembles another simple TPMS, the D surface.