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Materials science

Give a shell a break

Giant conches are seldom treated with the respect they deserve. Their impressive shells are prized as holiday souvenirs, but size and aesthetics are only half the story. At the microscopic scale, they are one of nature's greatest engineering masterpieces: a stunningly intricate hierarchical architecture of inorganic crystals, interwoven with organic molecules.

In search of fresh insight into the synthesis of such biomineral composites, and clues for how to design synthetic versions, Xiao-Wei Su and colleagues have performed a series of experiments with shells of Strombus gigas (right), the giant queen conch of the Caribbean. They drilled holes in the shells of live juvenile specimens, then, through X-ray diffraction and electron microscopy studies, monitored how the animals repaired their shell tissue (Chem. Mater. 16, 581–593; 2004).

In natural shell growth, an organic outer layer (the periostracum) is deposited first. It remains unmineralized but provides a base on which the mineralized shell is deposited. First, a micrometre-thick layer of perpendicularly oriented, elongated crystals forms, abutting the periostracum. Then comes the body of the shell, which grows to a thickness of a few millimetres and has a three-layer ‘crossed-lamellar’ structure (left image, right).

Su et al. found that the process of shell repair is somewhat different from natural growth. In a wounded conch, 24 hours after damage, a transparent organic membrane was deposited over the drilled hole. Once the membrane was in place, Su et al. saw that fine crystallites of aragonite — a particular crystal form of calcium carbonate — nucleated rapidly on the organic membrane. Each conch generated a 100-µm depth of such abnormal tissue in the damaged area (right image). After 6–8 days, elongated crystals were deposited, oriented perpendicular to the direction of shell growth. As in normal shell development, the setting down of these elongated crystals preceded the formation of the crossed-lamellar microstructure (seen at the bottom of the image).

In uncovering the development of the unusual organic–inorganic layers in shell repair, Su et al. have provided a window onto the complex process of shell formation. It remains to be discovered how the interplay of organic and inorganic components is controlled at the molecular level, in conch shells as well as in other mineralized structures.

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