For Fish, Looking Beautiful Takes Time

Ashley L. Brenon

In a guanine crystal growing contest, humans might be most prolific, but the Japanese koi fish (Cyprinus carpio) would take the prize for highest reflectivity. This species has been growing a supershiny version of the guanine photonic crystal structure for far longer than humans have been in the laboratory. Guanine crystals give the koi’s skin the metallic luster that researchers think is useful in protecting the fish from predators at the water’s surface.

This scanning electron micrograph image shows the contrast between the elongated semihexagonal platelike forms of isolated biogenic crystals from the skin of Japanese koi fish (A) and the stepped and much thicker morphology of anhydrous guanine crystals grown in vitro (B). Also shown is a schematic representation of the biogenic crystal’s primary crystal planes (C) and the multiple-layer construction of biogenic crystals in situ (D). Reprinted with permission of Crystal Growth & Design.

Often used as an iridescent additive to cosmetic products, guanine, as scientists have known for some time, is the substance responsible for the reflective quality of fish skin. However, until recently, the exact relationship between the morphology of biogenic and man-made crystals was unknown.

Researchers at the Weizmann Institute of Science and at Hebrew University, both in Rehovot, Israel, compared the differences between the fish’s guanine crystals and those formed in the laboratory, searching for clues about controlling biogenic crystal morphology.

This illustration compares the theoretically expected fish crystal (top) with the actual fish crystal morphology (bottom). Miller indices indicate theplanes and directions of the crystal lattices. Negative integers are written with a bar above the number.

They used powder x-ray diffraction, high-resolution scanning electron microscopy, transmission electron microscopy and theoretical morphology simulations to contrast guanine crystals grown naturally in koi with those grown in vitro.

Although the two types of samples are similar, the fish crystals are remarkably thin, measuring about 50 to 100 nm in thickness with well-defined crystal faces. Arranged according to their largest reflective surface, they form crystal stacks that cause the interference of visible light, which generates the metallic luster.

This image of the fish scale shows the reflection produced by guanine crystals.

By contrast, the laboratory-grown crystals are thicker and irregularly stepped. Bulkier and comparatively disorganized, they cannot form reflective crystal multilayers.

To study the crystals’ morphologies, the researchers examined the experimental and theoretically expected rates of growth in various directions. They were surprised to learn that, whereas in theory the morphology should demonstrate that crystals elongate quickly along their short crystal axes in the direction of stacking, the fish’s crystal-forming mechanism inhibits growth in the stacking direction. Although exactly how this occurs is not yet understood, the phenomenon of biogenic crystal formation creates thin, highly reflective plates more slowly than does in vivo crystal formation.