Crystals Create Iridescent Sheen

silver fish with black spots swimming in the water

Biological Strategy

Rainbow trout

AskNature Team

Image: Karel Bartik / Shutterstock / Some rights reserved

Modify Light/Color

Color in living systems comes from pigments and the interaction of light with surfaces. Color serves many purposes for living systems, such as attracting prey or mates, providing warnings, or protecting through camouflage. To create the effects needed for each purpose, living systems must control the expression or visibility of pigments and the interactions (such as reflectance and refraction) of light. To do so, they have strategies that modify color or light to increase or decrease the color’s position, intensity, opacity, and more. Male hummingbirds, for example, have brightly colored feather patches on their throats; the coloring comes from pigments, structures that refract light, or a combination of the two. When a male hummingbird hovers near a potential mate, it modifies the angle of these feathers to create a bright, colorful display. However, when it needs to mute the colors to reduce conspicuousness, such as to avoid a predator, it modifies the angle again.

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Fish

Class Agnatha (“without jaws”), Class Chondrichthyes (“cartilage fish”), Superclass Osteichthyes (“bone fish”): Sharks, eels, snapper, hagfish

The fish are a diverse group, comprising multiple classes within Phylum Animalia. The most well-known classes are Chondrichthyes, which has sharks and rays, and superclass Osteichthyes, which has all bony fish like cod and tuna. Unlike other vertebrates, fish only live in water. They use special adaptations like fins, gills, and swim bladders to survive. Most are ectothermic, meaning their body temperature depends on the water temperature around them. Over half of all vertebrates are fish. They’re found from the bottom of the sea to high mountain lakes.

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The scales of a rainbow trout have a silvery shine due to guanine crystals.

“The scales of a rainbow trout, reflective yet translucent. The silvery lustre is due to crystals of guanine, produced in the body as a waste product. The tiny black speckles are cells, and at intervals there are large clusters of these where the pigment is distributed right across each cell, instead of only in the centre; here we get the large black patches which produce colour changes in response to s released during the breeding season – hence the name rainbow trout. The overlapping scales form a waterproof armour which is flexible enough to allow for the flexing of the body during swimming.” (Foy and Oxford Scientific Films 1982:87)

Foy S, Oxford Scientific Films. The Grand Design: Form and Colour in Animals. Lingfield, Surrey, U.K.: BLA Publishing Limited for J. M. Dent & Sons Ltd, London; 1982. 238 p.