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  • José H. Leal

The Enigma of Hidden Colors

Colors in animals are almost always linked to visual behaviors and species interactions. Bright colors and patterns on mollusks may help, for instance, attract a mate, warn a predatory fish of a foul-tasting or poisonous sea slug, or camouflage a snail against its habitat. We expect that most colors “serve a purpose” based on another animal, “friend or foe,” being able to see those colors. But what if pigments are present inside a clam shell, in an area that is never seen by another animal? Take for instance, the two species in the illustration, the Knobby Scallop, Caribachlamys pellucens, and the Buttercup Lucine, Anodontia alba. The inside of the shell in both species is yellow, and we can assume that those colors are not advantageous for visual communication of any kind. But why is the color present in each case? Suzanne Williams from the Natural History Museum in London, who researches colors in mollusks, has recently started studying hidden colors in shells. She suggested a few hypotheses to explain their presence, all of which still need to be scientifically tested. For one, the hidden color could be associated with mechanical reinforcement of the shell. It could also be a way for the bivalve to get rid of metabolites, substances that result from the life processes. Last but not least, the color could be a left-over from the juvenile stage, when the shell was more “transparent,” with the color showing through. But there are no definitive answers yet. (Suzanne’s research has temporarily stopped following the onset of COVID-19 in the UK, and should resume in August, when the Natural History Museum re-opens for staff.)

Hidden yellow color inside the shells of (left) the Knobby Scallop and Buttercup Lucine.Views of the external surfaces (bottom) are included for comparison.  Photos by James F. Kelly.
Hidden yellow color inside the shells of (from left) the Knobby Scallop and Buttercup Lucine. Photos by James F. Kelly.
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