The inside of the clam's lip is packed with tiny spheres of silica, the primary component of glass. These spheres are only 340 nanometers in diameter, and they are ideal reflectors, particularly of the blue light that penetrates deeper into seawater than red light. The outside of the lip contains no silica nanospheres. As a result, when the clam lip is furled, no light is reflected.
This turns out to be the secret behind the light show. Previously it was thought that the flashing was due to bioluminescence, a chemical reaction. Other marine organisms, such as lantern sharks, use bioluminescence.
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In this case, when the clam unfurls its lip - typically twice a second - the millimeter-wide mirror is revealed and reflects the ambient light, like a disco ball.
Now the question is: Why? Dougherty and her team are now exploring whether the clam is trying to attract prey (mostly plankton) or other clams and potential breeding partners - or if it is trying to scare away predators.