Disco clams get their name from the rippling light show on their mirrored lips, visible even in the dim blue depths. Now their secret has been revealed.
Lindsey Dougherty, a graduate student at Berkeley University of California, has been studying the clams for four years and has confirmed that the flashing is not, as most people assumed, a form of bioluminescence — a chemical reaction inside animals like plankton that produces light similar to that of a glow stick. Instead, she found, the edge of the clam’s mantle lip is highly reflective on one side. When the clam unfurls its lip — typically twice a second — the millimetre-wide mirror is revealed and reflects the ambient light, like a disco ball.
The inside of the clam’s lip is packed with tiny spheres of silica, only 340 nanometers in diameter, that are ideal reflectors, particularly of the blue light that penetrates deeper into seawater than does red light. The outside of the lip contains no silica nanospheres, so when the lip is furled, no light is reflected.
By repeatedly unfurling and furling the lip, the clam produces a continual rippling light show. The non-reflective back of the lip strongly absorbs blue light, so it appears dark and makes the contrast between the sides even more striking.
The big question, Dougherty said, is why the clam flashes at all.
Disco clams (Ctenoides ales) are found in tropical areas of the Pacific Ocean, living in crevices in reefs and typically in clusters of two or more. Light is dim and blue-green at the clams’ typical depth, which ranges from 3 to 50m (10-150ft), but their rippling mirrored lips are visible even without artificial illumination. Dougherty said the question she is exploring is 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.
In ongoing experiments, she is studying the structure of the clam’s eyes — all 40 of them — to see whether they can even see the disco light and is raising clams in tanks to determine if they signal one another visually or chemically, and is testing their responses to fake predators.
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