Disco clams put on a streak show

FLASHING LINES  Up close, a disco clam native to Indo-Pacific waters flashes bright lines along the top and bottom edges of its mantle.

Courtesy L. Dougherty

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Blow a kiss. Then pinch your lips into a thin line. Now alternate kiss and pinch several times a second for a pathetic, low-wattage human attempt at mimicking a disco clam.

Scuba divers call Ctenoides ales the disco or electric clam because the restless, curling lips of its mantle flash bright streaks. “It’s very vivid and very dramatic,” says Lindsey Dougherty of the University of California, Berkeley. She has made progress discovering how the poorly understood clams create a streak show. But that only deepens the puzzle of why.

Dougherty helped bust the myth that the clams bioluminesce, an idea so reasonable and persistent that she makes sure to say it’s wrong at least twice in each scientific presentation. The clams don’t make light themselves, but unfurl a supremely reflective strip along the lips of the mantle. Reflected light winks off the strip, then the lip rolls up like a window shade, going dark for an instant before unfurling again.

Flashing clams are often found in multiples, like these two fastened in the same crevice. Courtesy L. Dougherty
Since it’s reflected light, Dougherty wonders why the clams end up in dimly lit spots. They range as deep as 50 meters and favor crevices within caves. “Holes within holes,” she says. And just as puzzling, the clams keep moving their lips at night when there’s virtually no light to reflect.

About half the clam crannies that Dougherty visited last summer housed more than one clam, so she wonders whether it’s possible that free-floating youngsters are drawn to the flashing beacons of adults. Adult clams have eyes (she has counted up to 40) but scientists haven’t seen the youngsters, much less figured out if they can see.

Tiny water creatures that clams eat might also be attracted to the flashing display, Dougherty speculates. “It reminds me of one of those ‘Eat Here’ signs,” she says. If this is molluscan deception, then the disco clam may also be the all-night-diner clam. 

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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