You can tell just by looking at Ctenoides ales that this is not the kind of bivalve you'd find in your clam chowder. This reddish-orange mollusk, which makes its home in clusters in the caves and crevices of Indo-Pacific coral reefs, creates flashing light shows so bright that they can be seen without artificial light—hence its common name, the disco clam. Scientists weren't quite sure why, or how, the mollusks flashed; they thought it might be bioluminescence, a chemical reaction that creates light within an animal. But recent research, conducted by University of California, Berkeley graduate student Lindsey Dougherty and scientists from Duke University and the University of Queensland, Brisbane, Australia, shows that there's something a little more complicated going on.
Dougherty used a number of high tech tools—including a transmission electron microscope, a spectrometer, an energy dispersive x-ray spectroscope, and high speed video—to examine the clam mantle lip, and found that the flashes are created not by bioluminescence but by a double layer of specialized tissues. The inside of the clam's lip is packed with spheres of silica that make the tissue reflective to light, like a mirror (or a disco ball!); on the other side of the lip, where no silica balls are present, light is absorbed. When the clams rapidly roll and unroll the tissues—typically at a rate of two times a second—it creates the appearance of flashing. Dougherty could find no other bivalves that have evolved this mechanism; the question is, why do they need it?
Dougherty and her team had a few hypotheses about why the clams flash. Examining the clams' eyes under a microscope showed that, although they have 40 tiny eyes, their eyesight is probably too weak to see displays from other clams, ruling out flashing for the purposes of finding a mate. "We did not find much chemical or visual attraction to one another, and research into their eyes suggests they may not be able to perceive the flashing in one another," Dougherty told LiveScience. But the other two hypotheses had more promise: Flashing to attract prey and repel predators.
To test the prey hypothesis, the scientists released phytoplankton into the tank in their lab. When the clams sensed the prey, their flashing increased. Though some plankton are attracted to light, it's unclear if this is true for the disco clam's prey, and researchers plan to study this question further in the field.
Natural predators of the disco clam include octopuses, mantis shrimp, and some species of snails. But for their first test of the predator hypothesis, scientists used a different kind of foe: A styrofoam lid, which they moved over the clams as if a predator was looming. The clams' flashing went from a rate of 1.5 times a second to 2.5 times a second when they sensed the lid.
Next, they unleashed an actual predator in the tank. Odontodactylus scyllarus, the peacock or harlequin mantis shrimp, uses its claws—which can deliver 160 pounds of force—to break open clams and other prey. The shrimp attacked the clam a few times, each time retreating from it and, eventually, going into what seemed to be a catatonic state (and then it got a little frisky with the mollusk). "They're very aggressive critters, and to have a clam open and flashing, and the mantis shrimp not attacking, is very weird," Dougherty told LiveScience. "That is very strange behavior [for the mantis shrimp]."
In both experiments, the researchers found high levels of sulfur in the water; Dougherty thinks the clams might be producing an acidic mucus in its tentacles that repels predators. "If you're flashing and saying, 'I'm distasteful; don't eat me,' that's one thing, but you have to sort of back it up," she said.