Gliding by a body of rocky reefs, a flash of light catches you from the corner of your eye. You swim closer, wondering if your imagination is taking you for a spin. The drift picks up and pushes your closer. There it is again, FLASH! A beacon luring your curious mind.
You reach out to the rock, steadying yourself. The flash came from the crevice, right there by your arm. Excited, with a hint of worry that some unsuspecting creature might be having you for a snack, you tilt your head lower, peeking at the crevice. Pulsing a smile of electric blue, a mollusk of some sort looks back at you.
It opens and closes its mouth, and the flash of blue becomes more frantic. Orange tendrils stick out from its mantle, like a warning to keep your distance, and you’re left mesmerized. What are they?
They go under many names from the literal electric clam, disco clam, and fire clams, to the more elaborate electric eye flashing fire shell, electric flame scallop and flashing file shell. And they are found throughout the indo-pacific.
It was long thought that they had the ability to produce bioluminescence, the biochemical emission of light by living organisms that occurs widely in marine vertebrates and invertebrates. But in the 1990s, it was discovered that they do not produce light but instead, reflect it.
These clams have highly reflective tissues on the very outer edge of their mantle that is exposed and then hidden very quickly, so the change back and forth from the white reflective tissue to the red tissue creates the appearance of flashing. They are also filter feeders, so their shells are constantly opening and closing in order to let water pass over their gills to pick up food. So you could imagine that stealing a bite of anything would be impossible for these clams without getting caught.
We are not sure yet if there is a purpose to the flashes. It could be a defense mechanism to ward off predators, a light to lure prey or even a signal to fellow clams. Heck, perhaps they don’t even know that are such flashy creatures. Until then, we shall continue to speculate that they’re simply the light of the party down there.