One of the more unusual species of clam is the colourful, flashy "disco clam", Ctenoides ales, which resides in the chthonic darkness of sub-aquatic caves. It's recognisable for its fringe of vivid orange tentacles -- and for the flashing ribbons of light that ripple along its lips.
This flashing effect, University of California at Berkeley researcher Lindsey Dougherty discovered last year, is a result of the unique composition of the clam's lip -- rather than bioluminescence, as had been assumed.
One side of the clam's lip is packed with highly reflective, tiny balls of silica, which work much like, well, reflective surfaces: cat's eye retroreflectors, for example, or, say, a mirror ball. When the clam ripples its lip, light bouncing off the silica causes a flashing light show. The balls of silica are so reflective that they work even with the very low levels of blue light found in a cave.
As for why the clams put on such a display? Well, further research has just revealed why, and it's not really surprising: it's for scaring off predators, and luring the sort of prey that's attracted to light.
Using disco clams in a lab setting, Dougherty and her colleagues -- Professor Roy Caldwell and undergraduate Alexandria Neibergall -- tested three hypotheses for why the clams flash: first, attracting a mate; second, scaring off predators; and third, attracting food.
To be used to attract a mate, the flashing needs to be visible to other clams. After studying the structure and proteins in the clams' eyes, the team determined that their vision is too poor to be able to detect other clams visually -- so that hypothesis was out.
Next, they tested how the clams might react to predators, by "looming" a false predator over the clams -- in this case a Styrofoam lid in mimicry of a real predator. The clams' flash rate, they found, doubled from just under 2Hz to just under 4Hz. The team also found high levels of sulphur in the clams' tentacles, indicating that the clams might be producing noxious acidic mucus to stun and repel predators.
Finally, the team introduced phytoplankton -- food -- into the tank, at which point the clams' flashing increased significantly. Some phytoplankton species are attracted to light, which indicates that the technique may indeed work to lure food -- much like an anglerfish's bioluminescent lure. However, the team needs to conduct further study to confirm that this is indeed the case.
Check out a video of the groovin' mollusc below.