The Ctenoides ales clam is also known as the ‘disco clam’ because of its flashing light display. The mantle, which forms the outer wall of clam’s body and encloses its internal organs, flashes in an impressive photic display. Often confused for bioluminescence, which is the production and emission of light by an animal, the disco lights of the Ctenoides ales clam is actually down to the reflection of beams of light.
A team of scientists from the US and Australia have set about testing the display of the clam in the lab and in their natural environments in Australia and Indonesia, to find out how and why it puts on its display. The researchers found that the C.Ales has a unique edge to their inner mantle folds which has two distinct sides. One side is strongly scattering of light, while the other side of the tissue is 16 times less reflective so scatters very little light.
Using high-speed video of 1000 frames per second, the team captured the wave-like motion of the clam which move its reflective tissue in and out. Unfurling the tissue exposes the highly reflective side while the furling motion exposes the poorly reflective side. The rapid wave-like movement creates the flashing appearance.
Transmission electron microscopy, or TEM, and X-ray spectroscopy team revealed that the reflective side of the clam’s mantle contains tiny electron dense spheres made out of silica. The duller side lacked any of these tiny glassy spheres. The beads were 0.30±0.04 μm in diameter, which the team say is close to the perfect size for scattering visible light at short wave lengths- around the same range of light the clams experience in their underwater habitats.
The team questioned why the clam unfurls in this way. Motions of the inner mantle of all sorts of bivalves help draw seawater into the gills for filter feeding. The motion of the disco clam’s mantle differs during its display in that only the very edge of the tissue moves rapidly, suggesting the primary reason for the wave-like movement is neither feeding nor respiration. Instead the team say the flashing might act as a warning signal to scare away predators or to attract other disco clams who tend to be found huddled in crevices in small groups.
Video credit: Lindsey F. Dougherty, Sönke Johnsen, Roy L. Caldwell and N. Justin Marshall