The 'disco' or 'electric' clam Ctenoides ales (Limidae) is the only species of bivalve known to have a behaviourally mediated photic display. This display is so vivid that it has been repeatedly confused for bioluminescence, but it is actually the result of scattered light. The flashing occurs on the mantle lip, where electron microscopy revealed two distinct tissue sides: one highly scattering side that contains dense aggregations of spheres composed of silica, and one highly absorbing side that does not. High-speed video confirmed that the two sides act in concert to alternate between vivid broadband reflectance and strong absorption in the blue region of the spectrum. Optical modelling suggests that the diameter of the spheres is nearly optimal for scattering visible light, especially at shorter wavelengths which predominate in their environment. This simple mechanism produces a striking optical effect that may function as a signal.

The ‘disco’ clam Ctenoides ales (Finlay, 1927) is a marine bivalve that has a unique, vivid flashing display that is a result of light scattering by silica nanospheres and rapid mantle movement. The eyes of C. ales were examined to determine their visual capabilities and whether the clams can see the flashing of conspecifics. Similar to the congener C. scaber, C. ales exhibits an off-response (shadow reflex) and an on-response (light reflex). In field observations, a shadow caused a significant increase in flash rate from a mean of 3.9 Hz to 4.7 Hz (P=0.0016). In laboratory trials, a looming stimulus, which increased light intensity, caused a significant increase in flash rate from a median of 1.8 Hz to 2.2 Hz (P=0.0001). Morphological analysis of the eyes of C. ales revealed coarsely-packed photoreceptors lacking sophisticated structure, resulting in visual resolution that is likely too low to detect the flashing of conspecifics. As the eyes of C. ales are incapable of perceiving conspecific flashing, it is likely that their vision is instead used to detect predators.

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