Generation of the streak-like flash in the inner-fold of the mantle in Ctenoides ales was investigated by video-camera recording, and stereo, light, fluorescence and electron microscopy. The stereo microscopy revealed the presence of a pale white band along the entire width of the marginal edge of the shell side surface of the inner fold. Since the flashing could not be seen in the dark, it was not due to the luminescence but the reflection of light. The light microscopy showed that the band region consisted of about ten rows of epithelial cells, cylindrical and 40 μm tall and 10 μm wide. The cytoplasm was densely filled with fine granules, eosinophilic in H-E staining. Under the electron microscope, those granules appeared as electron-dense vesicles, 0.5-0.6 μm in diameter, each containing a highly electron-dense spherical core, 0.2-0.3 μm in diameter. The cell had a nucleus, few mitochondria and lysosomes, however, other cellular organelles such as Golgi apparatus, rough and smooth endoplasmic reticulum were not evident, in the present observation. We assume that the electron-dense vesicles packed in the cytoplasm function to reflect light strongly. This highly reflecting structure found in C. ales is quite different from those have been reported in eyes of scallop and squid, and in iridophores of giant clam, cuttlefish, long-spined sea urchin and of fishes. The video observation showed that the mantle made a movement to roll the white band towards the shell-side and then, within a second, the rolling movement was released. The phase of the movement was different by the portions of the mantle, and the mantle edge made a wave-like motion. When the pale white band was hidden by the roll, the reflection of light disappeared. When the rolling was released, the reflection of light reappeared. Since the "roll and release movement" repeat quickly, it looks as though a streak-like flash run along the mantle margin.
