The number of scintillons, which are cellular organelles responsible for light emission in the marine alga Gonyaulax, were counted by both immunofluorescence and electron microscopic methods and found to vary tenfold between subjective day and subjective night. The number of scintillons peaks during the subjective night, as does stimulated bioluminescence (flashing). Furthermore, the number drops sharply at the time of the maximal spontaneous bioluminescence (glow), which occurs at the end of the night phase, suggesting that the breakdown of scintillons may be responsible for this mode of emission.

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