text: Cells observed under the electron microscope after a technique involving rapid freezing of the cells followed by substitution of water with a polymer (Fast-freeze Fixation/Freeze Substitution) contain a large number of electron dense bodies around the cell periphery. These structures correspond in size and location to the fluorescent bodies confirmed to be scintillons by their light emission, and they show colocalization of anti-luciferase and anti-LBP labeling meaning both bioluminescence proteins are found in the structures. Scintillons appear as cytoplasmic drops hanging in the vacuolar space, as they are almost completely surrounded by the vacuolar membrane. This structure led to the proposal that a voltage gated proton channel in the vacuolar membrane could allow an action potential to be propagated along the vacuolar membrane. This would in turn let protons enter into the cytoplasm around all the scintillons in the cells virtually simultaneously producing an intense but brief flash of light. Voltage gated proton channels were subsequently identified in a dinoflagellate confirming their predicted existence.