Chlamydodon mnemosyne, a brackish-water ciliate which feeds on cyanobacteria, is capable of sensing the direction of light. Cells are negatively phototactic in the well-fed state and tend to swim towards the light source when mildly starved. Severely starved cells normally fail to show phototactic responses. An autofluorescent substance, which is present in all life cycle stages, occurs in, or immediately beneath, the plasma membrane of this ciliate. It is located in the anterior left side of a cell, in the same region where mildly starved cells accumulate small orange globules that form a structure known as the stigma. The diameter of the whole area where the autofluorescent substance is located appears to be smaller than the stigma; typically, it consists of two rows of blue-green fluorescence, each row subdivided into 5–10 squares. Since the blue-green autofluorescence is excited by both blue (450–490 nm) and near-ultraviolet (340–380 nm) light, it possibly originates from flavin- and/or pterin-like molecules. We suggest that the autofluorescent substance located in or beneath the plasma membrane of Chlamydodon mnemosyne acts as a photoreceptor pigment in phototaxis and that photo-orientation of this ciliate is triggered by a combined mechanism involving the photoreceptor and either the stigma or a number of light-absorbing food vacuoles as a shading device.

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