Structure, fluorescent properties and proposed function in phototaxis of the stigma apparatus in the ciliate Chlamydodon mnemosyne

Chlamydodon mnemosyne, a brackish-water ciliate which feeds on cyanobacteri a, is capable of sensing the direction of light. Cells are negatively photo tactic in the well-fed state and tend to swim towards the light source when mildly starved. Severely starved cells normally fail to show phototactic r esponses. An autofluorescent substance, which is present in all life cycle stages, occurs in, or immediately beneath, the plasma membrane of this cili ate. 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 str ucture known as the stigma. The diameter of the whole area where the autofl uorescent substance is located appears to be smaller than the stigma; typic ally, it consists of two rows of blue-green fluorescence, each row subdivid ed into 5-10 squares, Since the blue-green autofluorescence is excited by b oth 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 a utofluorescent substance located in or beneath the plasma membrane of Chlam ydonon mnemosyne acts as a photoreceptor pigment in phototaxis and that pho to-orientation of this ciliate is triggered by a combined mechanism involvi ng the photoreceptor and either the stigma or a number of light-absorbing f ood vacuoles as a shading device.