DOUBLET Stentor DO NOT DISPLAY PHOTODISPERSAL

Normal Stentor, called singlets since they have a single membranellar band and oral groove surrounding their frontal field, swim away from light sources and collect in the darker areas of an unevenly illuminated container (photodispersal). Phenotypic variants, called doublets since they have 2 membranellar bands and 2 oral grooves, do not exhibit this behavior. Doublets produce photophobic responses and contractions when illuminated at the same fluence rates which produce those responses in singlets, hence their sensitivity to light is normal. Illumination of the frontal field of doublets produces a photophobic response at lower fluence rates than does illumination of their side or posterior. This directional sensitivity is quantitatively similar to that observed in singlets. However, doublets do not reorient their swimming direction after a phobic response as extensively as do singlets. This failure in reorientation is the probable reason that doublets fail to show photodispersal. These results imply that the mechanism producing photodispersal in singlets depends on photophobic responses or some other, presently undescribed, response which requires an asymmetric frontal field.