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Can we not see this even in things without anything like what we would consider a nervous system or senses? Don't plants also show photosensitivity?
There are quite a few unicellular organisms (single celled critters) that are sensitive to light, and many can direct themselves away or into light using photosensitive reactions. Just a quick scan of pubmed, a photo-reaction in the ciliated protozoan Blepharisma creates ion flow within the cytoplasm, not unlike the ion flows created across the membrane in mammalian retinal cells:
J Exp Zool. 2001 Jun 1;289(7):467-71. Related Articles, Links
Photosensory transduction in unicellular eukaryote Blepharisma.
Matsuoka T, Kotsuki H.
Department of Biology, Kochi University, Kochi 780-8520, Japan. tmatsuok@cc.kochi-u.ac.jp
In the ciliated protozoan Blepharisma, step-up photophobic response is mediated by a novel type of photosensory complex of pink-colored pigment "blepharismins" and 200-kDa membrane protein contained in the pigment granules located just beneath the plasma membrane. We found that the fluorescence intensity of isolated blepharismins decreased prominently with a decrease of H(+) concentration in the surrounding medium. In the present study, therefore, we utilized the endogenous pigment blepharismins as the pH indicator. Light stimulation evoked a sudden decrease in fluorescence intensity in a photosensitive anterior portion of the cell, suggesting that a drop in H(+) concentration occurred in the anterior region. The result indicates that the photosignal is transduced into cytoplasmic signaling of H(+) translocation across the outer membrane surrounding the pigment granules, so that cytosolic H(+) concentration in the vicinity of plasma membrane might be increased.
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