PHOTOSENSITIVITY IN INVERTEBRATES 



623 



400 424 



491 500 



575 585 600 



648 



700 m^i 



ultra- 

 violet 



vio- 

 let 



blue 



green 



yei-f 



llow: 



orange 



red 



Infra- 

 red 



410 



650 



FIG. I. The absorption maxima of extracted and synthesized photosensory pigments range across 

 much of the spectral range visible to man. Commonly accepted boundaries (jop) and representative 

 centers {holtom) of appropriate wavelengths of light arc shown for each hue sensation. Photosensory 

 pigments include: 463, euphausiopsin (Kampa, 1955); 491, rhodopsin (Kiihne, 1877) and cephalop- 

 sin (Bliss, 1948); 5.^2, porphyropsin (VVald, 1937); 5^.2, iodopsin (Wald, 1937); and 620, cyanopsin 

 (Wald, 1953). 



changes which accompany illumination of the organ 

 (6, 7, 87). Excised surviving eyes can be studied in 

 the same way although without gaining from them 

 any additional information (56, 211, 212). 



Far more unknowns arc encountered in trying to 

 learn about an animal's photosensitivity from its be- 

 havior either under laboratory conditions or undis- 

 turbed in its natural habitat. Yet the vast bulk of 

 physiological investigations on invertebrate vision 

 employ methods of this type. In them one advantage 

 can be seen : the reactions of the whole animal — even 

 in an artificial environment — must be closer to its 

 responses in normal life. By observing behavior, some- 

 thing more of the role of vision in ordinary situations 

 can be gathered. Isolated measurements of electrical 

 potentials are far more difficult to interpret on an 

 ecological basis. 



By far the most hazardous approach to photosensi- 

 tivity in animals is also the commonest. It is decep- 

 tively easy to examine their photosensory structures 

 anatomically and histologically and to infer how these 

 structures may be used. Valuable evidence can cer- 

 tainly be obtained as to limitations imposed by struc- 

 ture; but without careful experimentation with li\ing 

 individuals, there is no way to be sure that the aniinal 

 exploits its photo.sensory mechanism in its daily life. 



In most groups of in\'ertebrates the best that can 

 be done in summarizing findings on photosensitivity 

 is to relate the anatomical and behavior studies. This 



is approached most simply on a structural basis or on 

 a taxonomic framework (31, 77, 142, 193, 216, 269). 



PHOTOSENSITIVITY IN UNICELLULAR ORGANISMS 



Since both receptor and eff"ector are component 

 parts of the same cell in protozoans, photosensory 

 specializations are more limited than among meta- 

 zoans. Responses to light seem correspondingly re- 

 stricted to movements of the whole cell or of its loco- 

 motory structures, such as flagella. 



Cells Without Obvious Photoreceptors 



There is no a priori reason to assume that the re- 

 sponses to light found in amebas need correspond to 

 those in such flagellates as Peranema. In the former, an 

 increase in intensity of illumination is usually fol- 

 lowed by retraction of pseudopodia. The rate of 

 locomotion of amebas appears to be afifected signifi- 

 cantly by the intensity of continued illumination. 

 Initially the rate is modified by the state of dark 

 adaptation of the cell (183). Hertel, who investigated 

 the ultraviolet to 280 m// as a stimulus (104), postu- 

 lated that the radiations catalyzed the release of 

 hydrogen peroxide within the cell and that these 

 chemical changes accounted for behavior. Mast & 

 Stahler (183) believed, instead, that the light pro- 

 duced a physical change in the elastic strength of the 

 plasmagel, inhibiting the formation of pseudopodia. 



