330 



THE BIOLOGY OF MARINE ANIMALS 



around 522-533 m/u. A visual pigment found in chicken cones is iodopsin, 

 which shows maximal absorption at 560 m/i. Iodopsin contains the same 

 carotenoid fraction as rhodopsin, but differs in its protein moiety (30, 31, 

 35, 39a, 39b, 54, 78, 79, 143, 144). 



A systematic study of invertebrate rhodopsins is only beginning. The 

 retina of the squid (Loligo) contains a relatively stable red pigment, with 

 maximal absorption at 493 m//. Squid rhodopsin, like that of vertebrates, 

 contains a retinene chromophore linked to a protein, opsin. Rhodopsins 

 with similar properties are found in the eyes of blue crabs (Callinectes 

 hastatus) and horse-shoe crab (Limulus polyphemus). These have absorp- 

 tion maxima around 480 m//. Other photosensitive pigments have maxima 



450 



600 



500 550 



Wavelength 



Fig. 8.19. Absorption Spectra of Photosensitive Pigments from the Retinae 



of Fish, Illustrating the Transition from an Exclusively Rhodopsin to 



an Exclusively Porphyropsin System 



The dogfish possesses only rhodopsin; eel and killifish predominantly rhodopsin; 

 brook trout predominantly porphyropsin; white perch only porphyropsin. (From 

 Wald (143).) 



(derived from difference spectra) as follows: Euphausia, 462 mju; Nereis, 

 505 m^; Asterias, 505 m/u; Homarus, 515 mju, (8, 9, 14a, 80, 120, 144#). 



The absorption characteristics of the visual pigments determine the 

 spectral sensitivity of the retina. So long as there is not differential absorp- 

 tion elsewhere in the optical system, the absorption spectrum of visual 

 purple should be reflected in the action spectrum of the photoreceptor or 

 organism, and many studies offer confirmation. The scotopic visibility 

 curve for vertebrate eyes is a close replica of the spectral distribution of 

 visual purple sensitivity in the species concerned. 



The lenses of fishes generally transmit visible light above 400 mju equally 

 with regard to wave-length, although in some species there is differential 

 absorption in the blue at wave-lengths up to 440 m/u (38, 39). 



