SENSORY ORGANS AND RECEPTION 



329 



failed to discriminate varying intensities of grey. Blennies, it was found, 

 possess a definite and wide range of colour vision, and were able to 

 discriminate blue, green and red from grey (19, 69, 148). 



Photosensitive Pigments 



For light to affect photosensitive tissue it must be absorbed by some 

 pigment and produce a photochemical change leading eventually to sensory 

 excitation, The first of these photolabile pigments to be isolated was visual 

 purple or rhodopsin, associated with rod function in some vertebrate eyes. 

 Rhodopsin consists of a carotenoid retinenej (vitamin A 1 aldehyde) con- 

 jugated with a protein opsin. An allied pigment, porphyropsin, is found in 



400 



500 



BOO 



600 



600 400 



Wave- I ength (rn/J) 

 Fig. 8.18. (a) Average Visibility Curve for the Lateral Eye of Limulus. 

 Action Spectrum Determined from Impulse Discharge in Single Optic 

 Nerve Fibres (from Graham and Hartline, 1935). (b) Spectral Sensitivity 

 Curve for the Eye of Eledone. Data Derived from Retinal Electrical 

 Response (from Bliss (8), after Piper.) 



the rods of many fishes : its prosthetic carotenoid is retinene 2 (vitamin A 2 

 aldehyde). Marine fishes usually possess rhodopsin. Porphyropsins are 

 characteristic of freshwater teleosts, but also occur in some marine forms 

 (Labridae, Coridae) and in species which migrate to and from the sea 

 (alewife, salmon, trout). The eel (Anguilla) and killifish {Fundulus) have 

 both pigments, but predominantly rhodopsin (Fig. 8.19) (143). 



The absorption maximum of rhodopsin lies at about 500 m//, with 

 variations for different species between 490 and 502 m//. Some visual 

 purples from fish have peaks as follows: Petromyzon man'nus, 497 m//; 

 Squalus acanthias, Pleuronectes platessa, Trigla lucerna and Gadus polla- 

 chius, 500 m/n. Conger eels and deep-sea teleosts have golden rhodopsins, 

 called chrysopsins, with absorption maxima around 485 m//. The absorp- 

 tion bands of porphyropsins are displaced towards the red, with maxima 



M.A.— n* 



