CAROTENOIDS 



As realized by Steven, there is no reason why fish should absorb 

 astacin under any conditions. As was pointed out in Chapter V(see^. 170) 

 this is an artefact produced by oxidation of an astaxanthin and under 

 natural conditions fish would never be presented with it. Steven's 

 suggestion that failure to absorb lutein might be due to a similar 

 reason cannot be easily upheld ; with careful handling in extracting 

 this pigment from plant materials the only possible change it is likely 

 to undergo is cis-trans isomerization, and it is well known that cis 

 isomers of xanthophylls which are vitamin A precursors {e.g., crypto- 

 xanthin) are absorbed by rats and still possess some vitamin activity. ^ • 

 There is one report that the (? carotenoid) pigmentation of Esox 

 lucius is also of alimentary origin. * ° 



0-7 



EryChrophore 

 Xanthophore 



400 



450 



500 550 



Wavcleno!:'] (mp) 

 Fig. 27. — The absorption spectra of single Chromatophores of the trout {Salmo 

 trutta). 

 (From Steven, D. M. {1948) J. Exp. Biol., 25, 369.) 



Function 



(i) In Phototropic Responses 



Goodrich, Hill, and Arrick^^ examining a number of fresh water 

 tropical fish showed that the xanthophylls, lutein, zeaxanthin, and 

 taraxanthin were all concentrated in the xanthophores. The erythro- 

 phore pigment was not a carotenoid but a pterin-like pigment which 



206 



