COLOUR CHANGES 531 



to accentuate light and dark phases. They were then placed in white or 

 black tanks where they were exposed to attacks by the Galapagos penguin 

 Spheniscus mendiciilus. Pale and dark fish were placed in each tank, where 

 they were attacked by penguins, and the number of survivors recorded. 

 Of a total of 1,726 fish it was found that 43 % were eaten by the penguins. 

 In the white tank 61 % of the fish captured were in the dark phase and 

 39 % were pale. In the black tank 26 % of the fishes captured were dark and 

 74% were pale. The results show that pale fishes had an advantage in 

 escaping predation on a white background and dark fishes had an advan- 

 tage on a black background. 



Other functions of chromatophores may now be considered. The prim- 

 ary response, in which chromatophores expand when the animal is 

 illuminated, independently of background conditions, is found in the 

 larvae of some fish, for example pleuronectids, in which it is succeeded in 

 the adult by secondary background responses, and also occurs in some 

 lower forms. A primary response in the polychaete Platynereis dumerili, 

 and the echinoids Centrostephanus longispinus and Arbacia pustulosa, has 

 been described above. The black and white chromatophores of the fiddler 

 crab Uca pugilator are normally dispersed in the daytime, and concentrated 

 at night. These conditions are determined by several factors, an endogenous 

 diurnal rhythm, and a response to total illumination which results in 

 increased dispersion of both black and white pigments as the intensity of 

 illumination increases. Certain Crustacea living in brightly illuminated sur- 

 face waters among Sargassum weed, such as Latreutes fucorum, Palaemon 

 tenuicornis and Hippolyte acuminata, are richly provided with leucophores, 

 which form a very effective reflecting screen. Expansion of chromatophores 

 in these animals would seem to have the function of protecting their living 

 tissues against the harmful effects of actinic rays (14, 38). 



Another environmental response attributed to chromatophores is that 

 of temperature regulation. In a submerged animal this would be of little 

 consequence because of the great heat-absorbing capacity of water, but it 

 may be a physiological factor in littoral forms periodically exposed to the 

 air. In Uca the black pigments tend to concentrate as the temperature 

 is raised, and the white pigments to disperse: these chromatophore 

 movements lead to a condition in which there is reduced light absorption 

 and increased reflexion (21). 



REFERENCES 



1. Abramowitz, A. A., "The double innervation of caudal melanophores in 

 Fundulus," Proc. Nat. Acad. Sci., 22, 233 (1936). 



2. Abramowitz, A. A., "The pituitary control of chromatophores in the dog- 

 fish;' Amer. Nat., 73, 208 (1939). 



3. Abramowitz, A. A., "Color changes in animals," Tabid. Biol., 17, 267 

 (1939). 



4. Bliss, D. E., Durand, J. B. and Welsh, J. H., "Neurosecretory systems in 

 decapod Crustacea," Z. Zellforsch., 39, 520 (1954). 



