COLOUR CHANGES 



507 



TABLE 12.1 



Chromatophoral Responses of Normal and Blinded Specimens of 

 Ligia oceanica under Various Conditions of Illumination 



The results are expressed in terms of the chromatophore index in which 1 represents full 

 contraction, and 5 full expansion (from H. G. Smith (64).) 



light on the chromatophores of Ligia, which suggests that extra-ocular 

 photoreceptors are involved in this animal. 



The prawn Palaemon contains white reflecting chromatophores which 

 exhibit a direct primary response to change of illumination. In blinded 

 individuals of Palaemon serratus, P. adspersus and P. e/egans, the white 

 chromatophores continue to expand when illuminated, and to contract in 

 darkness. The degree of dispersion of the black chromatophores of the 

 fiddler crab Uca is affected by the total illumination, and in blinded 

 animals these cells behave as independent effectors, dispersing in bright 

 light and concentrating in darkness (18, 39). 



Some fishes also continue to show chromatophore activity when blinded. 

 These include species of Fundulus, Gobius, Ctenolabrus and Lepaclogaster, 

 which become pale in the dark, and darken when subsequently illumi- 

 nated. But the chromatophore response to illumination is absent in blinded 

 plaice Plcuronectes platessa. Yon Frisch found that when a spot of light 

 was focused on the skin of the wrasse Crenilabrus, a local dark area ap- 

 peared. No such effect was observed in the gurnard Trig/a, however. In 

 isolated scales of the tautog Tautoga, the melanophores respond by dis- 

 persion to a sudden increase in light intensity. It appears then that the 

 chromatophores of different animals show much variation in their sus- 

 ceptibility to direct photic stimulation. 



In indirect photic stimulation of fish, at least three possible receptor 

 surfaces are involved besides the eyes, namely, dermal photoreceptors, the 

 pineal complex and the photo-sensitive surface of the third ventricle. No 

 doubt their effective contributions vary greatly in different species of fish, 

 but insufficient data are available to distinguish between them. In one 

 study of pineal functioning in a large series of teleosts it was found that 

 the animals could be divided into three groups : those in which the tissues 

 overlying the pineal are sufficiently thin to allow the entry of light; those 

 in which the tissues are opaque; and those in which the behaviour of 

 appropriately placed chromatophores regulates the entrance of light to 

 the pineal organ. When the pineal was covered, fishes in the first group 

 reacted by expanding their melanophores, while fishes in the second and 

 third groups showed slighter effects. The pineal organ of the young salmon 



