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ALBINISM, AMBICOLORATION, REVERSAL 27 



accompanied by the characteristic hook above the eye, but individuals completely or 

 nearly completely pigmented on the blind side have two pelvic fins of similar form and 

 equal size (Fig. 18). ^ 



Mention may be made here of an interesting individual of the Turbot described 

 by Cunningham (1907). This fish was only 44 mm. in total length, the eyes were on 

 the right side, the ocular side was unpigmented except for an area in the region of 

 the eyes, and the blind side was deeply pigmented except for the head and the dorsal 

 region above it. This provides a unique case, albinism, ambicoloration and reversal 

 being combined in the same individual. 



Various theories have been brought forward to explain the phenomenon of ambi- 

 coloration, but these cannot be considered in any detail here. It has been suggested 

 that fish showing this condition swim as larva? in a vertical position for a longer period 

 than usual, and thus expose the future blind side to the light for a greater length of 

 time. Jlclntosh (1902) has observed, however, that in normal larvrc the pigmentation 



■ c > 



% 



% 



% 



Fic. 18. — RhnmbosoUa tapirtna. Anibicolorate specimen. (X I.) 



of the lower surface is paler even before the change to a demersal habit occurs, and 

 that even in cases where the pelagic life is unduly prolonged the same feature is notice- 

 able. Cunningham's views as to the effects of light on the pigmentation of the blind 

 side have been already mentioned, and other theories involve the consideration of 

 such matters as germinal factors, interference with the mechanism of embryonic 

 transformation, mutation in the direction of secondary bilateral symmetry, and so on. 

 The available evidence suggests that ambicoloration merely represents variation in 

 the direction of the original bilaterally symmetrical condition of the ancestors of the 

 Flatfishes. It is to be observed that the part of the fish last affected by ambicolora- 

 tion, namely, the anterior part of the head, represents the most asymmetrical portion. 

 There is absolutely no evidence that the behaviour of ambicolorate individuals is in 

 any respect different from that of normal Flatfishes. 



Reversal, or the occurrence of individuals with the eyes and colour on the side 

 which is generally eyeless and unpigmented in the species, is a not uncommon pheno- 

 menon in certain Flatfishes.^ In the great majority of Flatfishes all the individuals 



' Norman (1926B, pp. 278-280). 



" .\s Bateson (1894) has pointed out, discontinuous variation of this nature is well known in 

 other groups of animals. In Molluscs, for example, reversed or sinistral shells are not uncommon 

 III (lastropods, and may even characterise a race or subspecies. 



Ill 



