JO I'LATFISHES (HHTKKOSO.MATAl 



of trill' anibicdloraticiii arc very rare. Partially and nearly totally .imbicolorate 

 I'laice are frequently encountered, hut, as in the Turbot, those in which the pigmen- 

 tation of the blind side is most extensive nsnally retain the white patch on the head. 

 Ambicoloratc Flounders are fairly common, and in this species examples in which 

 the blind side is entirely pigmented occur from time to time. Partially ambicolorate 

 Dabs are not uncommon, but Soles or Lemon Soles exhibiting this condition are very 

 rare. Of the 51 ambicolorate Flatfishes examined by me, the species concerned and 

 the number of examples of each were as follows: Turbot 14, Brill j. Halibut i, Dab q, 

 l-'loiinder 10, Plaice 15. 



Where the amhicoloration is only partial no other variations in the structure 

 of the fish arc noticeable, but where the pigmentation of the blind side is complete 

 or almost complete it is nearly always accompanied by- other variations in the direction 

 of symmetry.' These are concerned with the scales and associated structures, the 

 "upper" eye, the anterior part of the dorsal fin, and the paired fins. In the Dab 

 \I.imanda) the scales are normally mostly ctenoid on the ocular side and cycloid or 

 feebly ctenoid on the blind side. In totally ambicolorate examples the scales of the 

 blind side are mostly distinctly ctenoid, and in specimens in which the pigmentation 

 of the bhnd side is confined to the posterior part of the body, the scales in the pigmented 

 .area are often more strongly spinulated than those in the unpigmented area. Similarly, 

 in ambicolorate Flounders (Platichthys) the bony tubercles tend to be more strongly 

 developed on the blind side than in normal examples. In normal Turbot (Scoph- 

 ihalmus) the characteristic bony tubercles are either absent, or but feebly developed 

 on the blind side of the head and body ; in total or nearly total ambicolorates these 

 are strongly developed on both sides (Fig. 17). 



In Flatfishes such as the Turbot, Dab, Flounder and Haice the migration of the 

 eye is completed liefore the forward extension of the dorsal fin takes place (p. 15), 

 but in ambicolorate examples this migration appears to be arrested or delayed .- 

 As a result the anterior movement of the fin is obstructed, and this grows forward 

 to form a fleshy hook above the eye.^ This hook, which forms so characteristic ,1 

 feature of ambicolorate Flatfishes, is present only where the pigmentation of the 

 filind side is complete or nearly so. As far as the Turbot is concerned, Cunningham* 

 has laid down the generalisation that hooking of the dorsal fin occurs in all examples 

 that exhibit pigmentation of the blind side in front of an imaginary line drawn through 

 the opercular bone (Fig. 17). 



Finally, Flatfishes with the blind side completely or almost completely pigmented 

 nearlv alwavs show some variations towards svmmetry in the paired fins In normal 

 Turbot. Plaice and Flounder the pectoral fin of the ocular side is larger than that of 

 the blind side, but in ambicolorate examples the fin of the blind side is often a little 

 larger than usual. Similarly the pelvic fins of ambicolorate Flounders and Plaice 

 are more nearly- exactly symmetrical in form and position than are those of normal 

 individuals. Of special interest in this connection is the Australian and New Zealand 

 genus Rlwmbosolea, in which normally only the pelvic fin of the ocular side (i. e. the 

 right side) is present ; this is elongate, placed on the median line of the body and 

 united with the anal. In members of this genus ambicoloration is not only 



' It seems probable that such variations al-ji-ays accompany complete or nearly complete ambi- 

 coloration. iLxamples of Lemon Sole, Dab and Sole have been described in which the blind side 

 was nearly entirely pi^'inented. but the tish were said to be normal in other respects. It is possible 

 that closer examination of these specimens would have revealed at least minor modifications of the 

 scales, etc. 



- Holt (iSf)4. pp. 4.^:1-445, figs.) lias described an adult Sole in whicli the eye of the blind side 

 had remained on that side of the head, nearly opposite to that of the ocular side, but was to a large 

 extent embedded in the skin. I'here was no trace of pigmentation on the blind side, and the skull 

 was almost normal. 



^ Sometimes in Turbot and Brill the eyes are normal in posuion. but a short portion of the 

 anterior part of the dorsal fin is free. This condition, which probably arcompanies dda\cd migration 

 of the eye, is not necessarily associated with ambicoloration. 



' LiinriinKh.im and .MarMiinn (1S94. p. 8nl,). 



