The mechanisms for asymmetric skin color formation in the Japanese flounder are studied with particular concerns to causes for pigment disorder (hypomelanosis) occurring under hatchery conditions. For an analysis of normal pigmentation, fish were raised with wild zooplanktons in an indoor hatchery, whilst for hypomelanosis, they were raised with Brazilian Artemia nauplii, a diet used in the hatcheries. Morphological observations, counting of melanophores, histochemical assay of DOPA-positive immature cells (melanoblasts), and radiometric estimation of tyrosinase activities in skins of developing larvae and juveniles indicate that 1) the structural plan for pigmentation in this species is bilaterally symmetric until metamorphosis, utilizing large-sized melanophores (hence larval melanophores) as main vehicles, and 2) an asymmetric coloration characteristic to metamorphosed juveniles is formed by an intensive development of smaller-sized melanophores (hence adult-type melanophores) appearing selectively in the ocular side at the later stages of metamorphosis and by an absence of it in the blind. These findings apparently indicate that 1) two types of melanophores occur in this species which differ with respect to morphological properties and developmental fate, and 2) selective differentiation of adult type melanophores in the ocular side of the body at or after metamorphosis is primarily responsible for an asymmetric coloration of its adult form. The similar assays on the fish fed with Artemia nauplii indicate that defective development of adult-type melanophores results in hypomelanosis in their ocular-sided skins, yielding a pigmentary pattern seen in the blind side of the metamorphosed juveniles with normal pigmentation.(ABSTRACT TRUNCATED AT 250 WORDS)

The larval stage of the flounder P. olivaceus, when their body color (normal coloration or albinism of juveniles) was determined, was estimated by two ways of feeding. One was to feed Brazilian Artemia nauplii and rotifers which induce albinism easily in a high percentage. The other was to feed wild zooplankton and rotifers which usually induce normal coloration in a high percentage. Larvae of 10 days old were divided into 8 lots of 1,000 larvae each and were fed for 29-30 days on different feeding schedules at different growth stages by a combination of the 2 feeding methods. Based upon the percentage of normal coloration and albinism at the end of experiment, 8 experimental lots were divided into 3 groups. In the first group (Tanks 1-3) in which Artemia nauplii were changed to wild zooplankton before the larvae grew up to D stage, the highest percentage of normal coloration was observed (60.2-75.4%). In the third group (Tanks 5-8) fish began to accept wild zooplankton from E stage, the highest percentage of albinism was observed (90.9-100.0%). In the second group (Tank 4 only), wild zooplankton were given to larvae in place of Artemia nauplii when the larval developmental stages consisted of 25% D and 75% E, and this composition agreed well with the percentage occurrences of normal coloration (27%) and albinism (73%). From these results, D stage was estimated to be the critical stage determining the juvenile's coloration in terms of kinds of live foods.The Japanese flounder, Paralichthys olivaceus, is one of the most important commercial species among Japanese preuronectiformes. Recently millions of the juveniles are produced artificially for cultivation to marketable size or release at a certain stage into the coastal water to promote coastal fisheries. Occurrence proportion of albinism in the hatchery-reared preuronectid fishes including this species is reported to be markedly higher than those in the wild.1-4) This abnormality involves frequently some other morphological abnormalities,5) and probably results in low survival rate after release into the sea. Thus it is very important to clarify the mechanism of this abnormality and establish effective methods to prevent this.As the causes of this abnormality, light condition during egg incubation,6) larval stocking density in rearing tank,3) feeding level during larval stage7) and so on have been already suggested by many workers. Recently we showed that nutritional factors correlated with appearance of the abnormality: feeding of Brazilian Artemia nauplii mixed with rotifers Brachionus plicatilis from 5mm TL resulted in high occurrence proportion of albinism (97.0-100.0%)4,8) and other geographically different strains of Artemia reduced the level of occurrence (81.3-84.9% on Tien-tsin strain and 21.1-26.1% on San Francisco Bay brand strain),8) whereas feeding of wild zooplankton mixed with rotifers, from the same stage resulted in extremely low percentages of occurrence (0-1.8%).9) These results indicate that occurrence of the abnormal coloration can be arbi...

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