An Association of Melanophores Appearing at Metamorphosis as Vehicles of Asymmetric Skin Color Formation With Pigment Anomalies Developed Under Hatchery Conditions in the Japanese Flounder, Paralichthys olivaceus

Abstract

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)

Origin of adult‐type pigment cells forming the asymmetric pigment pattern, in Japanese flounder (Paralichthys olivaceus)

“…Iridophores appear on both the left and right body sides, but they distribute differently between on the ocular side and on the blind side: on the ocular side iridophores distribute forming patches, whereas iridophores distribute dispersedly on the blind side (Matsumoto and Seikai, 1992). This spatial distribution pattern of iridophores also contributes to the specific coloration of flatfishes (Seikai et al, 1987;Matsumoto and Seikai, 1992).…”

“…In flatfish, also the body pigment pattern is formed of these various classes of neural crest-derived pigment cells. Previous studies have shown that, in flatfish like Japanese flounder, P. olivaceus, and stone flounder, Kareius bicoloratus, two morphologically distinct populations of melanophores appear at dual timing of developmental stages (Seikai et al, 1987(Seikai et al, , 1993Matsumoto and Seikai, 1992). The first group of melanophores, large-sized larval-type melanophores, appear sparsely and bilaterally around hatching.…”

“…Adult-type melanophores are thought to differentiate from melanoblasts or their precursors during metamorphosis (Seikai et al, 1987;Matsumoto and Seikai, 1992). The left-right asymmetric pigment pattern of adult flatfish is mainly brought about by these spatially restricted numerous appearances of adult-type melanophores (Seikai et al, 1987(Seikai et al, , 1993Matsumoto and Seikai, 1992;Hamre et al, 2007). Moreover, xanthophores remarkably increase on the ocular side during the course of metamorphosis and contribute to the asymmetric pigment pattern (Matsumoto and Seikai, 1992).…”

“…The second group of melanophores, small-sized adult-type melanophores, appear numerously only on the ocular side at the latest stage of metamorphosis. Adult-type melanophores are thought to differentiate from melanoblasts or their precursors during metamorphosis (Seikai et al, 1987;Matsumoto and Seikai, 1992). The left-right asymmetric pigment pattern of adult flatfish is mainly brought about by these spatially restricted numerous appearances of adult-type melanophores (Seikai et al, 1987(Seikai et al, , 1993Matsumoto and Seikai, 1992;Hamre et al, 2007).…”

“…After metamorphosis, two eyes located together on the ocular side that is left or right side, and the ocular side has a brownish color and the blind side has a pale color (Seikai et al, 1987;Matsumoto and Seikai, 1992;Seikai, 1992). It appears as if the axis conversion occurs from the dorso-ventral axis to the leftright axis.…”