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difference is observed between the skin structures of both sides including
pigmentation. During early larval period, some chromatoblasts give rise to
large-sized larval melanophores on both sides of the body. However, from
the onset of metamorphosis, remaining chromatoblasts start to proliferate
and differentiate into adult type pigment cells, such as small-sized adult
melanophores, xantophores and iridophores on the ocular side, while on
the blind side such adult-type chromatophores do not develop and even
cytolysis of chromatoblasts occurs at climax. Only iridophores, which are
supposed to be differentiated from surviving chromatoblasts remain on the
blind side. Thus, the dramatic asymmetry of dark ocular side and white-
colored blind side is established (Seikai et al., 1987; Seikai, 1992; Matsumoto
and Seikai, 1992; Seikai et al., 1993; Seikai and Matsumoto, 1994). Recently,
Watanabe et al. (2008) principally confi rmed the above mentioned process
by applying specifi c markers for the chromatoblast, melanoblast and
xanthoblast on the Japanese fl ounder and spotted halibut, Varasper variegatus
at different developmental stages.
Principally similar developmental pattern of pigmentation has been
found in summer fl ounder, Paralichthys dentatus with some species-specifi c
pigment distributions (Bolker et al., 2005).
Mucous cells also show asymmetrical development during
metamorphosis: the ratio of mucous cell density on the ocular side to that
of the blind side is the same before metamorphosis, but the ratio increases
steadily from the onset of metamorphosis to climax (Seikai, 1992; Seikai
and Matsumoto, 1994).
The squamation occurs differently on the ocular and blind side. Scale
formation starts earlier on the ocular side: scales start to develop along the
posterior part of the lateral line and spread to the anterior and then toward
the periphery of the trunk (Seikai, 1980; Fukuhara, 1986). All scales are
cycloid at fi rst but later they change to ctenoid following the developmental
order, until all the scales of the ocular side become ctenoid. Squamation on
the blind side occurs similarly to the ocular side but commences later and
progresses more slowly. On the blind side, all the scales are cycloid and
ctenii do not develop (Seikai, 1980; Kikuchi and Makino, 1990). Squamation
occurs after the metamorphosis (Seikai, 1980; Kikuchi and Makino, 1990).
However, the squamation seems to be related to the metamorphosis in a
way, since the squamation is affected by the adult-type pigmentation, which
occurs during early metamorphosis: severe abnormal pigmentation, which
occurs during metamorphosis strongly affects the subsequent development
of the scales (Seikai, 1980).
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