Biology Reference
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head, resulting in both eyes located on one side of the head. The whole
body structure is modifi ed accordingly. Pigmentation of the body becomes
also asymmetrical: the ocular side (the side on which the eyes are localized)
becomes dark with intensive pigmentation, and the blind side becomes
white. Concomitant with these changes, their life style changes from free
swimming, pelagic plankton feeders to sedentary carnivores, lying on the
bottom with both eyes facing up. Their swimming position also changes
from dorso-ventral upright to lateralized (Kyle, 1923; Norman, 1934; Hubbs
and Hubbs, 1945; Policansky, 1982a; Ahlstrom et al., 1984).
For this characteristic change of body form together with the change
in life style in early development, fl atfi sh are considered to undergo a
typical metamorphosis comparable to that in amphibians (Youson, 1988).
Youson (1988) categorized fi sh metamorphosis into two types: true or fi rst
metamorphosis, which occurs in the early development from larvae to
juvenile and second metamorphosis which takes place after the juvenile.
Thus, metamorphosis of fl atfi sh is considered as a representative of the fi rst
metamorphosis in teleosts.
In the last two decades, researches have shown that the flatfish
metamorphosis is primarily controlled by the pituitary-thyroid axis in
a comparable way to amphibian metamorphosis (Inui and Miwa, 1985;
Miwa and Inui, 1987a, b; Miwa et al., 1988; Inui et al., 1989; Inui et al.,
1994; Schreiber and Specker, 1998). These studies have disclosed the
developmental changes in various organs controlled by hormones during
metamorphosis (Miwa and Inui, 1991; Miwa et al., 1992; Yamano et al.,
1991, 1994b; Huang et al., 1998; Soffi entino and Specker, 2001; Schreiber
and Specker, 2000).
Furthermore, recently intriguing studies have been conducted on the
molecular pathway controlling the formation of left/right axis during early
development of fl atfi sh (Hashimoto et al., 2004; Hashimoto et al., 2007;
Suzuki et al., 2009).
This review describes the present status and perspectives of studies
of fl atfi sh metamorphosis. As recent major progress on this fi eld has been
achieved on the Japanese fl ounder,
Paralichthys olivaceus
, the article mainly
concerns with the results of these studies. However, we have also tried to
include studies of other fl atfi sh as much as possible.
4.1.1 Stage specifi c morphology during metamorphosis
Okiyama (1967, 1974) and Minami (1982) described developmental changes
in general morphology from post-larva to juvenile of the Japanese fl ounder,
defi ning 9 developmental stages with stage-specifi c criteria (Fig. 1). The
criteria have often been used in studies of the metamorphosis of the Japanese
fl ounder. Other criteria, such as the one which classifi es metamorphic stages
