Biology Reference
In-Depth Information
into premetamorphosis, early or prometamorphosis, midmetamorphosis or
climax, late or post metamorphosis and juvenile, have also been proposed
(Okiyama, 1967; Miwa et al., 1988; Tanangonan et al., 1989) and used
depending on the purpose of the study. Stage criteria similar to those of
the Japanese fl ounder have been presented for spotted halibut,
Varasper
variegatus
(Aritaki et al., 2001), brown sole,
Pseudopleuronectes herzenteini
(Aritaki and Seikai, 2004), barifi n fl ounder,
Varasper moseri
, (Aritaki et al.,
2000), slime fl ounder,
Microstomus achne
(Aritaki and Tanaka, 2003), summer
fl ounder,
Paralichthys dentatus
(Keefe and Able, 1993; Schreiber and Specker,
1998), a plaice,
Pleuronectes platessa
(Ryland, 1966), and a turbot,
Scophthalmus
maximus
(Al-Maghazachi and Gibson, 1984).
4.1.2 Cranium development and eye migration
The dramatic metamorphosis of the fl atfi sh, especially eye migration, has
attracted attention of many scientists. Various studies have been done to
elucidate how and by what mechanisms one of the eyes of the fl atfi sh moves
to the contralateral side of the head during early development (Tranquair,
1865; Williams, 1901; Kyle, 1923; Brewster, 1987; Wagemans et al., 1998;
Okada et al., 2001, 2003; Schreiber, 2006).
These studies can be grossly summarized as follows. The asymmetry
of fl atfi sh cranium is not due to any rotation of the cranium parts, but is
caused by the development and relocation of anterior cranium elements
on the blind side towards the ocular side (Brewster, 1987; Okada et al.,
2001, 2003). During the larval stage chondrocranium is formed, and the
only cranial element to ossify prior to metamorphosis is the parasphenoid
bone (Brewster, 1987; Okada et al., 2001). The cranial elements such as
trabecular cartilage, parasphenoid bone, lateral esmoids and supraorbital
bars originally appear in the position similar to those of other teleosts,
but they develop and relocate during metamorphosis, twisting toward
the ocular side and ossify after the start of eye migration (Brewster, 1987;
Okada et al., 2001, 2003). When the migrating eye reaches its new position,
the other cranial elements ossify and these elements share the same relative
position as those of other teleosts (Brewster, 1987).
Among cranial elements which become asymmetrical during eye
migration, the change in supraorbital bars is especially drastic. The bars
are originally located symmetrically between the eyes, but the one on the
prospective ocular side grows larger, fusing with trabecular cartilage,
while the blind side bar degrades and disappears by metamorphic climax
(Williams, 1901; Okada et al., 2001, 2003; Schreiber, 2006), possibly allowing
the passage of the migrating eye (Williams, 1901; Schreiber, 2006). Another
candidate of frontal element to be directly involved in eye migration is the
