Biology Reference
In-Depth Information
the larval body have the potential to metamorphose into either the juvenile
ocular or the blind side, and the timing of TH release and differential sen-
sitivity of each side to TH ultimately establishes flatfish metamorphic sided-
ness. Although TH receptors (
Galay-Burgos et al., 2008; Laudet, 2011;
Manchado et al., 2009; Marchand et al., 2004; Yamano & Miwa, 1998
)
and deiodinase enzymes (
Isorna et al., 2009; Itoh et al., 2010
) have been de-
scribed for flatfish, differences in expression or activity between the two sides
and among the various cranial bones and cartilages have not yet been
reported. One last aspect of craniofacial remodeling that has been virtually
ignored in the literature and hence deserves some mention are the changes
that take place to the orientation and positioning of the nonmigrating eye
during metamorphosis. The nonmigrating eye appears to move to a more
ventral position to make room for the opposite migrating eye (
Ballard
et al., 1987; Norman, 1934
), though this has not yet been established quan-
titatively. Furthermore, the nonmigrating eye also experiences a permanent
change in orientation as it rotates by 90
and establishes a more ventral visual
trajectory (see
Figs. 6.1, 6.2
A-C, and
6.8
). The rotation of the nonmigrating
eye is presumably mediated by permanent changes in its extraocular muscles,
though this process remains uncharacterized.
The development of symmetrical juvenile flatfish due to abnormal meta-
morphic craniofacial remodeling (“arrested development” or failure of the
eye to migrate) and symmetrical pigmentation (pseudoalbinism and
ambicoloration, discussed above) is a common phenomenon in mass-
cultured commercial flatfish species (reviewed in
Power et al., 2008
; also
see
Pittman, Jelmert, Naess, Harboe, & Watanabe, 1998
). Pseudoalbinos
possess the blind side pigment characteristics on both sides, whereas
ambicolored juveniles express ocular side pigmentation on both sides and
are considered malformations of asymmetry (
Aritaki & Tagawa, 2012
). In
halibut, juveniles with arrested eye migration have been shown to have ab-
normal remodeling of the frontal bones with symmetrical osteoclast activity
on either side, as well as the absence of the postlateral ethmoid (pseudomesial
bar) dermal bone and abnormally symmetrical paraethmoids (
Saele,
Smaradottir, et al., 2006
)(
Fig. 6.9
). Similar observations have been made
for
P. olivaceus
(
Okada, Takagi, et al., 2003; Okada, Tanaka, et al., 2003;
Okada et al., 2001
) and
P. lethostigma
(
Schreiber, 2006
) juveniles with
arrested eye migration.
Saele, Smaradottir, et al. (2006)
conclude that
“the lack of eye migration in flatfishes may not be a deformity like lordosis,
short jaw, etc., but may result from lack of remodeling, leaving it with the
neurocranial phenotype of its ancestors” (compare
Fig. 6.9
with
Fig. 6.10
in
Section 5
). These developmental abnormalities are known to be strongly
