Biology Reference
In-Depth Information
during metamorphosis or if they develop after behavioral change has already
been established.
The dorsal light reflex, which causes a typical upright swimming fish to
tilt toward a lateral light source until equilibrium is established between vi-
sual and gravitational sensory input, may contribute to the development of
flatfish lateralized swimming behavior as the migrating eye changes the per-
ceived angle of light incidence (
Finger, 1976; Graf & Baker, 1990; Neave,
1985
). Indeed, while premetamorphic larval flatfish have been shown to
exhibit a dorsal light response similar to that of a typical upright fish, after
one eye begins to migrate the larvae tilt toward their future blind side when
illuminated from above (
Neave, 1985
), a process that could be attributable to
the angle of perceived light caused by eye migration. Additionally,
Jansen
and Enger (1996b)
have shown that NADPH diaphorase activity is asym-
metrically distributed in the optic tectum during metamorphosis, a time
corresponding with the aforementioned transient morphological asymmetry
of the tectum described by
Brinon et al. (1993)
, though the functional
significance of this asymmetry is not clear. Ultimately, studies with southern
flounder (
P. lethostigma
) larvae have revealed three lines of evidence that sug-
gest lateralized swimming and settling behaviors in flatfish can be completely
dissociated from—and thus occur independently of—asymmetric eye posi-
tioning: (1) lateralized feeding and tilted swimming behaviors are apparent
prior to the start of eye migration during premetamorphosis, (2) treatment
of young premetamorphic larvae with TH induces acute tilted swimming
and settling behavior prior to its induction of eye migration, and (3) sym-
metrically metamorphosed flounder variants also display normal lateralized
swimming and settling behaviors despite the complete absence of eye migra-
tion (
Schreiber, 2006
). These findings suggest that both the transition to
a lateralized swim posture and metamorphic craniofacial remodeling/eye
migration are independent developmental events, each controlled autono-
mously by TH. The development of a lateralized swim posture occurs dur-
ing late premetamorphosis (when TH levels are known to be relatively low;
see
Fig. 6.3
) and precedes eye migration which takes place later during
metamorphic climax (when TH levels are known to be highest; see
Fig. 6.3
). Assuming that the transition to a lateralized swim posture is
ultimately mediated by central or peripheral changes in vestibular function,
these regions might be expected to exhibit a higher sensitivity to low levels
of TH (e.g., increased types 1 and 2 deiodinase activity or elevated TH
receptor levels) compared with other regions,
though this hypothesis
remains untested.
