Biology Reference
In-Depth Information
Unlike all other vertebrates, the axes of flatfish eyes and horizontal semi-
circular canals of the inner ear become oriented perpendicular to each other
during metamorphosis (
Graf & Baker, 1983
), a process that is accompanied
by the development of an increasingly tilted swimming posture (
Finger,
1976; Neave, 1985; Schreiber, 2006
) (see
Figs. 6.1 and 6.2
A-C). The spe-
cific mechanisms responsible for inducing the tilted swimming posture and
lateralized settling behaviors in flatfish remain essentially unknown, though
in principle the abrupt transition to lateralized behavior could be influenced
by asymmetric changes in eye position, inner ear morphology, and/or cen-
tral changes in vestibular connectivity or activity. Although changes in swim
posture have been shown to be accompanied by changes in central nervous
pathways (
Graf & Baker, 1983, 1985a, 1985b, 1990; Graf & Meyer, 1983;
Graf, Spencer, Baker, & Baker, 2001; Graff, Simmons, Meyer, Martin-
Morgan, & Kurtz, 1986; Platt, 1973a, 1973b
), none of the aforementioned
studies found evidence of an asymmetric remodeling of these pathways. Fur-
thermore, no morphological evidence of asymmetry has yet been identi-
fied for the flatfish labyrinths (
Graf et al., 2001; Jacob, 1928
), the
distribution of the vestibular nuclear complex in the hindbrain (
Jansen
& Enger, 1996a
), or the peripheral or central oculomotor apparatus
(
Graf & Baker, 1985a
), which all appear bilaterally symmetrical and similar
to those of other teleosts. Interestingly, all flatfish except for the most prim-
itive (i.e.,
Psettodes
, which have been observed to swim upright on occa-
sion; see
De Groot, 1970
) display nearly omnidirectional hair-cell
polarization of the inner ear's saccular and lagenar otoliths (
Jorgensen,
1976; Platt, 1973a, 1983
). This pattern, which is unique among the
vertebrates, is thought to accommodate, but not induce, the adult flatfish
posture. Some evidence for asymmetric peripheral and postural control in
flatfish adults (presumably established during metamorphosis) does exist,
though.
Meyer, von Seydlitz-Kurzbach, and Fiebig (1981)
found that
2-deoxyglucose is taken up differentially by the bilateral vestibular nuclei
suggesting that lateralized behavior may be caused by a “permanent imbalance
in vestibular neuron activity.” Left-right asymmetries for adult flatfish otolith
morphology (
Lychakov, 1996; Lychakov, Rebane, Lombarte, Demestre, &
Fuiman, 2008; Sogard, 1991
), mass (
Helling, Scherer, Hausmann, &
Clarke, 2005
), and chemical composition (
Loher, Wischniowski, & Martin,
2008
) have also been described. Indeed, THhas been shown to induce growth
and development of all three types of otoliths (saculus, utricle, and lagena) dur-
ing flatfishmetamorphosis (
Schreiber et al., 2010;Wang et al., 2011
), though it
remains to be seen if otolith morphological asymmetries develop specifically
