Biology Reference
In-Depth Information
genome sequencing projects have been initiated for several flatfish
species of commercial importance (e.g., Japanese flounder, Senegalese sole,
Atlantic halibut) (see reviews by
Cerda et al., 2008
,
Cerda et al., 2010
, and
Power et al., 2008
and studies by
Bao et al., 2005, Fu et al., 2011
, and
Xie
et al., 2011
).
5. FLATFISH EVOLUTION: A DEVELOPMENTAL
PERSPECTIVE
Flatfishes are thought to be descended from a bilaterally symmetrical,
upright swimming common ancestor from the clade Percomorpha, though
the specific sister group has not yet been identified (
Chapleau, 1993;
Friedman, 2012
). The unique positioning of independently mobile eyes
in juvenile and adult flatfish facilitates 360
vision, maximizing the ability
to spot both prey and predators from multiple trajectories. Also, the extra-
orbital positioning of the eyes allows these fish to burrow into the substrate
leaving the eyes exposed (
Gibson, 2005
). These features, in combination
with the ability to match their background color pattern through rapid adap-
tive camouflage, very effectively accommodate a predominantly benthic ex-
istence. From an ecological perspective, the retention of a bilaterally
symmetric pelagic larval stage promotes dispersal of the larvae away from
the benthic adults and toward future (often estuarine) nursery grounds
(
Bailey, Nakata, & Van der Veer, 2005
).
Fossil records of the most ancestral flatfishes known (genus
Amphistium
and
Heteronectes
) from the Eocene epoch (53 million years ago) show incom-
plete eye migration, with the eyes remaining on opposite sides of the head in
postmetamorphic fish (
Friedman, 2008; Friedman, 2012
;
Fig. 6.10
). The
findings by Friedman also indicate that dextral and sinistral morphs within
each ancestral species occurred in approximately equal frequency, in contrast
with modern flatfishes, most of which are monomorphic (
Munroe, 2005
).
An ancestral-like retention of dextral and sinistral polymorphism is still
found in the most primitive of living flatfishes,
Psettodes
(
Hubbs & Hubbs,
1944
). Therefore, it appears that eye-sidedness was initially random in an-
cestral flatfishes, until developmental systems (such as the NLP pathway)
evolved to control eye-sidedness (
Suzuki et al., 2009
) for reasons that remain
unknown. Interestingly, these ancestral flatfish with incomplete eye migra-
tion were also shown to lack a postlateral ethmoid (pseudomesial bar)
(
Friedman, 2008, 2012
), which appears to be required for normal eye mi-
gration in modern flatfishes. Friedman concludes “Thus, the evolutionary
