Biology Reference
In-Depth Information
1. INTRODUCTION
Flatfishes (flounder, sole, halibut, turbot, plaice, tonguefish) are
among the most easily recognizable of all groups of fishes, characterized
by a distinct laterally compressed body morphology with both eyes located
on the same side of the head. The f latfishes are a monophyletic group (Order
Pleuronectiformes) within the Percomorpha, consisting of over 700 species
with virtually global marine and estuarine distribution, including species that
inhabit deep sea thermal vents and fresh water rivers ( Friedman, 2012;
Munroe, 2005; Tunnicliffe, Koop, Tyler, & So, 2010 ). Adults can range
in size from 2 cm to over 2 m length, and they spend most of their lives
as benthic (bottom-dwelling) creatures that lay on one side, and are the most
left-right asymmetrically shaped and behaviorally lateralized vertebrates to
ever exist. In contrast with adults, larval flatfish resemble typical bilaterally
symmetrical fish that swim upright in the water column. Flatfish larvae meta-
morphose into their juvenile (immature adult) form, a process characterized in
part by the migration of one eye to the opposite side of the head and the tran-
sition to a lateralized swim posture ( Figs. 6.1 and 6.2 A-C) ( Schreiber, 2006 ).
This transformation is accompanied by the development of various dramatic
internal and external asymmetries, including extensive craniofacial remodeling
( Fig. 6.2 D-H) and development of pigmentation predominantly on one
side ( Fig. 6.2 I and J). As withmetamorphosis in amphibians, the developmental
programs of flatfish metamorphosis are mediated by thyroid hormone
(TH) ( Inui & Miwa, 1985; Miwa, Tagawa, Inui, & Hirano, 1988; Power
et al., 2008 ; Schreiber & Specker, 1998 ) and its nuclear-localized receptors
(TRs) ( Galay-Burgos, Power, Llewellyn, & Sweeney, 2008; Laudet, 2011;
Manchado, Infante, Rebordinos, & Canavate, 2009; Marchand, Duffraisse,
Triqueneaux, Safi, & Laudet, 2004; Yamano & Miwa, 1998 )( Fig. 6.3 ).
Deiodinases, enzymes that modulate TH activity in peripheral tissues, have also
been recently characterized in flatfish ( Isorna et al., 2009; Itoh, Watanabe, Wu,
&Suzuki,2010 ). Although all vertebrates undergo left-right asymmetric inter-
nal organ placement during embryogenesis ( Hamada, Meno, Watanabe, &
Saijoh, 2002; Levin, 2005 ), flatfish are unusual in that they experience an addi-
tional periodof postembryonic asymmetric remodeling duringmetamorphosis,
and thus deviate from a bilaterally symmetrical body plan more than other ver-
tebrates. This chapter highlights the current state of knowledge of the develop-
mental biology of flatfish metamorphosis with emphases on the genetic,
morphological, behavioral, and evolutionary origins of flatfish asymmetry.
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