Biology Reference
In-Depth Information
1. INTRODUCTION AND DEFINITIONS
Nearly half of all described vertebrates are teleosts (see Volff, 2005 ),
and the more than 23,500 known species of teleosts exhibit a vast diversity
of phenotypes, ecologies, and developmental life histories. Moreover, tele-
osts exhibit a tremendous range of metamorphic processes, sometimes un-
dergoing major phenotypic and physiological transitions that allow a single
species to exploit different habitats and niches during different life stages.
This diversity in teleost life history has long fascinated comparative embry-
ologists and marine ecologists, but the molecular mechanisms of these tran-
sitions remain almost entirely unexplored. With the methods of modern
genetics and developmental biology, we can begin to elucidate the molec-
ular, endocrinological, and morphogenetic processes underlying this incred-
ibly diverse array of postembryonic transitions.
Several families of teleosts undergo spectacular morphological metamor-
phoses, comparable to the metamorphosis of anuran amphibians. Other
teleosts undergo comparatively subtle phenotypic transformations, more
analogous to the fetal-to-adult transition of mammals. Accordingly, workers
in this area have adopted sometimes conflicting definitions of what should
rightly constitute a “metamorphosis.” For example, some count only the
most dramatic of transitions to be metamorphoses, requiring profound
morphological and ecological differences between life stages, and rapid tran-
sitions between these stages ( Balon, 1990, 1999; Just, Kraus-Just, & Check,
1981; Manzon, 2011; Youson, 1988, 2004 ). Such restrictive definitions
dismiss more subtle morphological, physiological, or ecological changes.
Alternatively, others have suggested that metamorphosis of teleosts and
other chordates can be defined as a conserved period of postembryonic rem-
odeling, how extensive it might be, that is regulated specifically by thyroid
hormone (TH; Laudet, 2011; Paris et al., 2008; Paris & Laudet, 2008;
Power, Silva, & Campinho, 2008 ). Yet, this definition precludes transfor-
mations that may rely on other hormonal axes.
For the purposes of this chapter, we define metamorphosis to be an irre-
versible developmental and physiological change that affects multiple traits during post-
embryonic development and is brought about by one or more systematically acting
endocrine mediators, but is independent of sexual maturation, sex-specific modifica-
tions, or senescence . This definition focuses on intrinsic features of an organism,
or how the environment impacts organismal functioning, and accommo-
dates what
is clearly a phylogenetic continuum in the magnitude of
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