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postembryonic remodeling. By excluding habitat changes
per se
, this defini-
tion allows for studies of how metamorphic processes have evolved in asso-
ciation with particular ecological conditions or life history modes. By our
definition, metamorphosis is not necessarily a specific event, but, rather,
any nonsexual, nonsenescent transition entailing developmental or physio-
logical remodeling that is systemically controlled. Thus, our criteria allow
one to pose testable hypotheses for whether any particular morphological
or physiological transformation constitutes a metamorphosis; that is,
whether or not it is controlled by one or more endocrine mediators. The
definition deliberately excludes morphological changes (e.g., acquisition
of nuptial coloration) that may be temporary, sex-specific, or both, and also
does not specify any requirement for overall somatic growth. Moreover, as
we have defined it, metamorphoses could occur over a range of time spans,
from less than a day to periods of several months, and could, in principle,
occur between a variety of life history phases, though metamorphosis is gen-
erally most prevalent during the larva-to-juvenile transition (
Fig. 5.1
).
Finally, in being agnostic to mechanism, our definition acknowledges that
additional factors mediating such transformations still may await discovery or
characterization. In proposing this definition, and in the review that follows,
our goal is to facilitate the study of postembryonic cellular, developmental,
and physiological processes, how they intersect with endocrine mechanisms,
and how they have evolved.
Egg
Adult
Embryo
Juvenile
Larva
Figure 5.1 Generalized life cycle of a teleost. Dashed lines indicate potential periods of
metamorphic transformations within the life cycle; the transformation from larva to
juvenile represents larval metamorphosis.
