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to become leaner and sleeker ( Bj¨rnsson, Einarsdottir, et al., 2012; Johnston &
Saunders, 1981; McCormick & Saunders, 1987; Winans & Nishioka, 1987 ).
In preparation for the hyperosmotic environment, smolts become able to ab-
sorb increased amounts of water through the intestine, and expression of ion
channels (aquaporins) and ion pumps (Na þ /K þ -ATPase) increases, allowing
active excretion of salt ( D'Cotta, Valotaire, le Gac, & Prunet, 2000; Tipsmark,
Sørensen, &Madsen, 2010 ). During smoltification, salmon also begin synthe-
sizing additional hemoglobin isoforms with enhanced oxygen affinity ( Seear
et al., 2010; Sullivan et al., 1985 ). If smolts do not reach salt water during a
critical “smolt window,” they undergo desmoltification, which includes a loss
of hypo-osmoregulatory abilities and metabolic adaptations, and a darkening
of skin color ( Stefansson, Bj¨rnsson, Ebbesson, & McCormick, 2008 ).
Although desmoltification includes the regression of certain physiological
functions, fish do not revert to an overall smolt morphology; thus,
desmoltification is not a reversal of metamorphosis. Returning to freshwater
environments to spawn, marine salmonids undergo yet another transforma-
tion into the life stage known as “grilse,” preparing them for migration in
fresh water and breeding. This transformation is controlled predominantly
by the gonadotropic axes ( Youngson, McLay, Wright, & Johnstone, 1988 ),
and the majority of the morphological changes are sex-specific, so by our
definition, this transformation primarily constitutes a puberty rather than
a metamorphosis.
Eels represent a second well-studied instance of a diadromous life history.
In contrast to salmonids, eels develop and disperse as marine larvae, then be-
gin to metamorphose in preparation for a salt-to-fresh water (catadromous)
migration. The leaf-shaped marine larvae (leptocephali) are dispersed by
ocean currents and after up to 18 months of larval development, transform
into transparent “glass eels” ( Fig. 5.5 ; Wang & Tzeng, 2000 ). Glass eels
undergo numerous morphological and physiological changes to preadapt
them to freshwater environments, suggesting that the entire glass eel stage
represents the metamorphic period. As glass eels are recruited to a freshwater
habitat, they complete metamorphosis to become juvenile “elvers.” To return
to oceanic environments for spawning, eels undergo the process of silvering
and regain saltwater tolerance. Silvering has been described as a secondary
metamorphic event, although it is stimulated by sex hormones and involves
maturation of the gonads ( Aroua et al., 2005; Rousseau, Aroua, Schmitz,
Elie, & Dufour, 2009 ). This event may represent a second metamorphosis
simultaneous with puberty.
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