Biology Reference
In-Depth Information
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Figure 5.4 Life stagesofAtlantic Salmon, Salmo salmar. (A) Alevin; (B) fry, 3
-
6weeks old; (C)
parr; (D) smolt, 1
-
3 years old; and (E) Breeding female andmale. Images frompublic domain.
The best-studied example of a fresh-to-salt water (anadromous) migration
is that of salmon. Salmon spawn in freshwater breeding grounds, where
embryonic “alevin” hatch into larval “fry.” After months to several years of
development, larval fry develop bars of pigmentation and are called “parr”
(
Fig. 5.4
). Swimming downstream, parr undergo smoltification, which
morphologically and physiologically prepares young “smolt” juveniles for
the marine habitat. By our definition, the entire parr-to-smolt period consti-
tutes a protracted larval metamorphosis, with different morphogenetic and
physiological processes stimulated by different hormones (insulin, prolactin,
TH, growth hormone (GH), and cortisol) that spike at different points during
the months of parr development (see
Bj¨rnsson, Einarsdottir, & Power, 2012;
Dickhoff, Brown, Sullivan, & Bern, 1990
). As fish metamorphose into smolts,
purines are deposited in the skin, producing a silvery appearance, body fat
decreases, gas bladder size increases, and body shape and condition change
