Biology Reference
In-Depth Information
6.3.1.2 Experimental induction
The administration of exogenous thyroid hormones to juvenile parr-status
salmonids results in morphological and physiological changes consistent
with the parr-smolt transformation (for reviews: Robertson, 1949; Fontaine,
1975; Donaldson et al., 1979; Higgs et al., 1982; McBride et al., 1982;
Sullivan et al., 1987). In addition, the use of anti-thyroid drugs such as
propylthiouracil (PTU) in drinking water or food, as well as by immersion
(Ebbesson et al., 1998) or intraperitoneal implants, in smolting salmon
induced hypothyroidism by inhibiting the naturally occurring increases
of plasma total T4 and total T3 and by affecting the conversion of T4 to
T3. Because T4-treated salmonids do not undergo all the metamorphic
changes necessary to pre-adapt them to saltwater, Eales (1979) referred
to them as “pseudosmolts”. Nevertheless, they remain a useful model for
studying metamorphic physiology, for specifi c changes can be observed in
isolation from the many other physiological factors that characterize true
smoltifi cation.
6.3.1.2.1 Body silvering/Fin darkening
Pioneer study by Robertson (1949) showed that intramuscular injection of
mammalian thyroid extract and TSH produced silvery smolt stage in rainbow
trout. Following studies confi rmed that exogenous thyroid hormones
(amago salmon: Miwa and Inui, 1983, 1985) and TSH (Premdas and Eales,
1976) induced silvery body color or guanine and hypoxanthine deposition
in the skin of salmonids. Diet T4 for 72 days to underyearling amago salmon
resulted in body silvering (Miwa and Inui, 1985). Administration of T4 from
July to December induced silvery skin in underyearling masu salmon, but
failed to induce seawater adaptability, darkening of dorsal and caudal fi n
margins and slimness of the body (Ikuta et al., 1985). T4 treatment, which
can induce silvering of the body surface, failed to induce darkening of dorsal
fi n margins in masu and amago salmon (Miwa and Inui, 1983; Ikuta et al.,
1985). T4 treatment increased the percentage of fi sh, which showed body
silvering (amago salmon: Miwa and Inui, 1985; rainbow trout: Coughlin
et al., 2001). T4 treatment caused increase in guanine levels in the skin and
serum in masu salmon (Kazuhiro et al., 1994).
6.3.1.2.2 Metabolism
Treatment of coho salmon with T3 in the diet accelerated the increase in
concentration of adult forms of blood hemoglobin, while dietary PTU
reduced this increase during smoltifi cation (Sullivan et al., 1985; for review:
Hoar, 1988). Thyroid hormones induce a shift towards slower isoforms
