Lamprey Metamorphosis - Metamorphosis in Fish - page 34

Partial clarifi cation of the importance of the TH decline arose from work
on goitrogen-induced metamorphosis in P. marinus and to a lesser extent
L. appendix . The induction of metamorphosis in P. marinus with KClO 4 was
fi rst demonstrated by Holmes and Youson (Holmes and Youson, 1993). Not
only were the larvae in this study induced to metamorphose at a time of
year when spontaneous metamorphosis does not naturally occur, but KClO 4
treatment superseded the size and CF requirements for metamorphosis.
Furthermore, it was discovered that KClO 4 -induced metamorphosis was size
dependent with 98%, 52%, and 22% induction of metamorphosis in larvae
> 130 mm, 110-119 mm and 65-95 mm in length, respectively (Holmes and
Youson, 1993). When the sera from the lampreys used in this study were
analyzed, it was found that KClO 4 signifi cantly suppressed T 3 concentrations
in all size groups and T 4 concentrations in the small and intermediate size
groups (Youson et al . , 1995b). The physiological link to the thyroid and the
absolute requirement for a decline in TH levels for induced metamorphosis
was confi rmed by observations that the KClO 4 -induced metamorphosis of
P. marinus larvae could be completely blocked by maintaining serum T 4
and T 3 concentrations at or above control levels with exposure to either
exogenous T 4 or T 3 (R.G. Manzon and Youson, 1997; R.G. Manzon et al . , 1998)
(Fig. 6). Moreover, a variety of different goitrogens (KClO 4 , NaClO 4 , potassium
thiocyanate, and methimazole) were effective at inducing metamorphosis
and the incidence of induction was correlated to the magnitude of the
decline in serum TH concentrations (R.G. Manzon et al . , 2001; Fig. 7).
Consistent with earlier work on G. australis (Leatherland et al . , 1990) and
Figure 6. Percent metamorphosing P. marinus following exposure to a high or low dose of
exogenous thyroxine (T4; 0.5 or 1 mg/L) or triiodothyronine (T3; 0.25 or 1 mg/L) in the
presence or absence of potassium perchlorate (KClO4; 0.05%) for 16 weeks. 80% of larvae
exposed to KClO4 alone were induced to metamorphose. Exogenous T4 and T3 did not induce
metamorphosis in any instance and completely blocked KClO4-induced metamorphosis at both
treatment doses. Sample for each treatment group is equal 30. (R.G. Manzon et al . , 1998).