Biology Reference
In-Depth Information
Secondly, whole body concentrations of thyroxine (T
4
) and
triiodothyronine (T
3
) became elevated during metamorphosis. In the conger
eel, the levels of T
4
increased from less than 5 ng/g to the maximum level
about 30 ng/g during the early metamorphic stage, and then declined toward
the end of metamorphosis (Yamano et al., 1991). On the other hand, levels
of T
3
(less than 0.15 ng/g in premetamorphic stage) increased gradually
during early metamorphosis and then steeply increased to about 2 ng/g in
Figure 9.
Histological sections of the thyroid glands of the conger eel (
Conger myriaster
). (a)
thyroid follicles of premetamorphic larva, (b) thyroid follicles of early metamorphic larva. Note
columnar cells and abundant capillaries (arrowheads) surrounding follicles. (c) active uptake
of colloid (arrowheads) by follicular cells. (d) thyroid follicles of late metamorphic larva. Scale
bars represent 50 µm. Reproduced from Yamano et al. (1991) with permission.
late metamorphosis (the profi le is illustrated in the upper fi gure of Fig. 13).
Thyroid hormone levels also increased during Japanese eel metamorphosis
but the patterns were different (Yamano et al., 2007). T
4
continued increasing
during metamorphosis and reached the highest level in fi sh at the end of
metamorphosis, or in fi sh that had just completed metamorphosis (Fig.
10, lower fi gure). In contrast, T
3
levels peaked during the late stages of
metamorphosis and declined toward the end of metamorphosis (Fig. 10,
upper fi gure). The signifi cance of the difference in the thyroid hormone
profi les between these two species, as well as functional divergence of T
3
and T
4
, are unknown.
Thirdly, the administration of thyroid hormone to premetamorphic
leptocephali induced or accelerated metamorphosis. Immersion of
premetamorphic leptocephali in seawater containing T
4
at 2 ppm and 6
ppm for one week stimulated the initiation of metamorphosis in the conger
