Biology Reference
In-Depth Information
lipase, all occurred before the start of feeding (8 days after hatching). The
stomach and gastric glands developed much later at metamorphosis. This
suggests that changes in dietary habits may accompany metamorphosis.
This success in producing glass eels is of great importance, both for
future commercial utilization of artifi cially produced glass eels for the seed
in aquaculture, and for understanding the developmental biology of eels.
However, improvements in the techniques for rearing leptocephali are still
required, because there still remain signifi cant problems that must be solved.
The survival rate was only 4% 50 days after hatching and declined to 1% by
100 days after hatching (Tanaka, 2003). Moreover, the growth rate of reared
larvae was less than half of that estimated for wild larvae (Tsukamoto et al.,
1994). The proportions of body dimension, particularly the ratio of body
depth to body length, and the number of pre-anal myomeres, also were
different between wild and artifi cial larvae (Tanaka et al., 2001).
3.5 Endocrine Control of Eel Metamorphosis
3.5.1 Thyroidal regulation
The involvement of the thyroid axis in fi sh metamorphosis in a manner
comparable to that in amphibian metamorphosis has been established in
various teleosts and is reviewed in other chapters in this topic. Although
endocrinological studies of elopomorph metamorphosis are scarce, and are
limited to anguilliforms, all of the data strongly support the regulation of
eel metamorphosis by thyroid hormone.
Firstly, histological observations of leptocephali before and during
metamorphosis clearly indicated that the development and activation of
the thyroid gland were related to the occurrence of metamorphosis. Thyroid
follicles were observed in all wild leptocephali of the Japanese eel ranging
from 19.8-32.6 mm in total length but they were smaller and less abundant
than in glass eels (Ozaki et al., 2000). A detailed examination using artifi cially
produced leptocephali of the Japanese eel further revealed that a single
thyroid follicle fi rst appeared in larvae 12.2 mm in total length and that the
number of follicles increased in proportion to total length from 25 to 45 mm
(Yamano et al., 2007). During metamorphosis of the conger eel, the thyroid
follicular cells became columnar with larger nuclei (Fig. 9b) (Yamano et
al., 1991). Active uptake of colloid by the epithelial cells was also notable
(Fig. 9c). These morphological features of the thyroid gland are evidence of
increased function of the gland. After the completion of metamorphosis the
thyroid gland decreased its histological activity (Fig. 9d). From histological
observations of wild
Anguilla
leptocephali, it has also been suggested that
TSH-producing cells in the pituitary appear just prior to the initiation of
metamorphosis (Ozaki et al., 2000).
