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but the tissue shows a strong immunoreaction for pepsinogen at the post
climax, indicating the stomach is functional at this stage. The volume of the
gastric glands continues to increase throughout the development (Miwa et
al., 1992). Along with the stomach development, pyloric appendages are
formed, and the larval compact pancreas start to develop into the diffuse
pancreas, invading the liver along the hepatic portal vein. Thus, the digestive
system of the fl ounder assumes the adult form in the early juvenile stage
(Kurokawa and Suzuki, 1996). Morphological and functional development
of the stomach during metamorphosis have also been reported in other
fl atfi shes, such as
Solea senegalensis
(Ribeiro et al., 1999), turbot,
Scophthalmus
maximus
(Segner et al., 1994), spotted halibut,
Varasper variegates
(Hotta et
al., 2001a) and summer fl ounder,
Paralichthys dentatus
(Bisbal and Bengtson,
1995; Soffi entino and Specker, 2001).
The above mentioned gastric development is also controlled by thyroid
hormone, and the gastric development can be manipulated by altering the
thyroid status. T4 administration to premetamorphic larvae of the Japanese
fl ounder markedly increased the total volume of the gastric glands in the
stomach, while TU treatment arrested the development of the gastric glands
(Fig. 13). In addition, T4 administration induced precocious appearance of
Figure 13.
Effects of 100 ppb thyroxine (T
4
) or 30 ppm thiourea in ambient seawater on increase
in volume of gastric glands of Japanese fl ounder larvae. T4 treatment was terminated 2 weeks
after the start of the experiment. [From Miwa et al., 1992, with permission].
