Kurokawa, T., Suzuki, T.
Abstract:
Flounder is a commercially important species in aquaculture of Japan.
Zooplanktons of rotifer and brine-shrimp nauplii are essential to rear
flounder larvae, because complete artificial diet for early larvae has
not yet been established. It is thought that larvae can not utilize the
artificial diet since their digestive organs have not fully developed. To
develop a complete diet, it is important to reveal the digestive ability
at the larval stage. In this study, to elucidate the developmental
process of digestive functions of the pancreas and the intestine during
the larval stage of flounder, distribution of trypsinogen and
aminopeptidase was traced by immunohistochemical methods;
trypsinogen is a proform of major protease synthesized in the
pancreas; aminopeptidase is one of the brush border digestive
enzymes of the intestine. In addition, the developmental process of
diffuse pancreas was followed from primordia stage at larva.
The larvae were kept in a 100 L tank supplied with running sea water
at 17 C, and fed on rotifers from 3 to 30 days post-hatching (dph), on
brine shrimp nauplii from 15 to 45 dph and on artificial diet from 20
dph.
From the pancreas, weak signals to antibody against trypsinogen
(anti-eTrg) were first detected at 2 dph. The signals increased
markedly from 2 to 3 dph. Zymogen granules containing trypsinogen
began to be secreted into the intestinal lumen at 3 dph. Thus, the
pancreas of flounder larva acquires an exocrine function by the time
of the first feeding at 3 dph. The pancreas was a small compact organ
at 3 dph. The pancreas started to elongate along the vein on the
intestine at 20 dph. After metamorphosis (45 dph), the pancreas had
branched out along the veins running to the porta hepatis from the
stomach, pyloric appendages, spleen and intestine. Thus, the pancreas
develops from compact-type organ of larvae to the diffuse-type organ
of the adult at the metamorphosis stage.
From the intestine, weak signals to antibody against aminopeptidase
(anti-ramp) were detected from the brush border of the posterior part
of the intestine at hatching. At 1 dph, the anterior part as well as the
posterior part of the intestine exhibited weak signals to anti-ramp of
intestine. At 2 dph, the rectum had differentiated morphologically,
and both intestine and rectum gave strong signals to anti-ramp. At 3
dph, the brush border of the intestine showed strong signals to
anti-ramp, but the signals of the rectum had been reduced. Thus, the
intestinal epithelial cells have already started to synthesize digestive
enzymes onto the brush border at hatch, and the functional
differentiation of the rectum from the intestine occurs at first feeding
at 3 dph.
It is known that the gastric glands start to synthesize digestive
enzyme (pepsinogen) at metamorphosis in flounder. Therefore, the
digestive organs of flounder larvae, including the pancreas and
intestine, other than the stomach, have acquired the ability to
synthesize digestive enzymes at first feeding.
(National Research Institute of Aquaculture Nansei, Mie, 516-01,
Japan)