Biology Reference
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width, body width, and pectoral fi n length, relative to body length increased,
whereas that of body height to body length decreased.
In the classifi cation of larva to adult stages described by Kubota (1961),
the metamorphic period ranged from stage IV to VII. Stage IV included
larvae at the early phase of metamorphosis with a leptocephalus-type
body (<126 mm in body length). Stage V (95-110 mm) was characterized
by a body appearance intermediate between leptocephalus and elver.
At stage VI (85-105 mm), the leptocephalus body shape was completely
transformed. The development of internal organs and erythrocytes were
visible through the skin. At stage VII (< 90 mm), the body became almost
juvenile-like although the pigmentation was incomplete. The gas bladder
was fi lled with gas.
Kubota (1961) examined the development of various tissues and organs
during conger eel metamorphosis. Adult-type melanophores (smaller than
larval melanophores) generally appeared from stage V or VI. However,
individual differences in the stage of the pigmentation were relatively large
compared with changes in other organs. Erythrocytes appeared at stage VI.
The epidermal layer of the skin became thick and mucous cells in the skin
increased in number at stages V and VI. The kidney of leptocephalus larvae
was composed of a protonephridium and a mesonephric kidney lacking
glomeruli. During metamorphosis, the protonephridium degenerated
from its posterior end and a number of glomeruli developed in the
mesonephric kidney. By the end of metamorphosis the internal structure of
the mesonephric kidney became similar to that of the adult. The digestive
tract transformed markedly during metamorphosis associated with
enlargement and torsion of the stomach and shortening and thickening
of the esophagus and intestines. A functional stomach was thus formed.
Ossifi cation of the vertebra, of the caudal regions, and of the head (except
for a few parts in the skull) took place at the beginning of metamorphosis
and was essentially complete by the end of metamorphosis. The ossifi cation
in the vertebra proceeded faster in the anterior and posterior regions than
in the mid region.
The metamorphic processes of Anguilla sp. are similar to those of
the conger eel except for the rate of body reduction; metamorphosing
Anguilla maintain their body length while body shape drastically changes.
Such metamorphic processes were fi rst elucidated from the collections of
wild metamorphosing Atlantic eels. Recently, after a challenging long-
term research program, glass eels of the Japanese eel, A. japonicus , were
successfully produced from artifi cially fertilized eggs by feeding larvae
with a slurry-type diet made from shark-egg powder and supplements
(Tanaka et al., 2003; Tanaka, 2003). This review introduces observations
on metamorphosis that occurred under aquarium conditions during this
study.
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