Biology Reference
In-Depth Information
3.6.4 Duration of metamorphosis
The duration of metamorphosis, estimated from otolith examinations,
ranged from 18-76 days for
A. anguilla
, 18-41 days for
A. rostrata
and
10-40 days for
A. japonica
(Table 2). Assuming that estimations by otolith
examination are reliable, such fl exibility in the duration of metamorphosis
appears to be unique to
Anguilla
species. Hitherto, such a high variability
had not previously been observed in any other elopomorph. Despite the
fact that each
Anguilla
species spawns in a single specifi c area of the ocean,
their habitats during freshwater life, which are remote from their spawning
grounds, are relatively widespread. Therefore, variation in metamorphic
duration, as well as in the day of metamorphic onset, may be adaptive to
their distribution over wide areas.
The daily otolith increments in the zone from the fi rst feeding check
(FFC; Fig. 16) to the metamorphosis check (MC) are wide. However,
the increments become very narrow and undistinguishable at MC. The
increment width then becomes wider outside of the MC zone. This implies
that the growth rate of otoliths is slower in metamorphosing larvae than in
premetamorphic larvae and glass eels. The energy for metabolism during
metamorphosis is, perhaps, utilized for remodeling rather than growth.
Conclusion
The elopomorph fi shes are characterized by the occurrence of a distinctive
leptocephalus larval stage despite great differences in the morphology of
adult elopomorphs. Recent phylogenetic analyses appear to support the
monophyly of the Elopomorpha. Because leptocephalus-like larvae are
not seen in any other teleosts, the elopomorphs have developed a strategy
specifi c for this taxonomic group. The fact that the elopomorphs are
distributed all over the ocean from the deep sea to coastal waters implies
that the strategy has been profi table in the evolution and divergence of
the elopomorphs. The morphology of leptocephali is characterized by a
particular leaf-like body shape. Compared with larva of other teleosts,
leptocephali grow to a very large size (approximately 3 cm in total length
even in the smallest case) and the larval period is exceptionally long, lasting
several months in cases where the life cycle has been elucidated. These
features strongly suggest adaptation to their distribution by ocean currents,
from their spawning grounds to habitats spread over a wide area.
When larvae cease planktonic life to settle in the adult habitat, they
metamorphose into juveniles. The metamorphosis is accompanied by the
degradation of larval tissues such as the gelatinous matrix, remodeling
of tissues from a larval to juvenile type, and development of adult-type
tissues. Thus, they acquire mobility and adaptability appropriate to their
